<!-- doc/src/sgml/func.sgml -->

 <chapter id="functions">
  <title>Functions and Operators</title>

  <indexterm zone="functions">
   <primary>function</primary>
  </indexterm>

  <indexterm zone="functions">
   <primary>operator</primary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname> provides a large number of
   functions and operators for the built-in data types.  This chapter
   describes most of them, although additional special-purpose functions
   appear in relevant sections of the manual.  Users can also
   define their own functions and operators, as described in
   <xref linkend="server-programming"/>.  The
   <application>psql</application> commands <command>\df</command> and
   <command>\do</command> can be used to list all
   available functions and operators, respectively.
  </para>

  <para>
   The notation used throughout this chapter to describe the argument and
   result data types of a function or operator is like this:
<synopsis>
<function>repeat</function> ( <type>text</type>, <type>integer</type> ) <returnvalue>text</returnvalue>
</synopsis>
   which says that the function <function>repeat</function> takes one text and
   one integer argument and returns a result of type text.  The right arrow
   is also used to indicate the result of an example, thus:
<programlisting>
repeat('Pg', 4) <returnvalue>PgPgPgPg</returnvalue>
</programlisting>
  </para>

  <para>
   If you are concerned about portability then note that most of
   the functions and operators described in this chapter, with the
   exception of the most trivial arithmetic and comparison operators
   and some explicitly marked functions, are not specified by the
   <acronym>SQL</acronym> standard. Some of this extended functionality
   is present in other <acronym>SQL</acronym> database management
   systems, and in many cases this functionality is compatible and
   consistent between the various implementations.
  </para>


  <sect1 id="functions-logical">
   <title>Logical Operators</title>

   <indexterm zone="functions-logical">
    <primary>operator</primary>
    <secondary>logical</secondary>
   </indexterm>

   <indexterm>
    <primary>Boolean</primary>
    <secondary>operators</secondary>
    <see>operators, logical</see>
   </indexterm>

   <para>
    The usual logical operators are available:

    <indexterm>
     <primary>AND (operator)</primary>
    </indexterm>

    <indexterm>
     <primary>OR (operator)</primary>
    </indexterm>

    <indexterm>
     <primary>NOT (operator)</primary>
    </indexterm>

    <indexterm>
     <primary>conjunction</primary>
    </indexterm>

    <indexterm>
     <primary>disjunction</primary>
    </indexterm>

    <indexterm>
     <primary>negation</primary>
    </indexterm>

<synopsis>
<type>boolean</type> <literal>AND</literal> <type>boolean</type> <returnvalue>boolean</returnvalue>
<type>boolean</type> <literal>OR</literal> <type>boolean</type> <returnvalue>boolean</returnvalue>
<literal>NOT</literal> <type>boolean</type> <returnvalue>boolean</returnvalue>
</synopsis>

    <acronym>SQL</acronym> uses a three-valued logic system with true,
    false, and <literal>null</literal>, which represents <quote>unknown</quote>.
    Observe the following truth tables:

    <informaltable>
     <tgroup cols="4">
      <thead>
       <row>
        <entry><replaceable>a</replaceable></entry>
        <entry><replaceable>b</replaceable></entry>
        <entry><replaceable>a</replaceable> AND <replaceable>b</replaceable></entry>
        <entry><replaceable>a</replaceable> OR <replaceable>b</replaceable></entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry>TRUE</entry>
        <entry>TRUE</entry>
        <entry>TRUE</entry>
        <entry>TRUE</entry>
       </row>

       <row>
        <entry>TRUE</entry>
        <entry>FALSE</entry>
        <entry>FALSE</entry>
        <entry>TRUE</entry>
       </row>

       <row>
        <entry>TRUE</entry>
        <entry>NULL</entry>
        <entry>NULL</entry>
        <entry>TRUE</entry>
       </row>

       <row>
        <entry>FALSE</entry>
        <entry>FALSE</entry>
        <entry>FALSE</entry>
        <entry>FALSE</entry>
       </row>

       <row>
        <entry>FALSE</entry>
        <entry>NULL</entry>
        <entry>FALSE</entry>
        <entry>NULL</entry>
       </row>

       <row>
        <entry>NULL</entry>
        <entry>NULL</entry>
        <entry>NULL</entry>
        <entry>NULL</entry>
       </row>
      </tbody>
     </tgroup>
    </informaltable>

    <informaltable>
     <tgroup cols="2">
      <thead>
       <row>
        <entry><replaceable>a</replaceable></entry>
        <entry>NOT <replaceable>a</replaceable></entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry>TRUE</entry>
        <entry>FALSE</entry>
       </row>

       <row>
        <entry>FALSE</entry>
        <entry>TRUE</entry>
       </row>

       <row>
        <entry>NULL</entry>
        <entry>NULL</entry>
       </row>
      </tbody>
     </tgroup>
    </informaltable>
   </para>

   <para>
    The operators <literal>AND</literal> and <literal>OR</literal> are
    commutative, that is, you can switch the left and right operands
    without affecting the result.  (However, it is not guaranteed that
    the left operand is evaluated before the right operand.  See <xref
    linkend="syntax-express-eval"/> for more information about the
    order of evaluation of subexpressions.)
   </para>
  </sect1>

  <sect1 id="functions-comparison">
   <title>Comparison Functions and Operators</title>

   <indexterm zone="functions-comparison">
    <primary>comparison</primary>
    <secondary>operators</secondary>
   </indexterm>

   <para>
    The usual comparison operators are available, as shown in <xref
    linkend="functions-comparison-op-table"/>.
   </para>

   <table id="functions-comparison-op-table">
    <title>Comparison Operators</title>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>Operator</entry>
       <entry>Description</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>
        <replaceable>datatype</replaceable> <literal>&lt;</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </entry>
       <entry>Less than</entry>
      </row>

      <row>
       <entry>
        <replaceable>datatype</replaceable> <literal>&gt;</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </entry>
       <entry>Greater than</entry>
      </row>

      <row>
       <entry>
        <replaceable>datatype</replaceable> <literal>&lt;=</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </entry>
       <entry>Less than or equal to</entry>
      </row>

      <row>
       <entry>
        <replaceable>datatype</replaceable> <literal>&gt;=</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </entry>
       <entry>Greater than or equal to</entry>
      </row>

      <row>
       <entry>
        <replaceable>datatype</replaceable> <literal>=</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </entry>
       <entry>Equal</entry>
      </row>

      <row>
       <entry>
        <replaceable>datatype</replaceable> <literal>&lt;&gt;</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </entry>
       <entry>Not equal</entry>
      </row>

      <row>
       <entry>
        <replaceable>datatype</replaceable> <literal>!=</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </entry>
       <entry>Not equal</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <note>
    <para>
     <literal>&lt;&gt;</literal> is the standard SQL notation for <quote>not
     equal</quote>.  <literal>!=</literal> is an alias, which is converted
     to <literal>&lt;&gt;</literal> at a very early stage of parsing.
     Hence, it is not possible to implement <literal>!=</literal>
     and <literal>&lt;&gt;</literal> operators that do different things.
    </para>
   </note>

   <para>
    These comparison operators are available for all built-in data types
    that have a natural ordering, including numeric, string, and date/time
    types.  In addition, arrays, composite types, and ranges can be compared
    if their component data types are comparable.
   </para>

   <para>
    It is usually possible to compare values of related data
    types as well; for example <type>integer</type> <literal>&gt;</literal>
    <type>bigint</type> will work.  Some cases of this sort are implemented
    directly by <quote>cross-type</quote> comparison operators, but if no
    such operator is available, the parser will coerce the less-general type
    to the more-general type and apply the latter's comparison operator.
   </para>

   <para>
    As shown above, all comparison operators are binary operators that
    return values of type <type>boolean</type>.  Thus, expressions like
    <literal>1 &lt; 2 &lt; 3</literal> are not valid (because there is
    no <literal>&lt;</literal> operator to compare a Boolean value with
    <literal>3</literal>).  Use the <literal>BETWEEN</literal> predicates
    shown below to perform range tests.
   </para>

   <para>
    There are also some comparison predicates, as shown in <xref
    linkend="functions-comparison-pred-table"/>.  These behave much like
    operators, but have special syntax mandated by the SQL standard.
   </para>

   <table id="functions-comparison-pred-table">
    <title>Comparison Predicates</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Predicate
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>datatype</replaceable> <literal>BETWEEN</literal> <replaceable>datatype</replaceable> <literal>AND</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Between (inclusive of the range endpoints).
       </para>
       <para>
        <literal>2 BETWEEN 1 AND 3</literal>
        <returnvalue>t</returnvalue>
       </para>
       <para>
        <literal>2 BETWEEN 3 AND 1</literal>
        <returnvalue>f</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>datatype</replaceable> <literal>NOT BETWEEN</literal> <replaceable>datatype</replaceable> <literal>AND</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Not between (the negation of <literal>BETWEEN</literal>).
       </para>
       <para>
        <literal>2 NOT BETWEEN 1 AND 3</literal>
        <returnvalue>f</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>datatype</replaceable> <literal>BETWEEN SYMMETRIC</literal> <replaceable>datatype</replaceable> <literal>AND</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Between, after sorting the two endpoint values.
       </para>
       <para>
        <literal>2 BETWEEN SYMMETRIC 3 AND 1</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>datatype</replaceable> <literal>NOT BETWEEN SYMMETRIC</literal> <replaceable>datatype</replaceable> <literal>AND</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Not between, after sorting the two endpoint values.
       </para>
       <para>
        <literal>2 NOT BETWEEN SYMMETRIC 3 AND 1</literal>
        <returnvalue>f</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>datatype</replaceable> <literal>IS DISTINCT FROM</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Not equal, treating null as a comparable value.
       </para>
       <para>
        <literal>1 IS DISTINCT FROM NULL</literal>
        <returnvalue>t</returnvalue> (rather than <literal>NULL</literal>)
       </para>
       <para>
        <literal>NULL IS DISTINCT FROM NULL</literal>
        <returnvalue>f</returnvalue> (rather than <literal>NULL</literal>)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>datatype</replaceable> <literal>IS NOT DISTINCT FROM</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Equal, treating null as a comparable value.
       </para>
       <para>
        <literal>1 IS NOT DISTINCT FROM NULL</literal>
        <returnvalue>f</returnvalue> (rather than <literal>NULL</literal>)
       </para>
       <para>
        <literal>NULL IS NOT DISTINCT FROM NULL</literal>
        <returnvalue>t</returnvalue> (rather than <literal>NULL</literal>)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>datatype</replaceable> <literal>IS NULL</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether value is null.
       </para>
       <para>
        <literal>1.5 IS NULL</literal>
        <returnvalue>f</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>datatype</replaceable> <literal>IS NOT NULL</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether value is not null.
       </para>
       <para>
        <literal>'null' IS NOT NULL</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>datatype</replaceable> <literal>ISNULL</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether value is null (nonstandard syntax).
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>datatype</replaceable> <literal>NOTNULL</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether value is not null (nonstandard syntax).
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>boolean</type> <literal>IS TRUE</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether boolean expression yields true.
       </para>
       <para>
        <literal>true IS TRUE</literal>
        <returnvalue>t</returnvalue>
       </para>
       <para>
        <literal>NULL::boolean IS TRUE</literal>
        <returnvalue>f</returnvalue> (rather than <literal>NULL</literal>)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>boolean</type> <literal>IS NOT TRUE</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether boolean expression yields false or unknown.
       </para>
       <para>
        <literal>true IS NOT TRUE</literal>
        <returnvalue>f</returnvalue>
       </para>
       <para>
        <literal>NULL::boolean IS NOT TRUE</literal>
        <returnvalue>t</returnvalue> (rather than <literal>NULL</literal>)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>boolean</type> <literal>IS FALSE</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether boolean expression yields false.
       </para>
       <para>
        <literal>true IS FALSE</literal>
        <returnvalue>f</returnvalue>
       </para>
       <para>
        <literal>NULL::boolean IS FALSE</literal>
        <returnvalue>f</returnvalue> (rather than <literal>NULL</literal>)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>boolean</type> <literal>IS NOT FALSE</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether boolean expression yields true or unknown.
       </para>
       <para>
        <literal>true IS NOT FALSE</literal>
        <returnvalue>t</returnvalue>
       </para>
       <para>
        <literal>NULL::boolean IS NOT FALSE</literal>
        <returnvalue>t</returnvalue> (rather than <literal>NULL</literal>)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>boolean</type> <literal>IS UNKNOWN</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether boolean expression yields unknown.
       </para>
       <para>
        <literal>true IS UNKNOWN</literal>
        <returnvalue>f</returnvalue>
       </para>
       <para>
        <literal>NULL::boolean IS UNKNOWN</literal>
        <returnvalue>t</returnvalue> (rather than <literal>NULL</literal>)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>boolean</type> <literal>IS NOT UNKNOWN</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether boolean expression yields true or false.
       </para>
       <para>
        <literal>true IS NOT UNKNOWN</literal>
        <returnvalue>t</returnvalue>
       </para>
       <para>
        <literal>NULL::boolean IS NOT UNKNOWN</literal>
        <returnvalue>f</returnvalue> (rather than <literal>NULL</literal>)
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    <indexterm>
     <primary>BETWEEN</primary>
    </indexterm>
    <indexterm>
     <primary>BETWEEN SYMMETRIC</primary>
    </indexterm>
    The <token>BETWEEN</token> predicate simplifies range tests:
<synopsis>
<replaceable>a</replaceable> BETWEEN <replaceable>x</replaceable> AND <replaceable>y</replaceable>
</synopsis>
    is equivalent to
<synopsis>
<replaceable>a</replaceable> &gt;= <replaceable>x</replaceable> AND <replaceable>a</replaceable> &lt;= <replaceable>y</replaceable>
</synopsis>
    Notice that <token>BETWEEN</token> treats the endpoint values as included
    in the range.
    <literal>BETWEEN SYMMETRIC</literal> is like <literal>BETWEEN</literal>
    except there is no requirement that the argument to the left of
    <literal>AND</literal> be less than or equal to the argument on the right.
    If it is not, those two arguments are automatically swapped, so that
    a nonempty range is always implied.
   </para>

   <para>
    The various variants of <literal>BETWEEN</literal> are implemented in
    terms of the ordinary comparison operators, and therefore will work for
    any data type(s) that can be compared.
   </para>

   <note>
    <para>
     The use of <literal>AND</literal> in the <literal>BETWEEN</literal>
     syntax creates an ambiguity with the use of <literal>AND</literal> as a
     logical operator.  To resolve this, only a limited set of expression
     types are allowed as the second argument of a <literal>BETWEEN</literal>
     clause.  If you need to write a more complex sub-expression
     in <literal>BETWEEN</literal>, write parentheses around the
     sub-expression.
    </para>
   </note>

   <para>
    <indexterm>
     <primary>IS DISTINCT FROM</primary>
    </indexterm>
    <indexterm>
     <primary>IS NOT DISTINCT FROM</primary>
    </indexterm>
    Ordinary comparison operators yield null (signifying <quote>unknown</quote>),
    not true or false, when either input is null.  For example,
    <literal>7 = NULL</literal> yields null, as does <literal>7 &lt;&gt; NULL</literal>.  When
    this behavior is not suitable, use the
    <literal>IS <optional> NOT </optional> DISTINCT FROM</literal> predicates:
<synopsis>
<replaceable>a</replaceable> IS DISTINCT FROM <replaceable>b</replaceable>
<replaceable>a</replaceable> IS NOT DISTINCT FROM <replaceable>b</replaceable>
</synopsis>
    For non-null inputs, <literal>IS DISTINCT FROM</literal> is
    the same as the <literal>&lt;&gt;</literal> operator.  However, if both
    inputs are null it returns false, and if only one input is
    null it returns true.  Similarly, <literal>IS NOT DISTINCT
    FROM</literal> is identical to <literal>=</literal> for non-null
    inputs, but it returns true when both inputs are null, and false when only
    one input is null. Thus, these predicates effectively act as though null
    were a normal data value, rather than <quote>unknown</quote>.
   </para>

   <para>
    <indexterm>
     <primary>IS NULL</primary>
    </indexterm>
    <indexterm>
     <primary>IS NOT NULL</primary>
    </indexterm>
    <indexterm>
     <primary>ISNULL</primary>
    </indexterm>
    <indexterm>
     <primary>NOTNULL</primary>
    </indexterm>
    To check whether a value is or is not null, use the predicates:
<synopsis>
<replaceable>expression</replaceable> IS NULL
<replaceable>expression</replaceable> IS NOT NULL
</synopsis>
    or the equivalent, but nonstandard, predicates:
<synopsis>
<replaceable>expression</replaceable> ISNULL
<replaceable>expression</replaceable> NOTNULL
</synopsis>
    <indexterm><primary>null value</primary><secondary>comparing</secondary></indexterm>
   </para>

   <para>
    Do <emphasis>not</emphasis> write
    <literal><replaceable>expression</replaceable> = NULL</literal>
    because <literal>NULL</literal> is not <quote>equal to</quote>
    <literal>NULL</literal>.  (The null value represents an unknown value,
    and it is not known whether two unknown values are equal.)
   </para>

  <tip>
   <para>
    Some applications might expect that
    <literal><replaceable>expression</replaceable> = NULL</literal>
    returns true if <replaceable>expression</replaceable> evaluates to
    the null value.  It is highly recommended that these applications
    be modified to comply with the SQL standard. However, if that
    cannot be done the <xref linkend="guc-transform-null-equals"/>
    configuration variable is available. If it is enabled,
    <productname>PostgreSQL</productname> will convert <literal>x =
    NULL</literal> clauses to <literal>x IS NULL</literal>.
   </para>
  </tip>

   <para>
    If the <replaceable>expression</replaceable> is row-valued, then
    <literal>IS NULL</literal> is true when the row expression itself is null
    or when all the row's fields are null, while
    <literal>IS NOT NULL</literal> is true when the row expression itself is non-null
    and all the row's fields are non-null.  Because of this behavior,
    <literal>IS NULL</literal> and <literal>IS NOT NULL</literal> do not always return
    inverse results for row-valued expressions; in particular, a row-valued
    expression that contains both null and non-null fields will return false
    for both tests.  In some cases, it may be preferable to
    write <replaceable>row</replaceable> <literal>IS DISTINCT FROM NULL</literal>
    or <replaceable>row</replaceable> <literal>IS NOT DISTINCT FROM NULL</literal>,
    which will simply check whether the overall row value is null without any
    additional tests on the row fields.
   </para>

   <para>
    <indexterm>
     <primary>IS TRUE</primary>
    </indexterm>
    <indexterm>
     <primary>IS NOT TRUE</primary>
    </indexterm>
    <indexterm>
     <primary>IS FALSE</primary>
    </indexterm>
    <indexterm>
     <primary>IS NOT FALSE</primary>
    </indexterm>
    <indexterm>
     <primary>IS UNKNOWN</primary>
    </indexterm>
    <indexterm>
     <primary>IS NOT UNKNOWN</primary>
    </indexterm>
    Boolean values can also be tested using the predicates
<synopsis>
<replaceable>boolean_expression</replaceable> IS TRUE
<replaceable>boolean_expression</replaceable> IS NOT TRUE
<replaceable>boolean_expression</replaceable> IS FALSE
<replaceable>boolean_expression</replaceable> IS NOT FALSE
<replaceable>boolean_expression</replaceable> IS UNKNOWN
<replaceable>boolean_expression</replaceable> IS NOT UNKNOWN
</synopsis>
    These will always return true or false, never a null value, even when the
    operand is null.
    A null input is treated as the logical value <quote>unknown</quote>.
    Notice that <literal>IS UNKNOWN</literal> and <literal>IS NOT UNKNOWN</literal> are
    effectively the same as <literal>IS NULL</literal> and
    <literal>IS NOT NULL</literal>, respectively, except that the input
    expression must be of Boolean type.
   </para>

<!-- IS OF does not conform to the ISO SQL behavior, so it is undocumented here
   <para>
    <indexterm>
     <primary>IS OF</primary>
    </indexterm>
    <indexterm>
     <primary>IS NOT OF</primary>
    </indexterm>
    It is possible to check the data type of an expression using the
    predicates
<synopsis>
<replaceable>expression</replaceable> IS OF (typename, ...)
<replaceable>expression</replaceable> IS NOT OF (typename, ...)
</synopsis>
    They return a boolean value based on whether the expression's data
    type is one of the listed data types.
   </para>
-->

   <para>
    Some comparison-related functions are also available, as shown in <xref
    linkend="functions-comparison-func-table"/>.
   </para>

  <table id="functions-comparison-func-table">
    <title>Comparison Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>num_nonnulls</primary>
        </indexterm>
        <function>num_nonnulls</function> ( <literal>VARIADIC</literal> <type>"any"</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the number of non-null arguments.
       </para>
       <para>
        <literal>num_nonnulls(1, NULL, 2)</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>num_nulls</primary>
        </indexterm>
        <function>num_nulls</function> ( <literal>VARIADIC</literal> <type>"any"</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the number of null arguments.
       </para>
       <para>
        <literal>num_nulls(1, NULL, 2)</literal>
        <returnvalue>1</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  </sect1>

  <sect1 id="functions-math">
   <title>Mathematical Functions and Operators</title>

   <para>
    Mathematical operators are provided for many
    <productname>PostgreSQL</productname> types. For types without
    standard mathematical conventions
    (e.g., date/time types) we
    describe the actual behavior in subsequent sections.
   </para>

   <para>
    <xref linkend="functions-math-op-table"/> shows the mathematical
    operators that are available for the standard numeric types.
    Unless otherwise noted, operators shown as
    accepting <replaceable>numeric_type</replaceable> are available for all
    the types <type>smallint</type>, <type>integer</type>,
    <type>bigint</type>, <type>numeric</type>, <type>real</type>,
    and <type>double precision</type>.
    Operators shown as accepting <replaceable>integral_type</replaceable>
    are available for the types <type>smallint</type>, <type>integer</type>,
    and <type>bigint</type>.
    Except where noted, each form of an operator returns the same data type
    as its argument(s).  Calls involving multiple argument data types, such
    as <type>integer</type> <literal>+</literal> <type>numeric</type>,
    are resolved by using the type appearing later in these lists.
   </para>

   <table id="functions-math-op-table">
    <title>Mathematical Operators</title>

    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Operator
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>numeric_type</replaceable> <literal>+</literal> <replaceable>numeric_type</replaceable>
        <returnvalue><replaceable>numeric_type</replaceable></returnvalue>
       </para>
       <para>
        Addition
       </para>
       <para>
        <literal>2 + 3</literal>
        <returnvalue>5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>+</literal> <replaceable>numeric_type</replaceable>
        <returnvalue><replaceable>numeric_type</replaceable></returnvalue>
       </para>
       <para>
        Unary plus (no operation)
       </para>
       <para>
        <literal>+ 3.5</literal>
        <returnvalue>3.5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>numeric_type</replaceable> <literal>-</literal> <replaceable>numeric_type</replaceable>
        <returnvalue><replaceable>numeric_type</replaceable></returnvalue>
       </para>
       <para>
        Subtraction
       </para>
       <para>
        <literal>2 - 3</literal>
        <returnvalue>-1</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>-</literal> <replaceable>numeric_type</replaceable>
        <returnvalue><replaceable>numeric_type</replaceable></returnvalue>
       </para>
       <para>
        Negation
       </para>
       <para>
        <literal>- (-4)</literal>
        <returnvalue>4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>numeric_type</replaceable> <literal>*</literal> <replaceable>numeric_type</replaceable>
        <returnvalue><replaceable>numeric_type</replaceable></returnvalue>
       </para>
       <para>
        Multiplication
       </para>
       <para>
        <literal>2 * 3</literal>
        <returnvalue>6</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>numeric_type</replaceable> <literal>/</literal> <replaceable>numeric_type</replaceable>
        <returnvalue><replaceable>numeric_type</replaceable></returnvalue>
       </para>
       <para>
        Division (for integral types, division truncates the result towards
        zero)
       </para>
       <para>
        <literal>5.0 / 2</literal>
        <returnvalue>2.5000000000000000</returnvalue>
       </para>
       <para>
        <literal>5 / 2</literal>
        <returnvalue>2</returnvalue>
       </para>
       <para>
        <literal>(-5) / 2</literal>
        <returnvalue>-2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>numeric_type</replaceable> <literal>%</literal> <replaceable>numeric_type</replaceable>
        <returnvalue><replaceable>numeric_type</replaceable></returnvalue>
       </para>
       <para>
        Modulo (remainder); available for <type>smallint</type>,
        <type>integer</type>, <type>bigint</type>, and <type>numeric</type>
       </para>
       <para>
        <literal>5 % 4</literal>
        <returnvalue>1</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>numeric</type> <literal>^</literal> <type>numeric</type>
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <type>double precision</type> <literal>^</literal> <type>double precision</type>
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Exponentiation (unlike typical mathematical practice, multiple uses of
        <literal>^</literal> will associate left to right)
       </para>
       <para>
        <literal>2 ^ 3</literal>
        <returnvalue>8</returnvalue>
       </para>
       <para>
        <literal>2 ^ 3 ^ 3</literal>
        <returnvalue>512</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>|/</literal> <type>double precision</type>
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Square root
       </para>
       <para>
        <literal>|/ 25.0</literal>
        <returnvalue>5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>||/</literal> <type>double precision</type>
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Cube root
       </para>
       <para>
        <literal>||/ 64.0</literal>
        <returnvalue>4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>@</literal> <replaceable>numeric_type</replaceable>
        <returnvalue><replaceable>numeric_type</replaceable></returnvalue>
       </para>
       <para>
        Absolute value
       </para>
       <para>
        <literal>@ -5.0</literal>
        <returnvalue>5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>integral_type</replaceable> <literal>&amp;</literal> <replaceable>integral_type</replaceable>
        <returnvalue><replaceable>integral_type</replaceable></returnvalue>
       </para>
       <para>
        Bitwise AND
       </para>
       <para>
        <literal>91 &amp; 15</literal>
        <returnvalue>11</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>integral_type</replaceable> <literal>|</literal> <replaceable>integral_type</replaceable>
        <returnvalue><replaceable>integral_type</replaceable></returnvalue>
       </para>
       <para>
        Bitwise OR
       </para>
       <para>
        <literal>32 | 3</literal>
        <returnvalue>35</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>integral_type</replaceable> <literal>#</literal> <replaceable>integral_type</replaceable>
        <returnvalue><replaceable>integral_type</replaceable></returnvalue>
       </para>
       <para>
        Bitwise exclusive OR
       </para>
       <para>
        <literal>17 # 5</literal>
        <returnvalue>20</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>~</literal> <replaceable>integral_type</replaceable>
        <returnvalue><replaceable>integral_type</replaceable></returnvalue>
       </para>
       <para>
        Bitwise NOT
       </para>
       <para>
        <literal>~1</literal>
        <returnvalue>-2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>integral_type</replaceable> <literal>&lt;&lt;</literal> <type>integer</type>
        <returnvalue><replaceable>integral_type</replaceable></returnvalue>
       </para>
       <para>
        Bitwise shift left
       </para>
       <para>
        <literal>1 &lt;&lt; 4</literal>
        <returnvalue>16</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>integral_type</replaceable> <literal>&gt;&gt;</literal> <type>integer</type>
        <returnvalue><replaceable>integral_type</replaceable></returnvalue>
       </para>
       <para>
        Bitwise shift right
       </para>
       <para>
        <literal>8 &gt;&gt; 2</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>

     </tbody>
    </tgroup>
   </table>

  <para>
   <xref linkend="functions-math-func-table"/> shows the available
   mathematical functions.
   Many of these functions are provided in multiple forms with different
   argument types.
   Except where noted, any given form of a function returns the same
   data type as its argument(s); cross-type cases are resolved in the
   same way as explained above for operators.
   The functions working with <type>double precision</type> data are mostly
   implemented on top of the host system's C library; accuracy and behavior in
   boundary cases can therefore vary depending on the host system.
  </para>

   <table id="functions-math-func-table">
    <title>Mathematical Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>abs</primary>
        </indexterm>
        <function>abs</function> ( <replaceable>numeric_type</replaceable> )
        <returnvalue><replaceable>numeric_type</replaceable></returnvalue>
       </para>
       <para>
        Absolute value
       </para>
       <para>
        <literal>abs(-17.4)</literal>
        <returnvalue>17.4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>cbrt</primary>
        </indexterm>
        <function>cbrt</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Cube root
       </para>
       <para>
        <literal>cbrt(64.0)</literal>
        <returnvalue>4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ceil</primary>
        </indexterm>
        <function>ceil</function> ( <type>numeric</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <function>ceil</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Nearest integer greater than or equal to argument
       </para>
       <para>
        <literal>ceil(42.2)</literal>
        <returnvalue>43</returnvalue>
       </para>
       <para>
        <literal>ceil(-42.8)</literal>
        <returnvalue>-42</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ceiling</primary>
        </indexterm>
        <function>ceiling</function> ( <type>numeric</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <function>ceiling</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Nearest integer greater than or equal to argument (same
        as <function>ceil</function>)
       </para>
       <para>
        <literal>ceiling(95.3)</literal>
        <returnvalue>96</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>degrees</primary>
        </indexterm>
        <function>degrees</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Converts radians to degrees
       </para>
       <para>
        <literal>degrees(0.5)</literal>
        <returnvalue>28.64788975654116</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>div</primary>
        </indexterm>
        <function>div</function> ( <parameter>y</parameter> <type>numeric</type>,
        <parameter>x</parameter> <type>numeric</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para>
        Integer quotient of <parameter>y</parameter>/<parameter>x</parameter>
        (truncates towards zero)
       </para>
       <para>
        <literal>div(9,4)</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>exp</primary>
        </indexterm>
        <function>exp</function> ( <type>numeric</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <function>exp</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Exponential (<literal>e</literal> raised to the given power)
       </para>
       <para>
        <literal>exp(1.0)</literal>
        <returnvalue>2.7182818284590452</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm id="function-factorial">
         <primary>factorial</primary>
        </indexterm>
        <function>factorial</function> ( <type>bigint</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para>
        Factorial
       </para>
       <para>
        <literal>factorial(5)</literal>
        <returnvalue>120</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>floor</primary>
        </indexterm>
        <function>floor</function> ( <type>numeric</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <function>floor</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Nearest integer less than or equal to argument
       </para>
       <para>
        <literal>floor(42.8)</literal>
        <returnvalue>42</returnvalue>
       </para>
       <para>
        <literal>floor(-42.8)</literal>
        <returnvalue>-43</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>gcd</primary>
        </indexterm>
        <function>gcd</function> ( <replaceable>numeric_type</replaceable>, <replaceable>numeric_type</replaceable> )
        <returnvalue><replaceable>numeric_type</replaceable></returnvalue>
       </para>
       <para>
        Greatest common divisor (the largest positive number that divides both
        inputs with no remainder); returns <literal>0</literal> if both inputs
        are zero; available for <type>integer</type>, <type>bigint</type>,
        and <type>numeric</type>
       </para>
       <para>
        <literal>gcd(1071, 462)</literal>
        <returnvalue>21</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>lcm</primary>
        </indexterm>
        <function>lcm</function> ( <replaceable>numeric_type</replaceable>, <replaceable>numeric_type</replaceable> )
        <returnvalue><replaceable>numeric_type</replaceable></returnvalue>
       </para>
       <para>
        Least common multiple (the smallest strictly positive number that is
        an integral multiple of both inputs); returns <literal>0</literal> if
        either input is zero; available for <type>integer</type>,
        <type>bigint</type>, and <type>numeric</type>
       </para>
       <para>
        <literal>lcm(1071, 462)</literal>
        <returnvalue>23562</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ln</primary>
        </indexterm>
        <function>ln</function> ( <type>numeric</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <function>ln</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Natural logarithm
       </para>
       <para>
        <literal>ln(2.0)</literal>
        <returnvalue>0.6931471805599453</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>log</primary>
        </indexterm>
        <function>log</function> ( <type>numeric</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <function>log</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Base 10 logarithm
       </para>
       <para>
        <literal>log(100)</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>log10</primary>
        </indexterm>
        <function>log10</function> ( <type>numeric</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <function>log10</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Base 10 logarithm (same as <function>log</function>)
       </para>
       <para>
        <literal>log10(1000)</literal>
        <returnvalue>3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>log</function> ( <parameter>b</parameter> <type>numeric</type>,
        <parameter>x</parameter> <type>numeric</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para>
        Logarithm of <parameter>x</parameter> to base <parameter>b</parameter>
       </para>
       <para>
       <literal>log(2.0, 64.0)</literal>
       <returnvalue>6.0000000000</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>min_scale</primary>
        </indexterm>
        <function>min_scale</function> ( <type>numeric</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Minimum scale (number of fractional decimal digits) needed
        to represent the supplied value precisely
       </para>
       <para>
        <literal>min_scale(8.4100)</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>mod</primary>
        </indexterm>
        <function>mod</function> ( <parameter>y</parameter> <replaceable>numeric_type</replaceable>,
        <parameter>x</parameter> <replaceable>numeric_type</replaceable> )
        <returnvalue><replaceable>numeric_type</replaceable></returnvalue>
       </para>
       <para>
        Remainder of <parameter>y</parameter>/<parameter>x</parameter>;
        available for <type>smallint</type>, <type>integer</type>,
        <type>bigint</type>, and <type>numeric</type>
       </para>
       <para>
        <literal>mod(9,4)</literal>
        <returnvalue>1</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pi</primary>
        </indexterm>
        <function>pi</function> (  )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Approximate value of <phrase role="symbol_font">&pi;</phrase>
       </para>
       <para>
        <literal>pi()</literal>
        <returnvalue>3.141592653589793</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>power</primary>
        </indexterm>
        <function>power</function> ( <parameter>a</parameter> <type>numeric</type>,
        <parameter>b</parameter> <type>numeric</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <function>power</function> ( <parameter>a</parameter> <type>double precision</type>,
        <parameter>b</parameter> <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        <parameter>a</parameter> raised to the power of <parameter>b</parameter>
       </para>
       <para>
        <literal>power(9, 3)</literal>
        <returnvalue>729</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>radians</primary>
        </indexterm>
        <function>radians</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Converts degrees to radians
       </para>
       <para>
        <literal>radians(45.0)</literal>
        <returnvalue>0.7853981633974483</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>round</primary>
        </indexterm>
        <function>round</function> ( <type>numeric</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <function>round</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Rounds to nearest integer
       </para>
       <para>
        <literal>round(42.4)</literal>
        <returnvalue>42</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>round</function> ( <parameter>v</parameter> <type>numeric</type>, <parameter>s</parameter> <type>integer</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para>
        Rounds <parameter>v</parameter> to <parameter>s</parameter> decimal
        places
       </para>
       <para>
        <literal>round(42.4382, 2)</literal>
        <returnvalue>42.44</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>scale</primary>
        </indexterm>
        <function>scale</function> ( <type>numeric</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Scale of the argument (the number of decimal digits in the fractional part)
       </para>
       <para>
        <literal>scale(8.4100)</literal>
        <returnvalue>4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>sign</primary>
        </indexterm>
        <function>sign</function> ( <type>numeric</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <function>sign</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Sign of the argument (-1, 0, or +1)
       </para>
       <para>
        <literal>sign(-8.4)</literal>
        <returnvalue>-1</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>sqrt</primary>
        </indexterm>
         <function>sqrt</function> ( <type>numeric</type> )
         <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
         <function>sqrt</function> ( <type>double precision</type> )
         <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Square root
       </para>
       <para>
        <literal>sqrt(2)</literal>
        <returnvalue>1.4142135623730951</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>trim_scale</primary>
        </indexterm>
        <function>trim_scale</function> ( <type>numeric</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para>
        Reduces the value's scale (number of fractional decimal digits) by
        removing trailing zeroes
       </para>
       <para>
        <literal>trim_scale(8.4100)</literal>
        <returnvalue>8.41</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>trunc</primary>
        </indexterm>
        <function>trunc</function> ( <type>numeric</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <function>trunc</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Truncates to integer (towards zero)
       </para>
       <para>
        <literal>trunc(42.8)</literal>
        <returnvalue>42</returnvalue>
       </para>
       <para>
        <literal>trunc(-42.8)</literal>
        <returnvalue>-42</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>trunc</function> ( <parameter>v</parameter> <type>numeric</type>, <parameter>s</parameter> <type>integer</type> )
       <returnvalue>numeric</returnvalue>
       </para>
       <para>
        Truncates <parameter>v</parameter> to <parameter>s</parameter>
        decimal places
       </para>
       <para>
        <literal>trunc(42.4382, 2)</literal>
        <returnvalue>42.43</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>width_bucket</primary>
        </indexterm>
        <function>width_bucket</function> ( <parameter>operand</parameter> <type>numeric</type>, <parameter>low</parameter> <type>numeric</type>, <parameter>high</parameter> <type>numeric</type>, <parameter>count</parameter> <type>integer</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para role="func_signature">
        <function>width_bucket</function> ( <parameter>operand</parameter> <type>double precision</type>, <parameter>low</parameter> <type>double precision</type>, <parameter>high</parameter> <type>double precision</type>, <parameter>count</parameter> <type>integer</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the number of the bucket in
        which <parameter>operand</parameter> falls in a histogram
        having <parameter>count</parameter> equal-width buckets spanning the
        range <parameter>low</parameter> to <parameter>high</parameter>.
        Returns <literal>0</literal>
        or <literal><parameter>count</parameter>+1</literal> for an input
        outside that range.
       </para>
       <para>
        <literal>width_bucket(5.35, 0.024, 10.06, 5)</literal>
        <returnvalue>3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>width_bucket</function> ( <parameter>operand</parameter> <type>anyelement</type>, <parameter>thresholds</parameter> <type>anyarray</type> )
       <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the number of the bucket in
        which <parameter>operand</parameter> falls given an array listing the
        lower bounds of the buckets.  Returns <literal>0</literal> for an
        input less than the first lower
        bound.  <parameter>operand</parameter> and the array elements can be
        of any type having standard comparison operators.
        The <parameter>thresholds</parameter> array <emphasis>must be
        sorted</emphasis>, smallest first, or unexpected results will be
        obtained.
       </para>
       <para>
        <literal>width_bucket(now(), array['yesterday', 'today', 'tomorrow']::timestamptz[])</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
    <xref linkend="functions-math-random-table"/> shows functions for
    generating random numbers.
  </para>

   <table id="functions-math-random-table">
    <title>Random Functions</title>

    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>random</primary>
        </indexterm>
        <function>random</function> ( )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Returns a random value in the range 0.0 &lt;= x &lt; 1.0
       </para>
       <para>
        <literal>random()</literal>
        <returnvalue>0.897124072839091</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>setseed</primary>
        </indexterm>
        <function>setseed</function> ( <type>double precision</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Sets the seed for subsequent <literal>random()</literal> calls;
        argument must be between -1.0 and 1.0, inclusive
       </para>
       <para>
        <literal>setseed(0.12345)</literal>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   The <function>random()</function> function uses a simple linear
   congruential algorithm.  It is fast but not suitable for cryptographic
   applications; see the <xref linkend="pgcrypto"/> module for a more
   secure alternative.
   If <function>setseed()</function> is called, the series of results of
   subsequent <function>random()</function> calls in the current session
   can be repeated by re-issuing <function>setseed()</function> with the same
   argument.
  </para>

  <para>
   <xref linkend="functions-math-trig-table"/> shows the
   available trigonometric functions.  Each of these functions comes in
   two variants, one that measures angles in radians and one that
   measures angles in degrees.
  </para>

   <table id="functions-math-trig-table">
    <title>Trigonometric Functions</title>

    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>acos</primary>
        </indexterm>
        <function>acos</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Inverse cosine, result in radians
       </para>
       <para>
        <literal>acos(1)</literal>
        <returnvalue>0</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>acosd</primary>
        </indexterm>
        <function>acosd</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Inverse cosine, result in degrees
       </para>
       <para>
        <literal>acosd(0.5)</literal>
        <returnvalue>60</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>asin</primary>
        </indexterm>
        <function>asin</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Inverse sine, result in radians
       </para>
       <para>
        <literal>asin(1)</literal>
        <returnvalue>1.5707963267948966</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>asind</primary>
        </indexterm>
        <function>asind</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Inverse sine, result in degrees
       </para>
       <para>
        <literal>asind(0.5)</literal>
        <returnvalue>30</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>atan</primary>
        </indexterm>
        <function>atan</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Inverse tangent, result in radians
       </para>
       <para>
        <literal>atan(1)</literal>
        <returnvalue>0.7853981633974483</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>atand</primary>
        </indexterm>
        <function>atand</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Inverse tangent, result in degrees
       </para>
       <para>
        <literal>atand(1)</literal>
        <returnvalue>45</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>atan2</primary>
        </indexterm>
        <function>atan2</function> ( <parameter>y</parameter> <type>double precision</type>,
        <parameter>x</parameter> <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Inverse tangent of
        <parameter>y</parameter>/<parameter>x</parameter>,
        result in radians
       </para>
       <para>
        <literal>atan2(1,0)</literal>
        <returnvalue>1.5707963267948966</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>atan2d</primary>
        </indexterm>
        <function>atan2d</function> ( <parameter>y</parameter> <type>double precision</type>,
        <parameter>x</parameter> <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Inverse tangent of
        <parameter>y</parameter>/<parameter>x</parameter>,
        result in degrees
       </para>
       <para>
        <literal>atan2d(1,0)</literal>
        <returnvalue>90</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>cos</primary>
        </indexterm>
        <function>cos</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Cosine, argument in radians
       </para>
       <para>
        <literal>cos(0)</literal>
        <returnvalue>1</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>cosd</primary>
        </indexterm>
        <function>cosd</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Cosine, argument in degrees
       </para>
       <para>
        <literal>cosd(60)</literal>
        <returnvalue>0.5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>cot</primary>
        </indexterm>
        <function>cot</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Cotangent, argument in radians
       </para>
       <para>
        <literal>cot(0.5)</literal>
        <returnvalue>1.830487721712452</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>cotd</primary>
        </indexterm>
        <function>cotd</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Cotangent, argument in degrees
       </para>
       <para>
        <literal>cotd(45)</literal>
        <returnvalue>1</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>sin</primary>
        </indexterm>
        <function>sin</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Sine, argument in radians
       </para>
       <para>
        <literal>sin(1)</literal>
        <returnvalue>0.8414709848078965</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>sind</primary>
        </indexterm>
        <function>sind</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Sine, argument in degrees
       </para>
       <para>
        <literal>sind(30)</literal>
        <returnvalue>0.5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>tan</primary>
        </indexterm>
        <function>tan</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Tangent, argument in radians
       </para>
       <para>
        <literal>tan(1)</literal>
        <returnvalue>1.5574077246549023</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>tand</primary>
        </indexterm>
        <function>tand</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Tangent, argument in degrees
       </para>
       <para>
        <literal>tand(45)</literal>
        <returnvalue>1</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <note>
   <para>
    Another way to work with angles measured in degrees is to use the unit
    transformation functions <literal><function>radians()</function></literal>
    and <literal><function>degrees()</function></literal> shown earlier.
    However, using the degree-based trigonometric functions is preferred,
    as that way avoids round-off error for special cases such
    as <literal>sind(30)</literal>.
   </para>
  </note>

  <para>
   <xref linkend="functions-math-hyp-table"/> shows the
   available hyperbolic functions.
  </para>

  <table id="functions-math-hyp-table">
    <title>Hyperbolic Functions</title>

    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>sinh</primary>
        </indexterm>
        <function>sinh</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Hyperbolic sine
       </para>
       <para>
        <literal>sinh(1)</literal>
        <returnvalue>1.1752011936438014</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>cosh</primary>
        </indexterm>
        <function>cosh</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Hyperbolic cosine
       </para>
       <para>
        <literal>cosh(0)</literal>
        <returnvalue>1</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>tanh</primary>
        </indexterm>
        <function>tanh</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Hyperbolic tangent
       </para>
       <para>
        <literal>tanh(1)</literal>
        <returnvalue>0.7615941559557649</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>asinh</primary>
        </indexterm>
        <function>asinh</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Inverse hyperbolic sine
       </para>
       <para>
        <literal>asinh(1)</literal>
        <returnvalue>0.881373587019543</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>acosh</primary>
        </indexterm>
        <function>acosh</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Inverse hyperbolic cosine
       </para>
       <para>
        <literal>acosh(1)</literal>
        <returnvalue>0</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>atanh</primary>
        </indexterm>
        <function>atanh</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Inverse hyperbolic tangent
       </para>
       <para>
        <literal>atanh(0.5)</literal>
        <returnvalue>0.5493061443340548</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  </sect1>


  <sect1 id="functions-string">
   <title>String Functions and Operators</title>

   <para>
    This section describes functions and operators for examining and
    manipulating string values.  Strings in this context include values
    of the types <type>character</type>, <type>character varying</type>,
    and <type>text</type>.  Except where noted, these functions and operators
    are declared to accept and return type <type>text</type>.  They will
    interchangeably accept <type>character varying</type> arguments.
    Values of type <type>character</type> will be converted
    to <type>text</type> before the function or operator is applied, resulting
    in stripping any trailing spaces in the <type>character</type> value.
   </para>

   <para>
    <acronym>SQL</acronym> defines some string functions that use
    key words, rather than commas, to separate
    arguments.  Details are in
    <xref linkend="functions-string-sql"/>.
    <productname>PostgreSQL</productname> also provides versions of these functions
    that use the regular function invocation syntax
    (see <xref linkend="functions-string-other"/>).
   </para>

   <note>
    <para>
     Before <productname>PostgreSQL</productname> 8.3, these functions would
     silently accept values of several non-string data types as well, due to
     the presence of implicit coercions from those data types to
     <type>text</type>.  Those coercions have been removed because they frequently
     caused surprising behaviors.  However, the string concatenation operator
     (<literal>||</literal>) still accepts non-string input, so long as at least one
     input is of a string type, as shown in <xref
     linkend="functions-string-sql"/>.  For other cases, insert an explicit
     coercion to <type>text</type> if you need to duplicate the previous behavior.
    </para>
   </note>

   <table id="functions-string-sql">
    <title><acronym>SQL</acronym> String Functions and Operators</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function/Operator
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>character string</primary>
         <secondary>concatenation</secondary>
        </indexterm>
        <type>text</type> <literal>||</literal> <type>text</type>
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Concatenates the two strings.
       </para>
       <para>
        <literal>'Post' || 'greSQL'</literal>
        <returnvalue>PostgreSQL</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>text</type> <literal>||</literal> <type>anynonarray</type>
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <type>anynonarray</type> <literal>||</literal> <type>text</type>
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the non-string input to text, then concatenates the two
        strings.  (The non-string input cannot be of an array type, because
        that would create ambiguity with the array <literal>||</literal>
        operators.  If you want to concatenate an array's text equivalent,
        cast it to <type>text</type> explicitly.)
       </para>
       <para>
        <literal>'Value: ' || 42</literal>
        <returnvalue>Value: 42</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>normalized</primary>
        </indexterm>
        <indexterm>
         <primary>Unicode normalization</primary>
        </indexterm>
         <type>text</type> <literal>IS</literal> <optional><literal>NOT</literal></optional> <optional><parameter>form</parameter></optional> <literal>NORMALIZED</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Checks whether the string is in the specified Unicode normalization
        form.  The optional <parameter>form</parameter> key word specifies the
        form: <literal>NFC</literal> (the default), <literal>NFD</literal>,
        <literal>NFKC</literal>, or <literal>NFKD</literal>.  This expression can
        only be used when the server encoding is <literal>UTF8</literal>.  Note
        that checking for normalization using this expression is often faster
        than normalizing possibly already normalized strings.
       </para>
       <para>
        <literal>U&amp;'\0061\0308bc' IS NFD NORMALIZED</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>bit_length</primary>
        </indexterm>
        <function>bit_length</function> ( <type>text</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns number of bits in the string (8
        times the <function>octet_length</function>).
       </para>
       <para>
        <literal>bit_length('jose')</literal>
        <returnvalue>32</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>char_length</primary>
        </indexterm>
        <indexterm>
         <primary>character string</primary>
         <secondary>length</secondary>
        </indexterm>
        <indexterm>
         <primary>length</primary>
         <secondary sortas="character string">of a character string</secondary>
         <see>character string, length</see>
        </indexterm>
        <function>char_length</function> ( <type>text</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>character_length</primary>
        </indexterm>
        <function>character_length</function> ( <type>text</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns number of characters in the string.
       </para>
       <para>
        <literal>char_length('jos&eacute;')</literal>
        <returnvalue>4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>lower</primary>
        </indexterm>
        <function>lower</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the string to all lower case, according to the rules of the
        database's locale.
       </para>
       <para>
        <literal>lower('TOM')</literal>
        <returnvalue>tom</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>normalize</primary>
        </indexterm>
        <indexterm>
         <primary>Unicode normalization</primary>
        </indexterm>
        <function>normalize</function> ( <type>text</type>
        <optional>, <parameter>form</parameter> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the string to the specified Unicode
        normalization form.  The optional <parameter>form</parameter> key word
        specifies the form: <literal>NFC</literal> (the default),
        <literal>NFD</literal>, <literal>NFKC</literal>, or
        <literal>NFKD</literal>.  This function can only be used when the
        server encoding is <literal>UTF8</literal>.
       </para>
       <para>
        <literal>normalize(U&amp;'\0061\0308bc', NFC)</literal>
        <returnvalue>U&amp;'\00E4bc'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>octet_length</primary>
        </indexterm>
        <function>octet_length</function> ( <type>text</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns number of bytes in the string.
       </para>
       <para>
        <literal>octet_length('jos&eacute;')</literal>
        <returnvalue>5</returnvalue> (if server encoding is UTF8)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>octet_length</primary>
        </indexterm>
        <function>octet_length</function> ( <type>character</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns number of bytes in the string.  Since this version of the
        function accepts type <type>character</type> directly, it will not
        strip trailing spaces.
       </para>
       <para>
        <literal>octet_length('abc '::character(4))</literal>
        <returnvalue>4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>overlay</primary>
        </indexterm>
        <function>overlay</function> ( <parameter>string</parameter> <type>text</type> <literal>PLACING</literal> <parameter>newsubstring</parameter> <type>text</type> <literal>FROM</literal> <parameter>start</parameter> <type>integer</type> <optional> <literal>FOR</literal> <parameter>count</parameter> <type>integer</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Replaces the substring of <parameter>string</parameter> that starts at
        the <parameter>start</parameter>'th character and extends
        for <parameter>count</parameter> characters
        with <parameter>newsubstring</parameter>.
        If <parameter>count</parameter> is omitted, it defaults to the length
        of <parameter>newsubstring</parameter>.
       </para>
       <para>
        <literal>overlay('Txxxxas' placing 'hom' from 2 for 4)</literal>
        <returnvalue>Thomas</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>position</primary>
        </indexterm>
        <function>position</function> ( <parameter>substring</parameter> <type>text</type> <literal>IN</literal> <parameter>string</parameter> <type>text</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns starting index of specified <parameter>substring</parameter>
        within <parameter>string</parameter>, or zero if it's not present.
       </para>
       <para>
        <literal>position('om' in 'Thomas')</literal>
        <returnvalue>3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>substring</primary>
        </indexterm>
        <function>substring</function> ( <parameter>string</parameter> <type>text</type> <optional> <literal>FROM</literal> <parameter>start</parameter> <type>integer</type> </optional> <optional> <literal>FOR</literal> <parameter>count</parameter> <type>integer</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Extracts the substring of <parameter>string</parameter> starting at
        the <parameter>start</parameter>'th character if that is specified,
        and stopping after <parameter>count</parameter> characters if that is
        specified.  Provide at least one of <parameter>start</parameter>
        and <parameter>count</parameter>.
       </para>
       <para>
        <literal>substring('Thomas' from 2 for 3)</literal>
        <returnvalue>hom</returnvalue>
       </para>
       <para>
        <literal>substring('Thomas' from 3)</literal>
        <returnvalue>omas</returnvalue>
       </para>
       <para>
        <literal>substring('Thomas' for 2)</literal>
        <returnvalue>Th</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>substring</function> ( <parameter>string</parameter> <type>text</type> <literal>FROM</literal> <parameter>pattern</parameter> <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Extracts substring matching POSIX regular expression; see
        <xref linkend="functions-posix-regexp"/>.
       </para>
       <para>
        <literal>substring('Thomas' from '...$')</literal>
        <returnvalue>mas</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>substring</function> ( <parameter>string</parameter> <type>text</type> <literal>SIMILAR</literal> <parameter>pattern</parameter> <type>text</type> <literal>ESCAPE</literal> <parameter>escape</parameter> <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <function>substring</function> ( <parameter>string</parameter> <type>text</type> <literal>FROM</literal> <parameter>pattern</parameter> <type>text</type> <literal>FOR</literal> <parameter>escape</parameter> <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Extracts substring matching <acronym>SQL</acronym> regular expression;
        see <xref linkend="functions-similarto-regexp"/>.  The first form has
        been specified since SQL:2003; the second form was only in SQL:1999
        and should be considered obsolete.
       </para>
       <para>
        <literal>substring('Thomas' similar '%#"o_a#"_' escape '#')</literal>
        <returnvalue>oma</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>trim</primary>
        </indexterm>
        <function>trim</function> ( <optional> <literal>LEADING</literal> | <literal>TRAILING</literal> | <literal>BOTH</literal> </optional>
        <optional> <parameter>characters</parameter> <type>text</type> </optional> <literal>FROM</literal>
        <parameter>string</parameter> <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Removes the longest string containing only characters in
        <parameter>characters</parameter> (a space by default) from the
        start, end, or both ends (<literal>BOTH</literal> is the default)
        of <parameter>string</parameter>.
       </para>
       <para>
        <literal>trim(both 'xyz' from 'yxTomxx')</literal>
        <returnvalue>Tom</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>trim</function> ( <optional> <literal>LEADING</literal> | <literal>TRAILING</literal> | <literal>BOTH</literal> </optional> <optional> <literal>FROM</literal> </optional>
        <parameter>string</parameter> <type>text</type> <optional>,
        <parameter>characters</parameter> <type>text</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        This is a non-standard syntax for <function>trim()</function>.
       </para>
       <para>
        <literal>trim(both from 'yxTomxx', 'xyz')</literal>
        <returnvalue>Tom</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>upper</primary>
        </indexterm>
        <function>upper</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the string to all upper case, according to the rules of the
        database's locale.
       </para>
       <para>
        <literal>upper('tom')</literal>
        <returnvalue>TOM</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    Additional string manipulation functions are available and are
    listed in <xref linkend="functions-string-other"/>.  Some of them are used internally to implement the
    <acronym>SQL</acronym>-standard string functions listed in <xref linkend="functions-string-sql"/>.
   </para>

   <table id="functions-string-other">
    <title>Other String Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ascii</primary>
        </indexterm>
        <function>ascii</function> ( <type>text</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the numeric code of the first character of the argument.
        In <acronym>UTF8</acronym> encoding, returns the Unicode code point
        of the character.  In other multibyte encodings, the argument must
        be an <acronym>ASCII</acronym> character.
       </para>
       <para>
        <literal>ascii('x')</literal>
        <returnvalue>120</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>btrim</primary>
        </indexterm>
        <function>btrim</function> ( <parameter>string</parameter> <type>text</type>
        <optional>, <parameter>characters</parameter> <type>text</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Removes the longest string containing only characters
        in <parameter>characters</parameter> (a space by default)
        from the start and end of <parameter>string</parameter>.
       </para>
       <para>
        <literal>btrim('xyxtrimyyx', 'xyz')</literal>
        <returnvalue>trim</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>chr</primary>
        </indexterm>
        <function>chr</function> ( <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the character with the given code. In <acronym>UTF8</acronym>
        encoding the argument is treated as a Unicode code point. In other
        multibyte encodings the argument must designate
        an <acronym>ASCII</acronym> character.  <literal>chr(0)</literal> is
        disallowed because text data types cannot store that character.
      </para>
      <para>
        <literal>chr(65)</literal>
        <returnvalue>A</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>concat</primary>
        </indexterm>
        <function>concat</function> ( <parameter>val1</parameter> <type>"any"</type>
         [, <parameter>val2</parameter> <type>"any"</type> [, ...] ] )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Concatenates the text representations of all the arguments.
        NULL arguments are ignored.
       </para>
       <para>
        <literal>concat('abcde', 2, NULL, 22)</literal>
        <returnvalue>abcde222</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>concat_ws</primary>
        </indexterm>
        <function>concat_ws</function> ( <parameter>sep</parameter> <type>text</type>,
        <parameter>val1</parameter> <type>"any"</type>
        [, <parameter>val2</parameter> <type>"any"</type> [, ...] ] )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Concatenates all but the first argument, with separators. The first
        argument is used as the separator string, and should not be NULL.
        Other NULL arguments are ignored.
       </para>
       <para>
        <literal>concat_ws(',', 'abcde', 2, NULL, 22)</literal>
        <returnvalue>abcde,2,22</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>format</primary>
        </indexterm>
        <function>format</function> ( <parameter>formatstr</parameter> <type>text</type>
        [, <parameter>formatarg</parameter> <type>"any"</type> [, ...] ] )
        <returnvalue>text</returnvalue>
       </para>
       <para>
         Formats arguments according to a format string;
         see <xref linkend="functions-string-format"/>.
         This function is similar to the C function <function>sprintf</function>.
       </para>
       <para>
        <literal>format('Hello %s, %1$s', 'World')</literal>
        <returnvalue>Hello World, World</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>initcap</primary>
        </indexterm>
        <function>initcap</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the first letter of each word to upper case and the
        rest to lower case. Words are sequences of alphanumeric
        characters separated by non-alphanumeric characters.
       </para>
       <para>
        <literal>initcap('hi THOMAS')</literal>
        <returnvalue>Hi Thomas</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>left</primary>
        </indexterm>
        <function>left</function> ( <parameter>string</parameter> <type>text</type>,
        <parameter>n</parameter> <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns first <parameter>n</parameter> characters in the
        string, or when <parameter>n</parameter> is negative, returns
        all but last |<parameter>n</parameter>| characters.
       </para>
       <para>
        <literal>left('abcde', 2)</literal>
        <returnvalue>ab</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>length</primary>
        </indexterm>
        <function>length</function> ( <type>text</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the number of characters in the string.
       </para>
       <para>
        <literal>length('jose')</literal>
        <returnvalue>4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>lpad</primary>
        </indexterm>
        <function>lpad</function> ( <parameter>string</parameter> <type>text</type>,
        <parameter>length</parameter> <type>integer</type>
        <optional>, <parameter>fill</parameter> <type>text</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Extends the <parameter>string</parameter> to length
        <parameter>length</parameter> by prepending the characters
        <parameter>fill</parameter> (a space by default).  If the
        <parameter>string</parameter> is already longer than
        <parameter>length</parameter> then it is truncated (on the right).
       </para>
       <para>
        <literal>lpad('hi', 5, 'xy')</literal>
        <returnvalue>xyxhi</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ltrim</primary>
        </indexterm>
        <function>ltrim</function> ( <parameter>string</parameter> <type>text</type>
        <optional>, <parameter>characters</parameter> <type>text</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Removes the longest string containing only characters in
        <parameter>characters</parameter> (a space by default) from the start of
        <parameter>string</parameter>.
       </para>
       <para>
        <literal>ltrim('zzzytest', 'xyz')</literal>
        <returnvalue>test</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>md5</primary>
        </indexterm>
        <function>md5</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Computes the MD5 <link linkend="functions-hash-note">hash</link> of
        the argument, with the result written in hexadecimal.
       </para>
       <para>
        <literal>md5('abc')</literal>
        <returnvalue>900150983cd24fb0&zwsp;d6963f7d28e17f72</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>parse_ident</primary>
        </indexterm>
        <function>parse_ident</function> ( <parameter>qualified_identifier</parameter> <type>text</type>
        [, <parameter>strict_mode</parameter> <type>boolean</type> <literal>DEFAULT</literal> <literal>true</literal> ] )
        <returnvalue>text[]</returnvalue>
       </para>
       <para>
        Splits <parameter>qualified_identifier</parameter> into an array of
        identifiers, removing any quoting of individual identifiers.  By
        default, extra characters after the last identifier are considered an
        error; but if the second parameter is <literal>false</literal>, then such
        extra characters are ignored. (This behavior is useful for parsing
        names for objects like functions.) Note that this function does not
        truncate over-length identifiers. If you want truncation you can cast
        the result to <type>name[]</type>.
       </para>
       <para>
        <literal>parse_ident('"SomeSchema".someTable')</literal>
        <returnvalue>{SomeSchema,sometable}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_client_encoding</primary>
        </indexterm>
        <function>pg_client_encoding</function> ( )
        <returnvalue>name</returnvalue>
       </para>
       <para>
        Returns current client encoding name.
       </para>
       <para>
        <literal>pg_client_encoding()</literal>
        <returnvalue>UTF8</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>quote_ident</primary>
        </indexterm>
        <function>quote_ident</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the given string suitably quoted to be used as an identifier
        in an <acronym>SQL</acronym> statement string.
        Quotes are added only if necessary (i.e., if the string contains
        non-identifier characters or would be case-folded).
        Embedded quotes are properly doubled.
        See also <xref linkend="plpgsql-quote-literal-example"/>.
       </para>
       <para>
        <literal>quote_ident('Foo bar')</literal>
        <returnvalue>"Foo bar"</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>quote_literal</primary>
        </indexterm>
        <function>quote_literal</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the given string suitably quoted to be used as a string literal
        in an <acronym>SQL</acronym> statement string.
        Embedded single-quotes and backslashes are properly doubled.
        Note that <function>quote_literal</function> returns null on null
        input; if the argument might be null,
        <function>quote_nullable</function> is often more suitable.
        See also <xref linkend="plpgsql-quote-literal-example"/>.
       </para>
       <para>
        <literal>quote_literal(E'O\'Reilly')</literal>
        <returnvalue>'O''Reilly'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>quote_literal</function> ( <type>anyelement</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the given value to text and then quotes it as a literal.
        Embedded single-quotes and backslashes are properly doubled.
       </para>
       <para>
        <literal>quote_literal(42.5)</literal>
        <returnvalue>'42.5'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>quote_nullable</primary>
        </indexterm>
        <function>quote_nullable</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the given string suitably quoted to be used as a string literal
        in an <acronym>SQL</acronym> statement string; or, if the argument
        is null, returns <literal>NULL</literal>.
        Embedded single-quotes and backslashes are properly doubled.
        See also <xref linkend="plpgsql-quote-literal-example"/>.
       </para>
       <para>
        <literal>quote_nullable(NULL)</literal>
        <returnvalue>NULL</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>quote_nullable</function> ( <type>anyelement</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the given value to text and then quotes it as a literal;
        or, if the argument is null, returns <literal>NULL</literal>.
        Embedded single-quotes and backslashes are properly doubled.
       </para>
       <para>
        <literal>quote_nullable(42.5)</literal>
        <returnvalue>'42.5'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regexp_match</primary>
        </indexterm>
        <function>regexp_match</function> ( <parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type> [, <parameter>flags</parameter> <type>text</type> ] )
        <returnvalue>text[]</returnvalue>
       </para>
       <para>
        Returns captured substring(s) resulting from the first match of a POSIX
        regular expression to the <parameter>string</parameter>; see
        <xref linkend="functions-posix-regexp"/>.
       </para>
       <para>
        <literal>regexp_match('foobarbequebaz', '(bar)(beque)')</literal>
        <returnvalue>{bar,beque}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regexp_matches</primary>
        </indexterm>
        <function>regexp_matches</function> ( <parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type> [, <parameter>flags</parameter> <type>text</type> ] )
        <returnvalue>setof text[]</returnvalue>
       </para>
       <para>
        Returns captured substring(s) resulting from matching a POSIX regular
        expression to the <parameter>string</parameter>; see
        <xref linkend="functions-posix-regexp"/>.
       </para>
       <para>
        <literal>regexp_matches('foobarbequebaz', 'ba.', 'g')</literal>
        <returnvalue></returnvalue>
<programlisting>
 {bar}
 {baz}
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regexp_replace</primary>
        </indexterm>
        <function>regexp_replace</function> ( <parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type>, <parameter>replacement</parameter> <type>text</type> [, <parameter>flags</parameter> <type>text</type> ] )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Replaces substring(s) matching a POSIX regular expression; see
        <xref linkend="functions-posix-regexp"/>.
       </para>
       <para>
        <literal>regexp_replace('Thomas', '.[mN]a.', 'M')</literal>
        <returnvalue>ThM</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regexp_split_to_array</primary>
        </indexterm>
        <function>regexp_split_to_array</function> ( <parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type> [, <parameter>flags</parameter> <type>text</type> ] )
        <returnvalue>text[]</returnvalue>
       </para>
       <para>
        Splits <parameter>string</parameter> using a POSIX regular
        expression as the delimiter, producing an array of results; see
        <xref linkend="functions-posix-regexp"/>.
       </para>
       <para>
        <literal>regexp_split_to_array('hello world', '\s+')</literal>
        <returnvalue>{hello,world}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regexp_split_to_table</primary>
        </indexterm>
        <function>regexp_split_to_table</function> ( <parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type> [, <parameter>flags</parameter> <type>text</type> ] )
        <returnvalue>setof text</returnvalue>
       </para>
       <para>
        Splits <parameter>string</parameter> using a POSIX regular
        expression as the delimiter, producing a set of results; see
        <xref linkend="functions-posix-regexp"/>.
       </para>
       <para>
        <literal>regexp_split_to_table('hello world', '\s+')</literal>
        <returnvalue></returnvalue>
<programlisting>
 hello
 world
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>repeat</primary>
        </indexterm>
        <function>repeat</function> ( <parameter>string</parameter> <type>text</type>, <parameter>number</parameter> <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Repeats <parameter>string</parameter> the specified
        <parameter>number</parameter> of times.
       </para>
       <para>
        <literal>repeat('Pg', 4)</literal>
        <returnvalue>PgPgPgPg</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>replace</primary>
        </indexterm>
        <function>replace</function> ( <parameter>string</parameter> <type>text</type>,
        <parameter>from</parameter> <type>text</type>,
        <parameter>to</parameter> <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Replaces all occurrences in <parameter>string</parameter> of
        substring <parameter>from</parameter> with
        substring <parameter>to</parameter>.
       </para>
       <para>
        <literal>replace('abcdefabcdef', 'cd', 'XX')</literal>
        <returnvalue>abXXefabXXef</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>reverse</primary>
        </indexterm>
        <function>reverse</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Reverses the order of the characters in the string.
       </para>
       <para>
        <literal>reverse('abcde')</literal>
        <returnvalue>edcba</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>right</primary>
        </indexterm>
        <function>right</function> ( <parameter>string</parameter> <type>text</type>,
         <parameter>n</parameter> <type>integer</type> )
<type></type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns last <parameter>n</parameter> characters in the string,
        or when <parameter>n</parameter> is negative, returns all but
        first |<parameter>n</parameter>| characters.
       </para>
       <para>
        <literal>right('abcde', 2)</literal>
        <returnvalue>de</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>rpad</primary>
        </indexterm>
        <function>rpad</function> ( <parameter>string</parameter> <type>text</type>,
        <parameter>length</parameter> <type>integer</type>
        <optional>, <parameter>fill</parameter> <type>text</type> </optional> )
<type></type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Extends the <parameter>string</parameter> to length
        <parameter>length</parameter> by appending the characters
        <parameter>fill</parameter> (a space by default).  If the
        <parameter>string</parameter> is already longer than
        <parameter>length</parameter> then it is truncated.
       </para>
       <para>
        <literal>rpad('hi', 5, 'xy')</literal>
        <returnvalue>hixyx</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>rtrim</primary>
        </indexterm>
        <function>rtrim</function> ( <parameter>string</parameter> <type>text</type>
         <optional>, <parameter>characters</parameter> <type>text</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Removes the longest string containing only characters in
        <parameter>characters</parameter> (a space by default) from the end of
        <parameter>string</parameter>.
       </para>
       <para>
        <literal>rtrim('testxxzx', 'xyz')</literal>
        <returnvalue>test</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>split_part</primary>
        </indexterm>
        <function>split_part</function> ( <parameter>string</parameter> <type>text</type>,
        <parameter>delimiter</parameter> <type>text</type>,
        <parameter>n</parameter> <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Splits <parameter>string</parameter> at occurrences
        of <parameter>delimiter</parameter> and returns
        the <parameter>n</parameter>'th field (counting from one).
       </para>
       <para>
        <literal>split_part('abc~@~def~@~ghi', '~@~', 2)</literal>
        <returnvalue>def</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>strpos</primary>
        </indexterm>
        <function>strpos</function> ( <parameter>string</parameter> <type>text</type>, <parameter>substring</parameter> <type>text</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns starting index of specified <parameter>substring</parameter>
        within <parameter>string</parameter>, or zero if it's not present.
        (Same as <literal>position(<parameter>substring</parameter> in
        <parameter>string</parameter>)</literal>, but note the reversed
        argument order.)
       </para>
       <para>
        <literal>strpos('high', 'ig')</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>substr</primary>
        </indexterm>
        <function>substr</function> ( <parameter>string</parameter> <type>text</type>, <parameter>start</parameter> <type>integer</type> <optional>, <parameter>count</parameter> <type>integer</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Extracts the substring of <parameter>string</parameter> starting at
        the <parameter>start</parameter>'th character,
        and extending for <parameter>count</parameter> characters if that is
        specified.  (Same
        as <literal>substring(<parameter>string</parameter>
        from <parameter>start</parameter>
        for <parameter>count</parameter>)</literal>.)
       </para>
       <para>
        <literal>substr('alphabet', 3)</literal>
        <returnvalue>phabet</returnvalue>
       </para>
       <para>
        <literal>substr('alphabet', 3, 2)</literal>
        <returnvalue>ph</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>starts_with</primary>
        </indexterm>
        <function>starts_with</function> ( <parameter>string</parameter> <type>text</type>, <parameter>prefix</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Returns true if <parameter>string</parameter> starts
        with <parameter>prefix</parameter>.
       </para>
       <para>
        <literal>starts_with('alphabet', 'alph')</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>string_to_array</primary>
        </indexterm>
        <function>string_to_array</function> ( <parameter>string</parameter> <type>text</type>, <parameter>delimiter</parameter> <type>text</type> <optional>, <parameter>null_string</parameter> <type>text</type> </optional> )
        <returnvalue>text[]</returnvalue>
       </para>
       <para>
        Splits the <parameter>string</parameter> at occurrences
        of <parameter>delimiter</parameter> and forms the resulting fields
        into a <type>text</type> array.
        If <parameter>delimiter</parameter> is <literal>NULL</literal>,
        each character in the <parameter>string</parameter> will become a
        separate element in the array.
        If <parameter>delimiter</parameter> is an empty string, then
        the <parameter>string</parameter> is treated as a single field.
        If <parameter>null_string</parameter> is supplied and is
        not <literal>NULL</literal>, fields matching that string are
        replaced by <literal>NULL</literal>.
       </para>
       <para>
        <literal>string_to_array('xx~~yy~~zz', '~~', 'yy')</literal>
        <returnvalue>{xx,NULL,zz}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>string_to_table</primary>
        </indexterm>
        <function>string_to_table</function> ( <parameter>string</parameter> <type>text</type>, <parameter>delimiter</parameter> <type>text</type> <optional>, <parameter>null_string</parameter> <type>text</type> </optional> )
        <returnvalue>setof text</returnvalue>
       </para>
       <para>
        Splits the <parameter>string</parameter> at occurrences
        of <parameter>delimiter</parameter> and returns the resulting fields
        as a set of <type>text</type> rows.
        If <parameter>delimiter</parameter> is <literal>NULL</literal>,
        each character in the <parameter>string</parameter> will become a
        separate row of the result.
        If <parameter>delimiter</parameter> is an empty string, then
        the <parameter>string</parameter> is treated as a single field.
        If <parameter>null_string</parameter> is supplied and is
        not <literal>NULL</literal>, fields matching that string are
        replaced by <literal>NULL</literal>.
       </para>
       <para>
        <literal>string_to_table('xx~^~yy~^~zz', '~^~', 'yy')</literal>
        <returnvalue></returnvalue>
<programlisting>
 xx
 NULL
 zz
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_ascii</primary>
        </indexterm>
        <function>to_ascii</function> ( <parameter>string</parameter> <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <function>to_ascii</function> ( <parameter>string</parameter> <type>text</type>,
        <parameter>encoding</parameter> <type>name</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <function>to_ascii</function> ( <parameter>string</parameter> <type>text</type>,
        <parameter>encoding</parameter> <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts <parameter>string</parameter> to <acronym>ASCII</acronym>
        from another encoding, which may be identified by name or number.
        If <parameter>encoding</parameter> is omitted the database encoding
        is assumed (which in practice is the only useful case).
        The conversion consists primarily of dropping accents.
        Conversion is only supported
        from <literal>LATIN1</literal>, <literal>LATIN2</literal>,
        <literal>LATIN9</literal>, and <literal>WIN1250</literal> encodings.
        (See the <xref linkend="unaccent"/> module for another, more flexible
        solution.)
       </para>
       <para>
        <literal>to_ascii('Kar&eacute;l')</literal>
        <returnvalue>Karel</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_hex</primary>
        </indexterm>
        <function>to_hex</function> ( <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <function>to_hex</function> ( <type>bigint</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the number to its equivalent hexadecimal representation.
       </para>
       <para>
        <literal>to_hex(2147483647)</literal>
        <returnvalue>7fffffff</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>translate</primary>
        </indexterm>
        <function>translate</function> ( <parameter>string</parameter> <type>text</type>,
        <parameter>from</parameter> <type>text</type>,
        <parameter>to</parameter> <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Replaces each character in <parameter>string</parameter> that
        matches a character in the <parameter>from</parameter> set with the
        corresponding character in the <parameter>to</parameter>
        set. If <parameter>from</parameter> is longer than
        <parameter>to</parameter>, occurrences of the extra characters in
        <parameter>from</parameter> are deleted.
       </para>
       <para>
        <literal>translate('12345', '143', 'ax')</literal>
        <returnvalue>a2x5</returnvalue>
       </para></entry>
      </row>

     </tbody>
    </tgroup>
   </table>

   <para>
    The <function>concat</function>, <function>concat_ws</function> and
    <function>format</function> functions are variadic, so it is possible to
    pass the values to be concatenated or formatted as an array marked with
    the <literal>VARIADIC</literal> keyword (see <xref
    linkend="xfunc-sql-variadic-functions"/>).  The array's elements are
    treated as if they were separate ordinary arguments to the function.
    If the variadic array argument is NULL, <function>concat</function>
    and <function>concat_ws</function> return NULL, but
    <function>format</function> treats a NULL as a zero-element array.
   </para>

   <para>
    See also the aggregate function <function>string_agg</function> in
    <xref linkend="functions-aggregate"/>, and the functions for
    converting between strings and the <type>bytea</type> type in
    <xref linkend="functions-binarystring-conversions"/>.
   </para>

   <sect2 id="functions-string-format">
    <title><function>format</function></title>

    <indexterm>
     <primary>format</primary>
    </indexterm>

    <para>
     The function <function>format</function> produces output formatted according to
     a format string, in a style similar to the C function
     <function>sprintf</function>.
    </para>

    <para>
<synopsis>
<function>format</function>(<parameter>formatstr</parameter> <type>text</type> [, <parameter>formatarg</parameter> <type>"any"</type> [, ...] ])
</synopsis>
     <parameter>formatstr</parameter> is a format string that specifies how the
     result should be formatted.  Text in the format string is copied
     directly to the result, except where <firstterm>format specifiers</firstterm> are
     used.  Format specifiers act as placeholders in the string, defining how
     subsequent function arguments should be formatted and inserted into the
     result.  Each <parameter>formatarg</parameter> argument is converted to text
     according to the usual output rules for its data type, and then formatted
     and inserted into the result string according to the format specifier(s).
    </para>

    <para>
     Format specifiers are introduced by a <literal>%</literal> character and have
     the form
<synopsis>
%[<parameter>position</parameter>][<parameter>flags</parameter>][<parameter>width</parameter>]<parameter>type</parameter>
</synopsis>
     where the component fields are:

     <variablelist>
      <varlistentry>
       <term><parameter>position</parameter> (optional)</term>
       <listitem>
        <para>
         A string of the form <literal><parameter>n</parameter>$</literal> where
         <parameter>n</parameter> is the index of the argument to print.
         Index 1 means the first argument after
         <parameter>formatstr</parameter>.  If the <parameter>position</parameter> is
         omitted, the default is to use the next argument in sequence.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><parameter>flags</parameter> (optional)</term>
       <listitem>
        <para>
         Additional options controlling how the format specifier's output is
         formatted.  Currently the only supported flag is a minus sign
         (<literal>-</literal>) which will cause the format specifier's output to be
         left-justified.  This has no effect unless the <parameter>width</parameter>
         field is also specified.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><parameter>width</parameter> (optional)</term>
       <listitem>
        <para>
         Specifies the <emphasis>minimum</emphasis> number of characters to use to
         display the format specifier's output.  The output is padded on the
         left or right (depending on the <literal>-</literal> flag) with spaces as
         needed to fill the width.  A too-small width does not cause
         truncation of the output, but is simply ignored.  The width may be
         specified using any of the following: a positive integer; an
         asterisk (<literal>*</literal>) to use the next function argument as the
         width; or a string of the form <literal>*<parameter>n</parameter>$</literal> to
         use the <parameter>n</parameter>th function argument as the width.
        </para>

        <para>
         If the width comes from a function argument, that argument is
         consumed before the argument that is used for the format specifier's
         value.  If the width argument is negative, the result is left
         aligned (as if the <literal>-</literal> flag had been specified) within a
         field of length <function>abs</function>(<parameter>width</parameter>).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><parameter>type</parameter> (required)</term>
       <listitem>
        <para>
         The type of format conversion to use to produce the format
         specifier's output.  The following types are supported:
         <itemizedlist>
          <listitem>
           <para>
            <literal>s</literal> formats the argument value as a simple
            string.  A null value is treated as an empty string.
           </para>
          </listitem>
          <listitem>
           <para>
            <literal>I</literal> treats the argument value as an SQL
            identifier, double-quoting it if necessary.
            It is an error for the value to be null (equivalent to
            <function>quote_ident</function>).
           </para>
          </listitem>
          <listitem>
           <para>
            <literal>L</literal> quotes the argument value as an SQL literal.
            A null value is displayed as the string <literal>NULL</literal>, without
            quotes (equivalent to <function>quote_nullable</function>).
           </para>
          </listitem>
         </itemizedlist>
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </para>

    <para>
     In addition to the format specifiers described above, the special sequence
     <literal>%%</literal> may be used to output a literal <literal>%</literal> character.
    </para>

    <para>
     Here are some examples of the basic format conversions:

<screen>
SELECT format('Hello %s', 'World');
<lineannotation>Result: </lineannotation><computeroutput>Hello World</computeroutput>

SELECT format('Testing %s, %s, %s, %%', 'one', 'two', 'three');
<lineannotation>Result: </lineannotation><computeroutput>Testing one, two, three, %</computeroutput>

SELECT format('INSERT INTO %I VALUES(%L)', 'Foo bar', E'O\'Reilly');
<lineannotation>Result: </lineannotation><computeroutput>INSERT INTO "Foo bar" VALUES('O''Reilly')</computeroutput>

SELECT format('INSERT INTO %I VALUES(%L)', 'locations', 'C:\Program Files');
<lineannotation>Result: </lineannotation><computeroutput>INSERT INTO locations VALUES('C:\Program Files')</computeroutput>
</screen>
    </para>

    <para>
     Here are examples using <parameter>width</parameter> fields
     and the <literal>-</literal> flag:

<screen>
SELECT format('|%10s|', 'foo');
<lineannotation>Result: </lineannotation><computeroutput>|       foo|</computeroutput>

SELECT format('|%-10s|', 'foo');
<lineannotation>Result: </lineannotation><computeroutput>|foo       |</computeroutput>

SELECT format('|%*s|', 10, 'foo');
<lineannotation>Result: </lineannotation><computeroutput>|       foo|</computeroutput>

SELECT format('|%*s|', -10, 'foo');
<lineannotation>Result: </lineannotation><computeroutput>|foo       |</computeroutput>

SELECT format('|%-*s|', 10, 'foo');
<lineannotation>Result: </lineannotation><computeroutput>|foo       |</computeroutput>

SELECT format('|%-*s|', -10, 'foo');
<lineannotation>Result: </lineannotation><computeroutput>|foo       |</computeroutput>
</screen>
    </para>

    <para>
     These examples show use of <parameter>position</parameter> fields:

<screen>
SELECT format('Testing %3$s, %2$s, %1$s', 'one', 'two', 'three');
<lineannotation>Result: </lineannotation><computeroutput>Testing three, two, one</computeroutput>

SELECT format('|%*2$s|', 'foo', 10, 'bar');
<lineannotation>Result: </lineannotation><computeroutput>|       bar|</computeroutput>

SELECT format('|%1$*2$s|', 'foo', 10, 'bar');
<lineannotation>Result: </lineannotation><computeroutput>|       foo|</computeroutput>
</screen>
    </para>

    <para>
     Unlike the standard C function <function>sprintf</function>,
     <productname>PostgreSQL</productname>'s <function>format</function> function allows format
     specifiers with and without <parameter>position</parameter> fields to be mixed
     in the same format string.  A format specifier without a
     <parameter>position</parameter> field always uses the next argument after the
     last argument consumed.
     In addition, the <function>format</function> function does not require all
     function arguments to be used in the format string.
     For example:

<screen>
SELECT format('Testing %3$s, %2$s, %s', 'one', 'two', 'three');
<lineannotation>Result: </lineannotation><computeroutput>Testing three, two, three</computeroutput>
</screen>
    </para>

    <para>
     The <literal>%I</literal> and <literal>%L</literal> format specifiers are particularly
     useful for safely constructing dynamic SQL statements.  See
     <xref linkend="plpgsql-quote-literal-example"/>.
    </para>
   </sect2>

  </sect1>


  <sect1 id="functions-binarystring">
   <title>Binary String Functions and Operators</title>

   <indexterm zone="functions-binarystring">
    <primary>binary data</primary>
    <secondary>functions</secondary>
   </indexterm>

   <para>
    This section describes functions and operators for examining and
    manipulating binary strings, that is values of type <type>bytea</type>.
    Many of these are equivalent, in purpose and syntax, to the
    text-string functions described in the previous section.
   </para>

   <para>
    <acronym>SQL</acronym> defines some string functions that use
    key words, rather than commas, to separate
    arguments.  Details are in
    <xref linkend="functions-binarystring-sql"/>.
    <productname>PostgreSQL</productname> also provides versions of these functions
    that use the regular function invocation syntax
    (see <xref linkend="functions-binarystring-other"/>).
   </para>

   <table id="functions-binarystring-sql">
    <title><acronym>SQL</acronym> Binary String Functions and Operators</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function/Operator
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>binary string</primary>
         <secondary>concatenation</secondary>
        </indexterm>
        <type>bytea</type> <literal>||</literal> <type>bytea</type>
        <returnvalue>bytea</returnvalue>
       </para>
       <para>
        Concatenates the two binary strings.
       </para>
       <para>
        <literal>'\x123456'::bytea || '\x789a00bcde'::bytea</literal>
        <returnvalue>\x123456789a00bcde</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>bit_length</primary>
        </indexterm>
        <function>bit_length</function> ( <type>bytea</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns number of bits in the binary string (8
        times the <function>octet_length</function>).
       </para>
       <para>
        <literal>bit_length('\x123456'::bytea)</literal>
        <returnvalue>24</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>octet_length</primary>
        </indexterm>
        <function>octet_length</function> ( <type>bytea</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns number of bytes in the binary string.
       </para>
       <para>
        <literal>octet_length('\x123456'::bytea)</literal>
        <returnvalue>3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>overlay</primary>
        </indexterm>
        <function>overlay</function> ( <parameter>bytes</parameter> <type>bytea</type> <literal>PLACING</literal> <parameter>newsubstring</parameter> <type>bytea</type> <literal>FROM</literal> <parameter>start</parameter> <type>integer</type> <optional> <literal>FOR</literal> <parameter>count</parameter> <type>integer</type> </optional> )
        <returnvalue>bytea</returnvalue>
       </para>
       <para>
        Replaces the substring of <parameter>bytes</parameter> that starts at
        the <parameter>start</parameter>'th byte and extends
        for <parameter>count</parameter> bytes
        with <parameter>newsubstring</parameter>.
        If <parameter>count</parameter> is omitted, it defaults to the length
        of <parameter>newsubstring</parameter>.
       </para>
       <para>
        <literal>overlay('\x1234567890'::bytea placing '\002\003'::bytea from 2 for 3)</literal>
        <returnvalue>\x12020390</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>position</primary>
        </indexterm>
        <function>position</function> ( <parameter>substring</parameter> <type>bytea</type> <literal>IN</literal> <parameter>bytes</parameter> <type>bytea</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns starting index of specified <parameter>substring</parameter>
        within <parameter>bytes</parameter>, or zero if it's not present.
       </para>
       <para>
        <literal>position('\x5678'::bytea in '\x1234567890'::bytea)</literal>
        <returnvalue>3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>substring</primary>
        </indexterm>
        <function>substring</function> ( <parameter>bytes</parameter> <type>bytea</type> <optional> <literal>FROM</literal> <parameter>start</parameter> <type>integer</type> </optional> <optional> <literal>FOR</literal> <parameter>count</parameter> <type>integer</type> </optional> )
        <returnvalue>bytea</returnvalue>
       </para>
       <para>
        Extracts the substring of <parameter>bytes</parameter> starting at
        the <parameter>start</parameter>'th byte if that is specified,
        and stopping after <parameter>count</parameter> bytes if that is
        specified.  Provide at least one of <parameter>start</parameter>
        and <parameter>count</parameter>.
       </para>
       <para>
        <literal>substring('\x1234567890'::bytea from 3 for 2)</literal>
        <returnvalue>\x5678</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>trim</primary>
        </indexterm>
        <function>trim</function> ( <optional> <literal>BOTH</literal> </optional>
        <parameter>bytesremoved</parameter> <type>bytea</type> <literal>FROM</literal>
        <parameter>bytes</parameter> <type>bytea</type> )
        <returnvalue>bytea</returnvalue>
       </para>
       <para>
        Removes the longest string containing only bytes appearing in
        <parameter>bytesremoved</parameter> from the start
        and end of <parameter>bytes</parameter>.
       </para>
       <para>
        <literal>trim('\x9012'::bytea from '\x1234567890'::bytea)</literal>
        <returnvalue>\x345678</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>trim</function> ( <optional> <literal>BOTH</literal> </optional> <optional> <literal>FROM</literal> </optional>
        <parameter>bytes</parameter> <type>bytea</type>,
        <parameter>bytesremoved</parameter> <type>bytea</type> )
        <returnvalue>bytea</returnvalue>
       </para>
       <para>
        This is a non-standard syntax for <function>trim()</function>.
       </para>
       <para>
        <literal>trim(both from '\x1234567890'::bytea, '\x9012'::bytea)</literal>
        <returnvalue>\x345678</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    Additional binary string manipulation functions are available and
    are listed in <xref linkend="functions-binarystring-other"/>.  Some
    of them are used internally to implement the
    <acronym>SQL</acronym>-standard string functions listed in <xref
    linkend="functions-binarystring-sql"/>.
   </para>

   <table id="functions-binarystring-other">
    <title>Other Binary String Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>btrim</primary>
        </indexterm>
        <function>btrim</function> ( <parameter>bytes</parameter> <type>bytea</type>,
        <parameter>bytesremoved</parameter> <type>bytea</type> )
        <returnvalue>bytea</returnvalue>
       </para>
       <para>
        Removes the longest string containing only bytes appearing in
        <parameter>bytesremoved</parameter> from the start and end of
        <parameter>bytes</parameter>.
       </para>
       <para>
       <literal>btrim('\x1234567890'::bytea, '\x9012'::bytea)</literal>
       <returnvalue>\x345678</returnvalue>
      </para></entry>
     </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>get_bit</primary>
        </indexterm>
        <function>get_bit</function> ( <parameter>bytes</parameter> <type>bytea</type>,
        <parameter>n</parameter> <type>bigint</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Extracts <link linkend="functions-zerobased-note">n'th</link> bit
        from binary string.
       </para>
       <para>
        <literal>get_bit('\x1234567890'::bytea, 30)</literal>
        <returnvalue>1</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>get_byte</primary>
        </indexterm>
        <function>get_byte</function> ( <parameter>bytes</parameter> <type>bytea</type>,
        <parameter>n</parameter> <type>integer</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Extracts <link linkend="functions-zerobased-note">n'th</link> byte
        from binary string.
       </para>
       <para>
        <literal>get_byte('\x1234567890'::bytea, 4)</literal>
        <returnvalue>144</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>length</primary>
        </indexterm>
        <indexterm>
         <primary>binary string</primary>
         <secondary>length</secondary>
        </indexterm>
        <indexterm>
         <primary>length</primary>
         <secondary sortas="binary string">of a binary string</secondary>
         <see>binary strings, length</see>
        </indexterm>
        <function>length</function> ( <type>bytea</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the number of bytes in the binary string.
       </para>
       <para>
        <literal>length('\x1234567890'::bytea)</literal>
        <returnvalue>5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>length</function> ( <parameter>bytes</parameter> <type>bytea</type>,
        <parameter>encoding</parameter> <type>name</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the number of characters in the binary string, assuming
        that it is text in the given <parameter>encoding</parameter>.
       </para>
       <para>
        <literal>length('jose'::bytea, 'UTF8')</literal>
        <returnvalue>4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>md5</primary>
        </indexterm>
        <function>md5</function> ( <type>bytea</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Computes the MD5 <link linkend="functions-hash-note">hash</link> of
        the binary string, with the result written in hexadecimal.
       </para>
       <para>
        <literal>md5('Th\000omas'::bytea)</literal>
        <returnvalue>8ab2d3c9689aaf18&zwsp;b4958c334c82d8b1</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>set_bit</primary>
        </indexterm>
        <function>set_bit</function> ( <parameter>bytes</parameter> <type>bytea</type>,
        <parameter>n</parameter> <type>bigint</type>,
        <parameter>newvalue</parameter> <type>integer</type> )
        <returnvalue>bytea</returnvalue>
       </para>
       <para>
        Sets <link linkend="functions-zerobased-note">n'th</link> bit in
        binary string to <parameter>newvalue</parameter>.
       </para>
       <para>
        <literal>set_bit('\x1234567890'::bytea, 30, 0)</literal>
        <returnvalue>\x1234563890</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>set_byte</primary>
        </indexterm>
        <function>set_byte</function> ( <parameter>bytes</parameter> <type>bytea</type>,
        <parameter>n</parameter> <type>integer</type>,
        <parameter>newvalue</parameter> <type>integer</type> )
        <returnvalue>bytea</returnvalue>
       </para>
       <para>
        Sets <link linkend="functions-zerobased-note">n'th</link> byte in
        binary string to <parameter>newvalue</parameter>.
       </para>
       <para>
        <literal>set_byte('\x1234567890'::bytea, 4, 64)</literal>
        <returnvalue>\x1234567840</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>sha224</primary>
        </indexterm>
        <function>sha224</function> ( <type>bytea</type> )
        <returnvalue>bytea</returnvalue>
       </para>
       <para>
        Computes the SHA-224 <link linkend="functions-hash-note">hash</link>
        of the binary string.
       </para>
       <para>
        <literal>sha224('abc'::bytea)</literal>
        <returnvalue>\x23097d223405d8228642a477bda2&zwsp;55b32aadbce4bda0b3f7e36c9da7</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>sha256</primary>
        </indexterm>
        <function>sha256</function> ( <type>bytea</type> )
        <returnvalue>bytea</returnvalue>
       </para>
       <para>
        Computes the SHA-256 <link linkend="functions-hash-note">hash</link>
        of the binary string.
       </para>
       <para>
        <literal>sha256('abc'::bytea)</literal>
        <returnvalue>\xba7816bf8f01cfea414140de5dae2223&zwsp;b00361a396177a9cb410ff61f20015ad</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>sha384</primary>
        </indexterm>
        <function>sha384</function> ( <type>bytea</type> )
        <returnvalue>bytea</returnvalue>
       </para>
       <para>
        Computes the SHA-384 <link linkend="functions-hash-note">hash</link>
        of the binary string.
       </para>
       <para>
        <literal>sha384('abc'::bytea)</literal>
        <returnvalue>\xcb00753f45a35e8bb5a03d699ac65007&zwsp;272c32ab0eded1631a8b605a43ff5bed&zwsp;8086072ba1e7cc2358baeca134c825a7</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>sha512</primary>
        </indexterm>
        <function>sha512</function> ( <type>bytea</type> )
        <returnvalue>bytea</returnvalue>
       </para>
       <para>
        Computes the SHA-512 <link linkend="functions-hash-note">hash</link>
        of the binary string.
       </para>
       <para>
        <literal>sha512('abc'::bytea)</literal>
        <returnvalue>\xddaf35a193617abacc417349ae204131&zwsp;12e6fa4e89a97ea20a9eeee64b55d39a&zwsp;2192992a274fc1a836ba3c23a3feebbd&zwsp;454d4423643ce80e2a9ac94fa54ca49f</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>substr</primary>
        </indexterm>
        <function>substr</function> ( <parameter>bytes</parameter> <type>bytea</type>, <parameter>start</parameter> <type>integer</type> <optional>, <parameter>count</parameter> <type>integer</type> </optional> )
        <returnvalue>bytea</returnvalue>
       </para>
       <para>
        Extracts the substring of <parameter>bytes</parameter> starting at
        the <parameter>start</parameter>'th byte,
        and extending for <parameter>count</parameter> bytes if that is
        specified.  (Same
        as <literal>substring(<parameter>bytes</parameter>
        from <parameter>start</parameter>
        for <parameter>count</parameter>)</literal>.)
       </para>
       <para>
        <literal>substr('\x1234567890'::bytea, 3, 2)</literal>
        <returnvalue>\x5678</returnvalue>
       </para></entry>
      </row>
    </tbody>
   </tgroup>
  </table>

  <para id="functions-zerobased-note">
   Functions <function>get_byte</function> and <function>set_byte</function>
   number the first byte of a binary string as byte 0.
   Functions <function>get_bit</function> and <function>set_bit</function>
   number bits from the right within each byte; for example bit 0 is the least
   significant bit of the first byte, and bit 15 is the most significant bit
   of the second byte.
  </para>

  <para id="functions-hash-note">
   For historical reasons, the function <function>md5</function>
   returns a hex-encoded value of type <type>text</type> whereas the SHA-2
   functions return type <type>bytea</type>.  Use the functions
   <link linkend="function-encode"><function>encode</function></link>
   and <link linkend="function-decode"><function>decode</function></link> to
   convert between the two.  For example write <literal>encode(sha256('abc'),
   'hex')</literal> to get a hex-encoded text representation,
   or <literal>decode(md5('abc'), 'hex')</literal> to get
   a <type>bytea</type> value.
  </para>

  <para>
   <indexterm>
    <primary>character string</primary>
    <secondary>converting to binary string</secondary>
   </indexterm>
   <indexterm>
    <primary>binary string</primary>
    <secondary>converting to character string</secondary>
   </indexterm>
   Functions for converting strings between different character sets
   (encodings), and for representing arbitrary binary data in textual
   form, are shown in
   <xref linkend="functions-binarystring-conversions"/>.  For these
   functions, an argument or result of type <type>text</type> is expressed
   in the database's default encoding, while arguments or results of
   type <type>bytea</type> are in an encoding named by another argument.
  </para>

  <table id="functions-binarystring-conversions">
   <title>Text/Binary String Conversion Functions</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="func_table_entry"><para role="func_signature">
       Function
      </para>
      <para>
       Description
      </para>
      <para>
       Example(s)
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="func_table_entry"><para role="func_signature">
       <indexterm>
        <primary>convert</primary>
       </indexterm>
       <function>convert</function> ( <parameter>bytes</parameter> <type>bytea</type>,
       <parameter>src_encoding</parameter> <type>name</type>,
       <parameter>dest_encoding</parameter> <type>name</type> )
       <returnvalue>bytea</returnvalue>
      </para>
      <para>
       Converts a binary string representing text in
       encoding <parameter>src_encoding</parameter>
       to a binary string in encoding <parameter>dest_encoding</parameter>
       (see <xref linkend="multibyte-conversions-supported"/> for
       available conversions).
      </para>
      <para>
       <literal>convert('text_in_utf8', 'UTF8', 'LATIN1')</literal>
       <returnvalue>\x746578745f696e5f75746638</returnvalue>
      </para></entry>
     </row>

     <row>
      <entry role="func_table_entry"><para role="func_signature">
       <indexterm>
        <primary>convert_from</primary>
       </indexterm>
       <function>convert_from</function> ( <parameter>bytes</parameter> <type>bytea</type>,
       <parameter>src_encoding</parameter> <type>name</type> )
       <returnvalue>text</returnvalue>
      </para>
      <para>
       Converts a binary string representing text in
       encoding <parameter>src_encoding</parameter>
       to <type>text</type> in the database encoding
       (see <xref linkend="multibyte-conversions-supported"/> for
       available conversions).
      </para>
      <para>
       <literal>convert_from('text_in_utf8', 'UTF8')</literal>
       <returnvalue>text_in_utf8</returnvalue>
      </para></entry>
     </row>

     <row>
      <entry role="func_table_entry"><para role="func_signature">
       <indexterm>
        <primary>convert_to</primary>
       </indexterm>
       <function>convert_to</function> ( <parameter>string</parameter> <type>text</type>,
       <parameter>dest_encoding</parameter> <type>name</type> )
       <returnvalue>bytea</returnvalue>
      </para>
      <para>
       Converts a <type>text</type> string (in the database encoding) to a
       binary string encoded in encoding <parameter>dest_encoding</parameter>
       (see <xref linkend="multibyte-conversions-supported"/> for
       available conversions).
      </para>
      <para>
       <literal>convert_to('some_text', 'UTF8')</literal>
       <returnvalue>\x736f6d655f74657874</returnvalue>
      </para></entry>
     </row>

     <row>
      <entry role="func_table_entry"><para role="func_signature">
       <indexterm id="function-encode">
        <primary>encode</primary>
       </indexterm>
       <function>encode</function> ( <parameter>bytes</parameter> <type>bytea</type>,
       <parameter>format</parameter> <type>text</type> )
       <returnvalue>text</returnvalue>
      </para>
      <para>
       Encodes binary data into a textual representation; supported
       <parameter>format</parameter> values are:
       <link linkend="encode-format-base64"><literal>base64</literal></link>,
       <link linkend="encode-format-escape"><literal>escape</literal></link>,
       <link linkend="encode-format-hex"><literal>hex</literal></link>.
      </para>
      <para>
       <literal>encode('123\000\001', 'base64')</literal>
       <returnvalue>MTIzAAE=</returnvalue>
      </para></entry>
     </row>

     <row>
      <entry role="func_table_entry"><para role="func_signature">
       <indexterm id="function-decode">
        <primary>decode</primary>
       </indexterm>
       <function>decode</function> ( <parameter>string</parameter> <type>text</type>,
       <parameter>format</parameter> <type>text</type> )
       <returnvalue>bytea</returnvalue>
      </para>
      <para>
       Decodes binary data from a textual representation; supported
       <parameter>format</parameter> values are the same as
       for <function>encode</function>.
      </para>
      <para>
       <literal>decode('MTIzAAE=', 'base64')</literal>
       <returnvalue>\x3132330001</returnvalue>
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   The <function>encode</function> and <function>decode</function>
   functions support the following textual formats:

   <variablelist>
    <varlistentry id="encode-format-base64">
     <term>base64
     <indexterm>
      <primary>base64 format</primary>
     </indexterm></term>
     <listitem>
      <para>
       The <literal>base64</literal> format is that
       of <ulink url="https://tools.ietf.org/html/rfc2045#section-6.8">RFC
       2045 Section 6.8</ulink>.  As per the RFC, encoded lines are
       broken at 76 characters.  However instead of the MIME CRLF
       end-of-line marker, only a newline is used for end-of-line.
       The <function>decode</function> function ignores carriage-return,
       newline, space, and tab characters.  Otherwise, an error is
       raised when <function>decode</function> is supplied invalid
       base64 data &mdash; including when trailing padding is incorrect.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="encode-format-escape">
     <term>escape
     <indexterm>
      <primary>escape format</primary>
     </indexterm></term>
     <listitem>
      <para>
       The <literal>escape</literal> format converts zero bytes and
       bytes with the high bit set into octal escape sequences
       (<literal>\</literal><replaceable>nnn</replaceable>), and it doubles
       backslashes.  Other byte values are represented literally.
       The <function>decode</function> function will raise an error if a
       backslash is not followed by either a second backslash or three
       octal digits; it accepts other byte values unchanged.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="encode-format-hex">
     <term>hex
     <indexterm>
      <primary>hex format</primary>
     </indexterm></term>
     <listitem>
      <para>
       The <literal>hex</literal> format represents each 4 bits of
       data as one hexadecimal digit, <literal>0</literal>
       through <literal>f</literal>, writing the higher-order digit of
       each byte first.  The <function>encode</function> function outputs
       the <literal>a</literal>-<literal>f</literal> hex digits in lower
       case.  Because the smallest unit of data is 8 bits, there are
       always an even number of characters returned
       by <function>encode</function>.
       The <function>decode</function> function
       accepts the <literal>a</literal>-<literal>f</literal> characters in
       either upper or lower case.  An error is raised
       when <function>decode</function> is given invalid hex data
       &mdash; including when given an odd number of characters.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <para>
   See also the aggregate function <function>string_agg</function> in
   <xref linkend="functions-aggregate"/> and the large object functions
   in <xref linkend="lo-funcs"/>.
  </para>
 </sect1>


  <sect1 id="functions-bitstring">
   <title>Bit String Functions and Operators</title>

   <indexterm zone="functions-bitstring">
    <primary>bit strings</primary>
    <secondary>functions</secondary>
   </indexterm>

   <para>
    This section describes functions and operators for examining and
    manipulating bit strings, that is values of the types
    <type>bit</type> and <type>bit varying</type>.  (While only
    type <type>bit</type> is mentioned in these tables, values of
    type <type>bit varying</type> can be used interchangeably.)
    Bit strings support the usual comparison operators shown in
    <xref linkend="functions-comparison-op-table"/>, as well as the
    operators shown in <xref linkend="functions-bit-string-op-table"/>.
   </para>

   <table id="functions-bit-string-op-table">
    <title>Bit String Operators</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Operator
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>bit</type> <literal>||</literal> <type>bit</type>
        <returnvalue>bit</returnvalue>
       </para>
       <para>
        Concatenation
       </para>
       <para>
        <literal>B'10001' || B'011'</literal>
        <returnvalue>10001011</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>bit</type> <literal>&amp;</literal> <type>bit</type>
        <returnvalue>bit</returnvalue>
       </para>
       <para>
        Bitwise AND (inputs must be of equal length)
       </para>
       <para>
        <literal>B'10001' &amp; B'01101'</literal>
        <returnvalue>00001</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>bit</type> <literal>|</literal> <type>bit</type>
        <returnvalue>bit</returnvalue>
       </para>
       <para>
        Bitwise OR (inputs must be of equal length)
       </para>
       <para>
        <literal>B'10001' | B'01101'</literal>
        <returnvalue>11101</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>bit</type> <literal>#</literal> <type>bit</type>
        <returnvalue>bit</returnvalue>
       </para>
       <para>
        Bitwise exclusive OR (inputs must be of equal length)
       </para>
       <para>
        <literal>B'10001' # B'01101'</literal>
        <returnvalue>11100</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>~</literal> <type>bit</type>
        <returnvalue>bit</returnvalue>
       </para>
       <para>
        Bitwise NOT
       </para>
       <para>
        <literal>~ B'10001'</literal>
        <returnvalue>01110</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>bit</type> <literal>&lt;&lt;</literal> <type>integer</type>
        <returnvalue>bit</returnvalue>
       </para>
       <para>
        Bitwise shift left
        (string length is preserved)
       </para>
       <para>
        <literal>B'10001' &lt;&lt; 3</literal>
        <returnvalue>01000</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>bit</type> <literal>&gt;&gt;</literal> <type>integer</type>
        <returnvalue>bit</returnvalue>
       </para>
       <para>
        Bitwise shift right
        (string length is preserved)
       </para>
       <para>
        <literal>B'10001' &gt;&gt; 2</literal>
        <returnvalue>00100</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    Some of the functions available for binary strings are also available
    for bit strings, as shown in <xref linkend="functions-bit-string-table"/>.
   </para>

   <table id="functions-bit-string-table">
    <title>Bit String Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>bit_length</primary>
        </indexterm>
        <function>bit_length</function> ( <type>bit</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns number of bits in the bit string.
       </para>
       <para>
        <literal>bit_length(B'10111')</literal>
        <returnvalue>5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>length</primary>
        </indexterm>
        <indexterm>
         <primary>bit string</primary>
         <secondary>length</secondary>
        </indexterm>
        <function>length</function> ( <type>bit</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns number of bits in the bit string.
       </para>
       <para>
        <literal>length(B'10111')</literal>
        <returnvalue>5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>octet_length</primary>
        </indexterm>
        <function>octet_length</function> ( <type>bit</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns number of bytes in the bit string.
       </para>
       <para>
        <literal>octet_length(B'1011111011')</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>overlay</primary>
        </indexterm>
        <function>overlay</function> ( <parameter>bits</parameter> <type>bit</type> <literal>PLACING</literal> <parameter>newsubstring</parameter> <type>bit</type> <literal>FROM</literal> <parameter>start</parameter> <type>integer</type> <optional> <literal>FOR</literal> <parameter>count</parameter> <type>integer</type> </optional> )
        <returnvalue>bit</returnvalue>
       </para>
       <para>
        Replaces the substring of <parameter>bits</parameter> that starts at
        the <parameter>start</parameter>'th bit and extends
        for <parameter>count</parameter> bits
        with <parameter>newsubstring</parameter>.
        If <parameter>count</parameter> is omitted, it defaults to the length
        of <parameter>newsubstring</parameter>.
       </para>
       <para>
        <literal>overlay(B'01010101010101010' placing B'11111' from 2 for 3)</literal>
        <returnvalue>0111110101010101010</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>position</primary>
        </indexterm>
        <function>position</function> ( <parameter>substring</parameter> <type>bit</type> <literal>IN</literal> <parameter>bits</parameter> <type>bit</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns starting index of specified <parameter>substring</parameter>
        within <parameter>bits</parameter>, or zero if it's not present.
       </para>
       <para>
        <literal>position(B'010' in B'000001101011')</literal>
        <returnvalue>8</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>substring</primary>
        </indexterm>
        <function>substring</function> ( <parameter>bits</parameter> <type>bit</type> <optional> <literal>FROM</literal> <parameter>start</parameter> <type>integer</type> </optional> <optional> <literal>FOR</literal> <parameter>count</parameter> <type>integer</type> </optional> )
        <returnvalue>bit</returnvalue>
       </para>
       <para>
        Extracts the substring of <parameter>bits</parameter> starting at
        the <parameter>start</parameter>'th bit if that is specified,
        and stopping after <parameter>count</parameter> bits if that is
        specified.  Provide at least one of <parameter>start</parameter>
        and <parameter>count</parameter>.
       </para>
       <para>
        <literal>substring(B'110010111111' from 3 for 2)</literal>
        <returnvalue>00</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>get_bit</primary>
        </indexterm>
        <function>get_bit</function> ( <parameter>bits</parameter> <type>bit</type>,
        <parameter>n</parameter> <type>integer</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Extracts <parameter>n</parameter>'th bit
        from bit string; the first (leftmost) bit is bit 0.
       </para>
       <para>
        <literal>get_bit(B'101010101010101010', 6)</literal>
        <returnvalue>1</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>set_bit</primary>
        </indexterm>
        <function>set_bit</function> ( <parameter>bits</parameter> <type>bit</type>,
        <parameter>n</parameter> <type>integer</type>,
        <parameter>newvalue</parameter> <type>integer</type> )
        <returnvalue>bit</returnvalue>
       </para>
       <para>
        Sets <parameter>n</parameter>'th bit in
        bit string to <parameter>newvalue</parameter>;
        the first (leftmost) bit is bit 0.
       </para>
       <para>
        <literal>set_bit(B'101010101010101010', 6, 0)</literal>
        <returnvalue>101010001010101010</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    In addition, it is possible to cast integral values to and from type
    <type>bit</type>.
    Casting an integer to <type>bit(n)</type> copies the rightmost
    <literal>n</literal> bits.  Casting an integer to a bit string width wider
    than the integer itself will sign-extend on the left.
    Some examples:
<programlisting>
44::bit(10)                    <lineannotation>0000101100</lineannotation>
44::bit(3)                     <lineannotation>100</lineannotation>
cast(-44 as bit(12))           <lineannotation>111111010100</lineannotation>
'1110'::bit(4)::integer        <lineannotation>14</lineannotation>
</programlisting>
    Note that casting to just <quote>bit</quote> means casting to
    <literal>bit(1)</literal>, and so will deliver only the least significant
    bit of the integer.
   </para>
  </sect1>


 <sect1 id="functions-matching">
  <title>Pattern Matching</title>

  <indexterm zone="functions-matching">
   <primary>pattern matching</primary>
  </indexterm>

   <para>
    There are three separate approaches to pattern matching provided
    by <productname>PostgreSQL</productname>: the traditional
    <acronym>SQL</acronym> <function>LIKE</function> operator, the
    more recent <function>SIMILAR TO</function> operator (added in
    SQL:1999), and <acronym>POSIX</acronym>-style regular
    expressions.  Aside from the basic <quote>does this string match
    this pattern?</quote> operators, functions are available to extract
    or replace matching substrings and to split a string at matching
    locations.
   </para>

   <tip>
    <para>
     If you have pattern matching needs that go beyond this,
     consider writing a user-defined function in Perl or Tcl.
    </para>
   </tip>

   <caution>
    <para>
     While most regular-expression searches can be executed very quickly,
     regular expressions can be contrived that take arbitrary amounts of
     time and memory to process.  Be wary of accepting regular-expression
     search patterns from hostile sources.  If you must do so, it is
     advisable to impose a statement timeout.
    </para>

    <para>
     Searches using <function>SIMILAR TO</function> patterns have the same
     security hazards, since <function>SIMILAR TO</function> provides many
     of the same capabilities as <acronym>POSIX</acronym>-style regular
     expressions.
    </para>

    <para>
     <function>LIKE</function> searches, being much simpler than the other
     two options, are safer to use with possibly-hostile pattern sources.
    </para>
   </caution>

   <para>
    The pattern matching operators of all three kinds do not support
    nondeterministic collations.  If required, apply a different collation to
    the expression to work around this limitation.
   </para>

  <sect2 id="functions-like">
   <title><function>LIKE</function></title>

   <indexterm>
    <primary>LIKE</primary>
   </indexterm>

<synopsis>
<replaceable>string</replaceable> LIKE <replaceable>pattern</replaceable> <optional>ESCAPE <replaceable>escape-character</replaceable></optional>
<replaceable>string</replaceable> NOT LIKE <replaceable>pattern</replaceable> <optional>ESCAPE <replaceable>escape-character</replaceable></optional>
</synopsis>

    <para>
     The <function>LIKE</function> expression returns true if the
     <replaceable>string</replaceable> matches the supplied
     <replaceable>pattern</replaceable>.  (As
     expected, the <function>NOT LIKE</function> expression returns
     false if <function>LIKE</function> returns true, and vice versa.
     An equivalent expression is
     <literal>NOT (<replaceable>string</replaceable> LIKE
      <replaceable>pattern</replaceable>)</literal>.)
    </para>

    <para>
     If <replaceable>pattern</replaceable> does not contain percent
     signs or underscores, then the pattern only represents the string
     itself; in that case <function>LIKE</function> acts like the
     equals operator.  An underscore (<literal>_</literal>) in
     <replaceable>pattern</replaceable> stands for (matches) any single
     character; a percent sign (<literal>%</literal>) matches any sequence
     of zero or more characters.
    </para>

   <para>
    Some examples:
<programlisting>
'abc' LIKE 'abc'    <lineannotation>true</lineannotation>
'abc' LIKE 'a%'     <lineannotation>true</lineannotation>
'abc' LIKE '_b_'    <lineannotation>true</lineannotation>
'abc' LIKE 'c'      <lineannotation>false</lineannotation>
</programlisting>
   </para>

   <para>
    <function>LIKE</function> pattern matching always covers the entire
    string.  Therefore, if it's desired to match a sequence anywhere within
    a string, the pattern must start and end with a percent sign.
   </para>

   <para>
    To match a literal underscore or percent sign without matching
    other characters, the respective character in
    <replaceable>pattern</replaceable> must be
    preceded by the escape character.  The default escape
    character is the backslash but a different one can be selected by
    using the <literal>ESCAPE</literal> clause.  To match the escape
    character itself, write two escape characters.
   </para>

   <note>
    <para>
     If you have <xref linkend="guc-standard-conforming-strings"/> turned off,
     any backslashes you write in literal string constants will need to be
     doubled.  See <xref linkend="sql-syntax-strings"/> for more information.
    </para>
   </note>

   <para>
    It's also possible to select no escape character by writing
    <literal>ESCAPE ''</literal>.  This effectively disables the
    escape mechanism, which makes it impossible to turn off the
    special meaning of underscore and percent signs in the pattern.
   </para>

   <para>
    According to the SQL standard, omitting <literal>ESCAPE</literal>
    means there is no escape character (rather than defaulting to a
    backslash), and a zero-length <literal>ESCAPE</literal> value is
    disallowed.  <productname>PostgreSQL</productname>'s behavior in
    this regard is therefore slightly nonstandard.
   </para>

   <para>
    The key word <token>ILIKE</token> can be used instead of
    <token>LIKE</token> to make the match case-insensitive according
    to the active locale.  This is not in the <acronym>SQL</acronym> standard but is a
    <productname>PostgreSQL</productname> extension.
   </para>

   <para>
    The operator <literal>~~</literal> is equivalent to
    <function>LIKE</function>, and <literal>~~*</literal> corresponds to
    <function>ILIKE</function>.  There are also
    <literal>!~~</literal> and <literal>!~~*</literal> operators that
    represent <function>NOT LIKE</function> and <function>NOT
    ILIKE</function>, respectively.  All of these operators are
    <productname>PostgreSQL</productname>-specific.  You may see these
    operator names in <command>EXPLAIN</command> output and similar
    places, since the parser actually translates <function>LIKE</function>
    et al. to these operators.
   </para>

   <para>
    The phrases <function>LIKE</function>, <function>ILIKE</function>,
    <function>NOT LIKE</function>, and <function>NOT ILIKE</function> are
    generally treated as operators
    in <productname>PostgreSQL</productname> syntax; for example they can
    be used in <replaceable>expression</replaceable>
    <replaceable>operator</replaceable> ANY
    (<replaceable>subquery</replaceable>) constructs, although
    an <literal>ESCAPE</literal> clause cannot be included there.  In some
    obscure cases it may be necessary to use the underlying operator names
    instead.
   </para>

   <para>
    Also see the prefix operator <literal>^@</literal> and corresponding
    <function>starts_with</function> function, which are useful in cases
    where simply matching the beginning of a string is needed.
   </para>
  </sect2>


  <sect2 id="functions-similarto-regexp">
   <title><function>SIMILAR TO</function> Regular Expressions</title>

   <indexterm>
    <primary>regular expression</primary>
    <!-- <seealso>pattern matching</seealso> breaks index build -->
   </indexterm>

   <indexterm>
    <primary>SIMILAR TO</primary>
   </indexterm>
   <indexterm>
    <primary>substring</primary>
   </indexterm>

<synopsis>
<replaceable>string</replaceable> SIMILAR TO <replaceable>pattern</replaceable> <optional>ESCAPE <replaceable>escape-character</replaceable></optional>
<replaceable>string</replaceable> NOT SIMILAR TO <replaceable>pattern</replaceable> <optional>ESCAPE <replaceable>escape-character</replaceable></optional>
</synopsis>

   <para>
    The <function>SIMILAR TO</function> operator returns true or
    false depending on whether its pattern matches the given string.
    It is similar to <function>LIKE</function>, except that it
    interprets the pattern using the SQL standard's definition of a
    regular expression.  SQL regular expressions are a curious cross
    between <function>LIKE</function> notation and common (POSIX) regular
    expression notation.
   </para>

   <para>
    Like <function>LIKE</function>, the <function>SIMILAR TO</function>
    operator succeeds only if its pattern matches the entire string;
    this is unlike common regular expression behavior where the pattern
    can match any part of the string.
    Also like
    <function>LIKE</function>, <function>SIMILAR TO</function> uses
    <literal>_</literal> and <literal>%</literal> as wildcard characters denoting
    any single character and any string, respectively (these are
    comparable to <literal>.</literal> and <literal>.*</literal> in POSIX regular
    expressions).
   </para>

   <para>
    In addition to these facilities borrowed from <function>LIKE</function>,
    <function>SIMILAR TO</function> supports these pattern-matching
    metacharacters borrowed from POSIX regular expressions:

   <itemizedlist>
    <listitem>
     <para>
      <literal>|</literal> denotes alternation (either of two alternatives).
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>*</literal> denotes repetition of the previous item zero
      or more times.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>+</literal> denotes repetition of the previous item one
      or more times.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>?</literal> denotes repetition of the previous item zero
      or one time.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>{</literal><replaceable>m</replaceable><literal>}</literal> denotes repetition
      of the previous item exactly <replaceable>m</replaceable> times.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>{</literal><replaceable>m</replaceable><literal>,}</literal> denotes repetition
      of the previous item <replaceable>m</replaceable> or more times.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>{</literal><replaceable>m</replaceable><literal>,</literal><replaceable>n</replaceable><literal>}</literal>
      denotes repetition of the previous item at least <replaceable>m</replaceable> and
      not more than <replaceable>n</replaceable> times.
     </para>
    </listitem>
    <listitem>
     <para>
      Parentheses <literal>()</literal> can be used to group items into
      a single logical item.
     </para>
    </listitem>
    <listitem>
     <para>
      A bracket expression <literal>[...]</literal> specifies a character
      class, just as in POSIX regular expressions.
     </para>
    </listitem>
   </itemizedlist>

    Notice that the period (<literal>.</literal>) is not a metacharacter
    for <function>SIMILAR TO</function>.
   </para>

   <para>
    As with <function>LIKE</function>, a backslash disables the special
    meaning of any of these metacharacters.  A different escape character
    can be specified with <literal>ESCAPE</literal>, or the escape
    capability can be disabled by writing <literal>ESCAPE ''</literal>.
   </para>

   <para>
    According to the SQL standard, omitting <literal>ESCAPE</literal>
    means there is no escape character (rather than defaulting to a
    backslash), and a zero-length <literal>ESCAPE</literal> value is
    disallowed.  <productname>PostgreSQL</productname>'s behavior in
    this regard is therefore slightly nonstandard.
   </para>

   <para>
    Another nonstandard extension is that following the escape character
    with a letter or digit provides access to the escape sequences
    defined for POSIX regular expressions; see
    <xref linkend="posix-character-entry-escapes-table"/>,
    <xref linkend="posix-class-shorthand-escapes-table"/>, and
    <xref linkend="posix-constraint-escapes-table"/> below.
   </para>

   <para>
    Some examples:
<programlisting>
'abc' SIMILAR TO 'abc'          <lineannotation>true</lineannotation>
'abc' SIMILAR TO 'a'            <lineannotation>false</lineannotation>
'abc' SIMILAR TO '%(b|d)%'      <lineannotation>true</lineannotation>
'abc' SIMILAR TO '(b|c)%'       <lineannotation>false</lineannotation>
'-abc-' SIMILAR TO '%\mabc\M%'  <lineannotation>true</lineannotation>
'xabcy' SIMILAR TO '%\mabc\M%'  <lineannotation>false</lineannotation>
</programlisting>
   </para>

   <para>
    The <function>substring</function> function with three parameters
    provides extraction of a substring that matches an SQL
    regular expression pattern.  The function can be written according
    to standard SQL syntax:
<synopsis>
substring(<replaceable>string</replaceable> similar <replaceable>pattern</replaceable> escape <replaceable>escape-character</replaceable>)
</synopsis>
    or using the now obsolete SQL:1999 syntax:
<synopsis>
substring(<replaceable>string</replaceable> from <replaceable>pattern</replaceable> for <replaceable>escape-character</replaceable>)
</synopsis>
    or as a plain three-argument function:
<synopsis>
substring(<replaceable>string</replaceable>, <replaceable>pattern</replaceable>, <replaceable>escape-character</replaceable>)
</synopsis>
    As with <literal>SIMILAR TO</literal>, the
    specified pattern must match the entire data string, or else the
    function fails and returns null.  To indicate the part of the
    pattern for which the matching data sub-string is of interest,
    the pattern should contain
    two occurrences of the escape character followed by a double quote
    (<literal>"</literal>). <!-- " font-lock sanity -->
    The text matching the portion of the pattern
    between these separators is returned when the match is successful.
   </para>

   <para>
    The escape-double-quote separators actually
    divide <function>substring</function>'s pattern into three independent
    regular expressions; for example, a vertical bar (<literal>|</literal>)
    in any of the three sections affects only that section.  Also, the first
    and third of these regular expressions are defined to match the smallest
    possible amount of text, not the largest, when there is any ambiguity
    about how much of the data string matches which pattern.  (In POSIX
    parlance, the first and third regular expressions are forced to be
    non-greedy.)
   </para>

   <para>
    As an extension to the SQL standard, <productname>PostgreSQL</productname>
    allows there to be just one escape-double-quote separator, in which case
    the third regular expression is taken as empty; or no separators, in which
    case the first and third regular expressions are taken as empty.
   </para>

   <para>
    Some examples, with <literal>#&quot;</literal> delimiting the return string:
<programlisting>
substring('foobar' similar '%#"o_b#"%' escape '#')   <lineannotation>oob</lineannotation>
substring('foobar' similar '#"o_b#"%' escape '#')    <lineannotation>NULL</lineannotation>
</programlisting>
   </para>
  </sect2>

  <sect2 id="functions-posix-regexp">
   <title><acronym>POSIX</acronym> Regular Expressions</title>

   <indexterm zone="functions-posix-regexp">
    <primary>regular expression</primary>
    <seealso>pattern matching</seealso>
   </indexterm>
   <indexterm>
    <primary>substring</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_replace</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_match</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_matches</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_split_to_table</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_split_to_array</primary>
   </indexterm>

   <para>
    <xref linkend="functions-posix-table"/> lists the available
    operators for pattern matching using POSIX regular expressions.
   </para>

   <table id="functions-posix-table">
    <title>Regular Expression Match Operators</title>

    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Operator
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>text</type> <literal>~</literal> <type>text</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        String matches regular expression, case sensitively
       </para>
       <para>
        <literal>'thomas' ~ '.*thom.*'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
       </row>

       <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>text</type> <literal>~*</literal> <type>text</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        String matches regular expression, case insensitively
       </para>
       <para>
        <literal>'thomas' ~* '.*Thom.*'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
       </row>

       <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>text</type> <literal>!~</literal> <type>text</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        String does not match regular expression, case sensitively
       </para>
       <para>
        <literal>'thomas' !~ '.*thomas.*'</literal>
        <returnvalue>f</returnvalue>
       </para></entry>
       </row>

       <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>text</type> <literal>!~*</literal> <type>text</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        String does not match regular expression, case insensitively
       </para>
       <para>
        <literal>'thomas' !~* '.*vadim.*'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <para>
     <acronym>POSIX</acronym> regular expressions provide a more
     powerful means for pattern matching than the <function>LIKE</function> and
     <function>SIMILAR TO</function> operators.
     Many Unix tools such as <command>egrep</command>,
     <command>sed</command>, or <command>awk</command> use a pattern
     matching language that is similar to the one described here.
    </para>

    <para>
     A regular expression is a character sequence that is an
     abbreviated definition of a set of strings (a <firstterm>regular
     set</firstterm>).  A string is said to match a regular expression
     if it is a member of the regular set described by the regular
     expression.  As with <function>LIKE</function>, pattern characters
     match string characters exactly unless they are special characters
     in the regular expression language &mdash; but regular expressions use
     different special characters than <function>LIKE</function> does.
     Unlike <function>LIKE</function> patterns, a
     regular expression is allowed to match anywhere within a string, unless
     the regular expression is explicitly anchored to the beginning or
     end of the string.
    </para>

    <para>
     Some examples:
<programlisting>
'abc' ~ 'abc'    <lineannotation>true</lineannotation>
'abc' ~ '^a'     <lineannotation>true</lineannotation>
'abc' ~ '(b|d)'  <lineannotation>true</lineannotation>
'abc' ~ '^(b|c)' <lineannotation>false</lineannotation>
</programlisting>
    </para>

    <para>
     The <acronym>POSIX</acronym> pattern language is described in much
     greater detail below.
    </para>

    <para>
     The <function>substring</function> function with two parameters,
     <function>substring(<replaceable>string</replaceable> from
     <replaceable>pattern</replaceable>)</function>, provides extraction of a
     substring
     that matches a POSIX regular expression pattern.  It returns null if
     there is no match, otherwise the portion of the text that matched the
     pattern.  But if the pattern contains any parentheses, the portion
     of the text that matched the first parenthesized subexpression (the
     one whose left parenthesis comes first) is
     returned.  You can put parentheses around the whole expression
     if you want to use parentheses within it without triggering this
     exception.  If you need parentheses in the pattern before the
     subexpression you want to extract, see the non-capturing parentheses
     described below.
    </para>

   <para>
    Some examples:
<programlisting>
substring('foobar' from 'o.b')     <lineannotation>oob</lineannotation>
substring('foobar' from 'o(.)b')   <lineannotation>o</lineannotation>
</programlisting>
   </para>

    <para>
     The <function>regexp_replace</function> function provides substitution of
     new text for substrings that match POSIX regular expression patterns.
     It has the syntax
     <function>regexp_replace</function>(<replaceable>source</replaceable>,
     <replaceable>pattern</replaceable>, <replaceable>replacement</replaceable>
     <optional>, <replaceable>flags</replaceable> </optional>).
     The <replaceable>source</replaceable> string is returned unchanged if
     there is no match to the <replaceable>pattern</replaceable>.  If there is a
     match, the <replaceable>source</replaceable> string is returned with the
     <replaceable>replacement</replaceable> string substituted for the matching
     substring.  The <replaceable>replacement</replaceable> string can contain
     <literal>\</literal><replaceable>n</replaceable>, where <replaceable>n</replaceable> is 1
     through 9, to indicate that the source substring matching the
     <replaceable>n</replaceable>'th parenthesized subexpression of the pattern should be
     inserted, and it can contain <literal>\&amp;</literal> to indicate that the
     substring matching the entire pattern should be inserted.  Write
     <literal>\\</literal> if you need to put a literal backslash in the replacement
     text.
     The <replaceable>flags</replaceable> parameter is an optional text
     string containing zero or more single-letter flags that change the
     function's behavior.  Flag <literal>i</literal> specifies case-insensitive
     matching, while flag <literal>g</literal> specifies replacement of each matching
     substring rather than only the first one.  Supported flags (though
     not <literal>g</literal>) are
     described in <xref linkend="posix-embedded-options-table"/>.
    </para>

   <para>
    Some examples:
<programlisting>
regexp_replace('foobarbaz', 'b..', 'X')
                                   <lineannotation>fooXbaz</lineannotation>
regexp_replace('foobarbaz', 'b..', 'X', 'g')
                                   <lineannotation>fooXX</lineannotation>
regexp_replace('foobarbaz', 'b(..)', 'X\1Y', 'g')
                                   <lineannotation>fooXarYXazY</lineannotation>
</programlisting>
   </para>

    <para>
     The <function>regexp_match</function> function returns a text array of
     captured substring(s) resulting from the first match of a POSIX
     regular expression pattern to a string.  It has the syntax
     <function>regexp_match</function>(<replaceable>string</replaceable>,
     <replaceable>pattern</replaceable> <optional>, <replaceable>flags</replaceable> </optional>).
     If there is no match, the result is <literal>NULL</literal>.
     If a match is found, and the <replaceable>pattern</replaceable> contains no
     parenthesized subexpressions, then the result is a single-element text
     array containing the substring matching the whole pattern.
     If a match is found, and the <replaceable>pattern</replaceable> contains
     parenthesized subexpressions, then the result is a text array
     whose <replaceable>n</replaceable>'th element is the substring matching
     the <replaceable>n</replaceable>'th parenthesized subexpression of
     the <replaceable>pattern</replaceable> (not counting <quote>non-capturing</quote>
     parentheses; see below for details).
     The <replaceable>flags</replaceable> parameter is an optional text string
     containing zero or more single-letter flags that change the function's
     behavior.  Supported flags are described
     in <xref linkend="posix-embedded-options-table"/>.
    </para>

   <para>
    Some examples:
<programlisting>
SELECT regexp_match('foobarbequebaz', 'bar.*que');
 regexp_match
--------------
 {barbeque}
(1 row)

SELECT regexp_match('foobarbequebaz', '(bar)(beque)');
 regexp_match
--------------
 {bar,beque}
(1 row)
</programlisting>
    In the common case where you just want the whole matching substring
    or <literal>NULL</literal> for no match, write something like
<programlisting>
SELECT (regexp_match('foobarbequebaz', 'bar.*que'))[1];
 regexp_match
--------------
 barbeque
(1 row)
</programlisting>
   </para>

    <para>
     The <function>regexp_matches</function> function returns a set of text arrays
     of captured substring(s) resulting from matching a POSIX regular
     expression pattern to a string.  It has the same syntax as
     <function>regexp_match</function>.
     This function returns no rows if there is no match, one row if there is
     a match and the <literal>g</literal> flag is not given, or <replaceable>N</replaceable>
     rows if there are <replaceable>N</replaceable> matches and the <literal>g</literal> flag
     is given.  Each returned row is a text array containing the whole
     matched substring or the substrings matching parenthesized
     subexpressions of the <replaceable>pattern</replaceable>, just as described above
     for <function>regexp_match</function>.
     <function>regexp_matches</function> accepts all the flags shown
     in <xref linkend="posix-embedded-options-table"/>, plus
     the <literal>g</literal> flag which commands it to return all matches, not
     just the first one.
    </para>

   <para>
    Some examples:
<programlisting>
SELECT regexp_matches('foo', 'not there');
 regexp_matches
----------------
(0 rows)

SELECT regexp_matches('foobarbequebazilbarfbonk', '(b[^b]+)(b[^b]+)', 'g');
 regexp_matches
----------------
 {bar,beque}
 {bazil,barf}
(2 rows)
</programlisting>
   </para>

   <tip>
    <para>
     In most cases <function>regexp_matches()</function> should be used with
     the <literal>g</literal> flag, since if you only want the first match, it's
     easier and more efficient to use <function>regexp_match()</function>.
     However, <function>regexp_match()</function> only exists
     in <productname>PostgreSQL</productname> version 10 and up.  When working in older
     versions, a common trick is to place a <function>regexp_matches()</function>
     call in a sub-select, for example:
<programlisting>
SELECT col1, (SELECT regexp_matches(col2, '(bar)(beque)')) FROM tab;
</programlisting>
     This produces a text array if there's a match, or <literal>NULL</literal> if
     not, the same as <function>regexp_match()</function> would do.  Without the
     sub-select, this query would produce no output at all for table rows
     without a match, which is typically not the desired behavior.
    </para>
   </tip>

    <para>
     The <function>regexp_split_to_table</function> function splits a string using a POSIX
     regular expression pattern as a delimiter.  It has the syntax
     <function>regexp_split_to_table</function>(<replaceable>string</replaceable>, <replaceable>pattern</replaceable>
     <optional>, <replaceable>flags</replaceable> </optional>).
     If there is no match to the <replaceable>pattern</replaceable>, the function returns the
     <replaceable>string</replaceable>.  If there is at least one match, for each match it returns
     the text from the end of the last match (or the beginning of the string)
     to the beginning of the match.  When there are no more matches, it
     returns the text from the end of the last match to the end of the string.
     The <replaceable>flags</replaceable> parameter is an optional text string containing
     zero or more single-letter flags that change the function's behavior.
     <function>regexp_split_to_table</function> supports the flags described in
     <xref linkend="posix-embedded-options-table"/>.
    </para>

    <para>
     The <function>regexp_split_to_array</function> function behaves the same as
     <function>regexp_split_to_table</function>, except that <function>regexp_split_to_array</function>
     returns its result as an array of <type>text</type>.  It has the syntax
     <function>regexp_split_to_array</function>(<replaceable>string</replaceable>, <replaceable>pattern</replaceable>
     <optional>, <replaceable>flags</replaceable> </optional>).
     The parameters are the same as for <function>regexp_split_to_table</function>.
    </para>

   <para>
    Some examples:
<programlisting>

SELECT foo FROM regexp_split_to_table('the quick brown fox jumps over the lazy dog', '\s+') AS foo;
  foo   
-------
 the    
 quick  
 brown  
 fox    
 jumps 
 over   
 the    
 lazy   
 dog    
(9 rows)

SELECT regexp_split_to_array('the quick brown fox jumps over the lazy dog', '\s+');
              regexp_split_to_array             
-----------------------------------------------
 {the,quick,brown,fox,jumps,over,the,lazy,dog}
(1 row)

SELECT foo FROM regexp_split_to_table('the quick brown fox', '\s*') AS foo;
 foo 
-----
 t         
 h         
 e         
 q         
 u         
 i         
 c         
 k         
 b         
 r         
 o         
 w         
 n         
 f         
 o         
 x         
(16 rows)
</programlisting>
   </para>

   <para>
    As the last example demonstrates, the regexp split functions ignore
    zero-length matches that occur at the start or end of the string
    or immediately after a previous match.  This is contrary to the strict
    definition of regexp matching that is implemented by
    <function>regexp_match</function> and
    <function>regexp_matches</function>, but is usually the most convenient behavior
    in practice.  Other software systems such as Perl use similar definitions.
   </para>

<!-- derived from the re_syntax.n man page -->

   <sect3 id="posix-syntax-details">
    <title>Regular Expression Details</title>

   <para>
    <productname>PostgreSQL</productname>'s regular expressions are implemented
    using a software package written by Henry Spencer.  Much of
    the description of regular expressions below is copied verbatim from his
    manual.
   </para>

   <para>
    Regular expressions (<acronym>RE</acronym>s), as defined in
    <acronym>POSIX</acronym> 1003.2, come in two forms:
    <firstterm>extended</firstterm> <acronym>RE</acronym>s or <acronym>ERE</acronym>s
    (roughly those of <command>egrep</command>), and
    <firstterm>basic</firstterm> <acronym>RE</acronym>s or <acronym>BRE</acronym>s
    (roughly those of <command>ed</command>).
    <productname>PostgreSQL</productname> supports both forms, and
    also implements some extensions
    that are not in the POSIX standard, but have become widely used
    due to their availability in programming languages such as Perl and Tcl.
    <acronym>RE</acronym>s using these non-POSIX extensions are called
    <firstterm>advanced</firstterm> <acronym>RE</acronym>s or <acronym>ARE</acronym>s
    in this documentation.  AREs are almost an exact superset of EREs,
    but BREs have several notational incompatibilities (as well as being
    much more limited).
    We first describe the ARE and ERE forms, noting features that apply
    only to AREs, and then describe how BREs differ.
   </para>

   <note>
    <para>
     <productname>PostgreSQL</productname> always initially presumes that a regular
     expression follows the ARE rules.  However, the more limited ERE or
     BRE rules can be chosen by prepending an <firstterm>embedded option</firstterm>
     to the RE pattern, as described in <xref linkend="posix-metasyntax"/>.
     This can be useful for compatibility with applications that expect
     exactly the <acronym>POSIX</acronym> 1003.2 rules.
    </para>
   </note>

   <para>
    A regular expression is defined as one or more
    <firstterm>branches</firstterm>, separated by
    <literal>|</literal>.  It matches anything that matches one of the
    branches.
   </para>

   <para>
    A branch is zero or more <firstterm>quantified atoms</firstterm> or
    <firstterm>constraints</firstterm>, concatenated.
    It matches a match for the first, followed by a match for the second, etc;
    an empty branch matches the empty string.
   </para>

   <para>
    A quantified atom is an <firstterm>atom</firstterm> possibly followed
    by a single <firstterm>quantifier</firstterm>.
    Without a quantifier, it matches a match for the atom.
    With a quantifier, it can match some number of matches of the atom.
    An <firstterm>atom</firstterm> can be any of the possibilities
    shown in <xref linkend="posix-atoms-table"/>.
    The possible quantifiers and their meanings are shown in
    <xref linkend="posix-quantifiers-table"/>.
   </para>

   <para>
    A <firstterm>constraint</firstterm> matches an empty string, but matches only when
    specific conditions are met.  A constraint can be used where an atom
    could be used, except it cannot be followed by a quantifier.
    The simple constraints are shown in
    <xref linkend="posix-constraints-table"/>;
    some more constraints are described later.
   </para>


   <table id="posix-atoms-table">
    <title>Regular Expression Atoms</title>

    <tgroup cols="2">
     <thead>
      <row>
       <entry>Atom</entry>
       <entry>Description</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>(</literal><replaceable>re</replaceable><literal>)</literal> </entry>
       <entry> (where <replaceable>re</replaceable> is any regular expression)
       matches a match for
       <replaceable>re</replaceable>, with the match noted for possible reporting </entry>
       </row>

       <row>
       <entry> <literal>(?:</literal><replaceable>re</replaceable><literal>)</literal> </entry>
       <entry> as above, but the match is not noted for reporting
       (a <quote>non-capturing</quote> set of parentheses)
       (AREs only) </entry>
       </row>

       <row>
       <entry> <literal>.</literal> </entry>
       <entry> matches any single character </entry>
       </row>

       <row>
       <entry> <literal>[</literal><replaceable>chars</replaceable><literal>]</literal> </entry>
       <entry> a <firstterm>bracket expression</firstterm>,
       matching any one of the <replaceable>chars</replaceable> (see
       <xref linkend="posix-bracket-expressions"/> for more detail) </entry>
       </row>

       <row>
       <entry> <literal>\</literal><replaceable>k</replaceable> </entry>
       <entry> (where <replaceable>k</replaceable> is a non-alphanumeric character)
       matches that character taken as an ordinary character,
       e.g., <literal>\\</literal> matches a backslash character </entry>
       </row>

       <row>
       <entry> <literal>\</literal><replaceable>c</replaceable> </entry>
       <entry> where <replaceable>c</replaceable> is alphanumeric
       (possibly followed by other characters)
       is an <firstterm>escape</firstterm>, see <xref linkend="posix-escape-sequences"/>
       (AREs only; in EREs and BREs, this matches <replaceable>c</replaceable>) </entry>
       </row>

       <row>
       <entry> <literal>{</literal> </entry>
       <entry> when followed by a character other than a digit,
       matches the left-brace character <literal>{</literal>;
       when followed by a digit, it is the beginning of a
       <replaceable>bound</replaceable> (see below) </entry>
       </row>

       <row>
       <entry> <replaceable>x</replaceable> </entry>
       <entry> where <replaceable>x</replaceable> is a single character with no other
       significance, matches that character </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    An RE cannot end with a backslash (<literal>\</literal>).
   </para>

   <note>
    <para>
     If you have <xref linkend="guc-standard-conforming-strings"/> turned off,
     any backslashes you write in literal string constants will need to be
     doubled.  See <xref linkend="sql-syntax-strings"/> for more information.
    </para>
   </note>

   <table id="posix-quantifiers-table">
    <title>Regular Expression Quantifiers</title>

    <tgroup cols="2">
     <thead>
      <row>
       <entry>Quantifier</entry>
       <entry>Matches</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>*</literal> </entry>
       <entry> a sequence of 0 or more matches of the atom </entry>
       </row>

       <row>
       <entry> <literal>+</literal> </entry>
       <entry> a sequence of 1 or more matches of the atom </entry>
       </row>

       <row>
       <entry> <literal>?</literal> </entry>
       <entry> a sequence of 0 or 1 matches of the atom </entry>
       </row>

       <row>
       <entry> <literal>{</literal><replaceable>m</replaceable><literal>}</literal> </entry>
       <entry> a sequence of exactly <replaceable>m</replaceable> matches of the atom </entry>
       </row>

       <row>
       <entry> <literal>{</literal><replaceable>m</replaceable><literal>,}</literal> </entry>
       <entry> a sequence of <replaceable>m</replaceable> or more matches of the atom </entry>
       </row>

       <row>
       <entry>
       <literal>{</literal><replaceable>m</replaceable><literal>,</literal><replaceable>n</replaceable><literal>}</literal> </entry>
       <entry> a sequence of <replaceable>m</replaceable> through <replaceable>n</replaceable>
       (inclusive) matches of the atom; <replaceable>m</replaceable> cannot exceed
       <replaceable>n</replaceable> </entry>
       </row>

       <row>
       <entry> <literal>*?</literal> </entry>
       <entry> non-greedy version of <literal>*</literal> </entry>
       </row>

       <row>
       <entry> <literal>+?</literal> </entry>
       <entry> non-greedy version of <literal>+</literal> </entry>
       </row>

       <row>
       <entry> <literal>??</literal> </entry>
       <entry> non-greedy version of <literal>?</literal> </entry>
       </row>

       <row>
       <entry> <literal>{</literal><replaceable>m</replaceable><literal>}?</literal> </entry>
       <entry> non-greedy version of <literal>{</literal><replaceable>m</replaceable><literal>}</literal> </entry>
       </row>

       <row>
       <entry> <literal>{</literal><replaceable>m</replaceable><literal>,}?</literal> </entry>
       <entry> non-greedy version of <literal>{</literal><replaceable>m</replaceable><literal>,}</literal> </entry>
       </row>

       <row>
       <entry>
       <literal>{</literal><replaceable>m</replaceable><literal>,</literal><replaceable>n</replaceable><literal>}?</literal> </entry>
       <entry> non-greedy version of <literal>{</literal><replaceable>m</replaceable><literal>,</literal><replaceable>n</replaceable><literal>}</literal> </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    The forms using <literal>{</literal><replaceable>...</replaceable><literal>}</literal>
    are known as <firstterm>bounds</firstterm>.
    The numbers <replaceable>m</replaceable> and <replaceable>n</replaceable> within a bound are
    unsigned decimal integers with permissible values from 0 to 255 inclusive.
   </para>

    <para>
     <firstterm>Non-greedy</firstterm> quantifiers (available in AREs only) match the
     same possibilities as their corresponding normal (<firstterm>greedy</firstterm>)
     counterparts, but prefer the smallest number rather than the largest
     number of matches.
     See <xref linkend="posix-matching-rules"/> for more detail.
   </para>

   <note>
    <para>
     A quantifier cannot immediately follow another quantifier, e.g.,
     <literal>**</literal> is invalid.
     A quantifier cannot
     begin an expression or subexpression or follow
     <literal>^</literal> or <literal>|</literal>.
    </para>
   </note>

   <table id="posix-constraints-table">
    <title>Regular Expression Constraints</title>

    <tgroup cols="2">
     <thead>
      <row>
       <entry>Constraint</entry>
       <entry>Description</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>^</literal> </entry>
       <entry> matches at the beginning of the string </entry>
       </row>

       <row>
       <entry> <literal>$</literal> </entry>
       <entry> matches at the end of the string </entry>
       </row>

       <row>
       <entry> <literal>(?=</literal><replaceable>re</replaceable><literal>)</literal> </entry>
       <entry> <firstterm>positive lookahead</firstterm> matches at any point
       where a substring matching <replaceable>re</replaceable> begins
       (AREs only) </entry>
       </row>

       <row>
       <entry> <literal>(?!</literal><replaceable>re</replaceable><literal>)</literal> </entry>
       <entry> <firstterm>negative lookahead</firstterm> matches at any point
       where no substring matching <replaceable>re</replaceable> begins
       (AREs only) </entry>
       </row>

       <row>
       <entry> <literal>(?&lt;=</literal><replaceable>re</replaceable><literal>)</literal> </entry>
       <entry> <firstterm>positive lookbehind</firstterm> matches at any point
       where a substring matching <replaceable>re</replaceable> ends
       (AREs only) </entry>
       </row>

       <row>
       <entry> <literal>(?&lt;!</literal><replaceable>re</replaceable><literal>)</literal> </entry>
       <entry> <firstterm>negative lookbehind</firstterm> matches at any point
       where no substring matching <replaceable>re</replaceable> ends
       (AREs only) </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    Lookahead and lookbehind constraints cannot contain <firstterm>back
    references</firstterm> (see <xref linkend="posix-escape-sequences"/>),
    and all parentheses within them are considered non-capturing.
   </para>
   </sect3>

   <sect3 id="posix-bracket-expressions">
    <title>Bracket Expressions</title>

   <para>
    A <firstterm>bracket expression</firstterm> is a list of
    characters enclosed in <literal>[]</literal>.  It normally matches
    any single character from the list (but see below).  If the list
    begins with <literal>^</literal>, it matches any single character
    <emphasis>not</emphasis> from the rest of the list.
    If two characters
    in the list are separated by <literal>-</literal>, this is
    shorthand for the full range of characters between those two
    (inclusive) in the collating sequence,
    e.g., <literal>[0-9]</literal> in <acronym>ASCII</acronym> matches
    any decimal digit.  It is illegal for two ranges to share an
    endpoint, e.g.,  <literal>a-c-e</literal>.  Ranges are very
    collating-sequence-dependent, so portable programs should avoid
    relying on them.
   </para>

   <para>
    To include a literal <literal>]</literal> in the list, make it the
    first character (after <literal>^</literal>, if that is used).  To
    include a literal <literal>-</literal>, make it the first or last
    character, or the second endpoint of a range.  To use a literal
    <literal>-</literal> as the first endpoint of a range, enclose it
    in <literal>[.</literal> and <literal>.]</literal> to make it a
    collating element (see below).  With the exception of these characters,
    some combinations using <literal>[</literal>
    (see next paragraphs), and escapes (AREs only), all other special
    characters lose their special significance within a bracket expression.
    In particular, <literal>\</literal> is not special when following
    ERE or BRE rules, though it is special (as introducing an escape)
    in AREs.
   </para>

   <para>
    Within a bracket expression, a collating element (a character, a
    multiple-character sequence that collates as if it were a single
    character, or a collating-sequence name for either) enclosed in
    <literal>[.</literal> and <literal>.]</literal> stands for the
    sequence of characters of that collating element.  The sequence is
    treated as a single element of the bracket expression's list.  This
    allows a bracket
    expression containing a multiple-character collating element to
    match more than one character, e.g., if the collating sequence
    includes a <literal>ch</literal> collating element, then the RE
    <literal>[[.ch.]]*c</literal> matches the first five characters of
    <literal>chchcc</literal>.
   </para>

   <note>
    <para>
     <productname>PostgreSQL</productname> currently does not support multi-character collating
     elements. This information describes possible future behavior.
    </para>
   </note>

   <para>
    Within a bracket expression, a collating element enclosed in
    <literal>[=</literal> and <literal>=]</literal> is an <firstterm>equivalence
    class</firstterm>, standing for the sequences of characters of all collating
    elements equivalent to that one, including itself.  (If there are
    no other equivalent collating elements, the treatment is as if the
    enclosing delimiters were <literal>[.</literal> and
    <literal>.]</literal>.)  For example, if <literal>o</literal> and
    <literal>^</literal> are the members of an equivalence class, then
    <literal>[[=o=]]</literal>, <literal>[[=^=]]</literal>, and
    <literal>[o^]</literal> are all synonymous.  An equivalence class
    cannot be an endpoint of a range.
   </para>

   <para>
    Within a bracket expression, the name of a character class
    enclosed in <literal>[:</literal> and <literal>:]</literal> stands
    for the list of all characters belonging to that class.  A character
    class cannot be used as an endpoint of a range.
    The <acronym>POSIX</acronym> standard defines these character class
    names:
    <literal>alnum</literal> (letters and numeric digits),
    <literal>alpha</literal> (letters),
    <literal>blank</literal> (space and tab),
    <literal>cntrl</literal> (control characters),
    <literal>digit</literal> (numeric digits),
    <literal>graph</literal> (printable characters except space),
    <literal>lower</literal> (lower-case letters),
    <literal>print</literal> (printable characters including space),
    <literal>punct</literal> (punctuation),
    <literal>space</literal> (any white space),
    <literal>upper</literal> (upper-case letters),
    and <literal>xdigit</literal> (hexadecimal digits).
    The behavior of these standard character classes is generally
    consistent across platforms for characters in the 7-bit ASCII set.
    Whether a given non-ASCII character is considered to belong to one
    of these classes depends on the <firstterm>collation</firstterm>
    that is used for the regular-expression function or operator
    (see <xref linkend="collation"/>), or by default on the
    database's <envar>LC_CTYPE</envar> locale setting (see
    <xref linkend="locale"/>).  The classification of non-ASCII
    characters can vary across platforms even in similarly-named
    locales.  (But the <literal>C</literal> locale never considers any
    non-ASCII characters to belong to any of these classes.)
    In addition to these standard character
    classes, <productname>PostgreSQL</productname> defines
    the <literal>ascii</literal> character class, which contains exactly
    the 7-bit ASCII set.
   </para>

   <para>
    There are two special cases of bracket expressions:  the bracket
    expressions <literal>[[:&lt;:]]</literal> and
    <literal>[[:&gt;:]]</literal> are constraints,
    matching empty strings at the beginning
    and end of a word respectively.  A word is defined as a sequence
    of word characters that is neither preceded nor followed by word
    characters.  A word character is an <literal>alnum</literal> character (as
    defined by the <acronym>POSIX</acronym> character class described above)
    or an underscore.  This is an extension, compatible with but not
    specified by <acronym>POSIX</acronym> 1003.2, and should be used with
    caution in software intended to be portable to other systems.
    The constraint escapes described below are usually preferable; they
    are no more standard, but are easier to type.
   </para>
   </sect3>

   <sect3 id="posix-escape-sequences">
    <title>Regular Expression Escapes</title>

   <para>
    <firstterm>Escapes</firstterm> are special sequences beginning with <literal>\</literal>
    followed by an alphanumeric character. Escapes come in several varieties:
    character entry, class shorthands, constraint escapes, and back references.
    A <literal>\</literal> followed by an alphanumeric character but not constituting
    a valid escape is illegal in AREs.
    In EREs, there are no escapes: outside a bracket expression,
    a <literal>\</literal> followed by an alphanumeric character merely stands for
    that character as an ordinary character, and inside a bracket expression,
    <literal>\</literal> is an ordinary character.
    (The latter is the one actual incompatibility between EREs and AREs.)
   </para>

   <para>
    <firstterm>Character-entry escapes</firstterm> exist to make it easier to specify
    non-printing and other inconvenient characters in REs.  They are
    shown in <xref linkend="posix-character-entry-escapes-table"/>.
   </para>

   <para>
    <firstterm>Class-shorthand escapes</firstterm> provide shorthands for certain
    commonly-used character classes.  They are
    shown in <xref linkend="posix-class-shorthand-escapes-table"/>.
   </para>

   <para>
    A <firstterm>constraint escape</firstterm> is a constraint,
    matching the empty string if specific conditions are met,
    written as an escape.  They are
    shown in <xref linkend="posix-constraint-escapes-table"/>.
   </para>

   <para>
    A <firstterm>back reference</firstterm> (<literal>\</literal><replaceable>n</replaceable>) matches the
    same string matched by the previous parenthesized subexpression specified
    by the number <replaceable>n</replaceable>
    (see <xref linkend="posix-constraint-backref-table"/>).  For example,
    <literal>([bc])\1</literal> matches <literal>bb</literal> or <literal>cc</literal>
    but not <literal>bc</literal> or <literal>cb</literal>.
    The subexpression must entirely precede the back reference in the RE.
    Subexpressions are numbered in the order of their leading parentheses.
    Non-capturing parentheses do not define subexpressions.
   </para>

   <table id="posix-character-entry-escapes-table">
    <title>Regular Expression Character-Entry Escapes</title>

    <tgroup cols="2">
     <thead>
      <row>
       <entry>Escape</entry>
       <entry>Description</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>\a</literal> </entry>
       <entry> alert (bell) character, as in C </entry>
       </row>

       <row>
       <entry> <literal>\b</literal> </entry>
       <entry> backspace, as in C </entry>
       </row>

       <row>
       <entry> <literal>\B</literal> </entry>
       <entry> synonym for backslash (<literal>\</literal>) to help reduce the need for backslash
       doubling </entry>
       </row>

       <row>
       <entry> <literal>\c</literal><replaceable>X</replaceable> </entry>
       <entry> (where <replaceable>X</replaceable> is any character) the character whose
       low-order 5 bits are the same as those of
       <replaceable>X</replaceable>, and whose other bits are all zero </entry>
       </row>

       <row>
       <entry> <literal>\e</literal> </entry>
       <entry> the character whose collating-sequence name
       is <literal>ESC</literal>,
       or failing that, the character with octal value <literal>033</literal> </entry>
       </row>

       <row>
       <entry> <literal>\f</literal> </entry>
       <entry> form feed, as in C </entry>
       </row>

       <row>
       <entry> <literal>\n</literal> </entry>
       <entry> newline, as in C </entry>
       </row>

       <row>
       <entry> <literal>\r</literal> </entry>
       <entry> carriage return, as in C </entry>
       </row>

       <row>
       <entry> <literal>\t</literal> </entry>
       <entry> horizontal tab, as in C </entry>
       </row>

       <row>
       <entry> <literal>\u</literal><replaceable>wxyz</replaceable> </entry>
       <entry> (where <replaceable>wxyz</replaceable> is exactly four hexadecimal digits)
       the character whose hexadecimal value is
       <literal>0x</literal><replaceable>wxyz</replaceable>
       </entry>
       </row>

       <row>
       <entry> <literal>\U</literal><replaceable>stuvwxyz</replaceable> </entry>
       <entry> (where <replaceable>stuvwxyz</replaceable> is exactly eight hexadecimal
       digits)
       the character whose hexadecimal value is
       <literal>0x</literal><replaceable>stuvwxyz</replaceable>
       </entry>
       </row>

       <row>
       <entry> <literal>\v</literal> </entry>
       <entry> vertical tab, as in C </entry>
       </row>

       <row>
       <entry> <literal>\x</literal><replaceable>hhh</replaceable> </entry>
       <entry> (where <replaceable>hhh</replaceable> is any sequence of hexadecimal
       digits)
       the character whose hexadecimal value is
       <literal>0x</literal><replaceable>hhh</replaceable>
       (a single character no matter how many hexadecimal digits are used)
       </entry>
       </row>

       <row>
       <entry> <literal>\0</literal> </entry>
       <entry> the character whose value is <literal>0</literal> (the null byte)</entry>
       </row>

       <row>
       <entry> <literal>\</literal><replaceable>xy</replaceable> </entry>
       <entry> (where <replaceable>xy</replaceable> is exactly two octal digits,
       and is not a <firstterm>back reference</firstterm>)
       the character whose octal value is
       <literal>0</literal><replaceable>xy</replaceable> </entry>
       </row>

       <row>
       <entry> <literal>\</literal><replaceable>xyz</replaceable> </entry>
       <entry> (where <replaceable>xyz</replaceable> is exactly three octal digits,
       and is not a <firstterm>back reference</firstterm>)
       the character whose octal value is
       <literal>0</literal><replaceable>xyz</replaceable> </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    Hexadecimal digits are <literal>0</literal>-<literal>9</literal>,
    <literal>a</literal>-<literal>f</literal>, and <literal>A</literal>-<literal>F</literal>.
    Octal digits are <literal>0</literal>-<literal>7</literal>.
   </para>

   <para>
    Numeric character-entry escapes specifying values outside the ASCII range
    (0&ndash;127) have meanings dependent on the database encoding.  When the
    encoding is UTF-8, escape values are equivalent to Unicode code points,
    for example <literal>\u1234</literal> means the character <literal>U+1234</literal>.
    For other multibyte encodings, character-entry escapes usually just
    specify the concatenation of the byte values for the character.  If the
    escape value does not correspond to any legal character in the database
    encoding, no error will be raised, but it will never match any data.
   </para>

   <para>
    The character-entry escapes are always taken as ordinary characters.
    For example, <literal>\135</literal> is <literal>]</literal> in ASCII, but
    <literal>\135</literal> does not terminate a bracket expression.
   </para>

   <table id="posix-class-shorthand-escapes-table">
    <title>Regular Expression Class-Shorthand Escapes</title>

    <tgroup cols="2">
     <thead>
      <row>
       <entry>Escape</entry>
       <entry>Description</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>\d</literal> </entry>
       <entry> <literal>[[:digit:]]</literal> </entry>
       </row>

       <row>
       <entry> <literal>\s</literal> </entry>
       <entry> <literal>[[:space:]]</literal> </entry>
       </row>

       <row>
       <entry> <literal>\w</literal> </entry>
       <entry> <literal>[[:alnum:]_]</literal>
       (note underscore is included) </entry>
       </row>

       <row>
       <entry> <literal>\D</literal> </entry>
       <entry> <literal>[^[:digit:]]</literal> </entry>
       </row>

       <row>
       <entry> <literal>\S</literal> </entry>
       <entry> <literal>[^[:space:]]</literal> </entry>
       </row>

       <row>
       <entry> <literal>\W</literal> </entry>
       <entry> <literal>[^[:alnum:]_]</literal>
       (note underscore is included) </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    Within bracket expressions, <literal>\d</literal>, <literal>\s</literal>,
    and <literal>\w</literal> lose their outer brackets,
    and <literal>\D</literal>, <literal>\S</literal>, and <literal>\W</literal> are illegal.
    (So, for example, <literal>[a-c\d]</literal> is equivalent to
    <literal>[a-c[:digit:]]</literal>.
    Also, <literal>[a-c\D]</literal>, which is equivalent to
    <literal>[a-c^[:digit:]]</literal>, is illegal.)
   </para>

   <table id="posix-constraint-escapes-table">
    <title>Regular Expression Constraint Escapes</title>

    <tgroup cols="2">
     <thead>
      <row>
       <entry>Escape</entry>
       <entry>Description</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>\A</literal> </entry>
       <entry> matches only at the beginning of the string
       (see <xref linkend="posix-matching-rules"/> for how this differs from
       <literal>^</literal>) </entry>
       </row>

       <row>
       <entry> <literal>\m</literal> </entry>
       <entry> matches only at the beginning of a word </entry>
       </row>

       <row>
       <entry> <literal>\M</literal> </entry>
       <entry> matches only at the end of a word </entry>
       </row>

       <row>
       <entry> <literal>\y</literal> </entry>
       <entry> matches only at the beginning or end of a word </entry>
       </row>

       <row>
       <entry> <literal>\Y</literal> </entry>
       <entry> matches only at a point that is not the beginning or end of a
       word </entry>
       </row>

       <row>
       <entry> <literal>\Z</literal> </entry>
       <entry> matches only at the end of the string
       (see <xref linkend="posix-matching-rules"/> for how this differs from
       <literal>$</literal>) </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    A word is defined as in the specification of
    <literal>[[:&lt;:]]</literal> and <literal>[[:&gt;:]]</literal> above.
    Constraint escapes are illegal within bracket expressions.
   </para>

   <table id="posix-constraint-backref-table">
    <title>Regular Expression Back References</title>

    <tgroup cols="2">
     <thead>
      <row>
       <entry>Escape</entry>
       <entry>Description</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>\</literal><replaceable>m</replaceable> </entry>
       <entry> (where <replaceable>m</replaceable> is a nonzero digit)
       a back reference to the <replaceable>m</replaceable>'th subexpression </entry>
       </row>

       <row>
       <entry> <literal>\</literal><replaceable>mnn</replaceable> </entry>
       <entry> (where <replaceable>m</replaceable> is a nonzero digit, and
       <replaceable>nn</replaceable> is some more digits, and the decimal value
       <replaceable>mnn</replaceable> is not greater than the number of closing capturing
       parentheses seen so far)
       a back reference to the <replaceable>mnn</replaceable>'th subexpression </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <note>
    <para>
     There is an inherent ambiguity between octal character-entry
     escapes and back references, which is resolved by the following heuristics,
     as hinted at above.
     A leading zero always indicates an octal escape.
     A single non-zero digit, not followed by another digit,
     is always taken as a back reference.
     A multi-digit sequence not starting with a zero is taken as a back
     reference if it comes after a suitable subexpression
     (i.e., the number is in the legal range for a back reference),
     and otherwise is taken as octal.
    </para>
   </note>
   </sect3>

   <sect3 id="posix-metasyntax">
    <title>Regular Expression Metasyntax</title>

   <para>
    In addition to the main syntax described above, there are some special
    forms and miscellaneous syntactic facilities available.
   </para>

   <para>
    An RE can begin with one of two special <firstterm>director</firstterm> prefixes.
    If an RE begins with <literal>***:</literal>,
    the rest of the RE is taken as an ARE.  (This normally has no effect in
    <productname>PostgreSQL</productname>, since REs are assumed to be AREs;
    but it does have an effect if ERE or BRE mode had been specified by
    the <replaceable>flags</replaceable> parameter to a regex function.)
    If an RE begins with <literal>***=</literal>,
    the rest of the RE is taken to be a literal string,
    with all characters considered ordinary characters.
   </para>

   <para>
    An ARE can begin with <firstterm>embedded options</firstterm>:
    a sequence <literal>(?</literal><replaceable>xyz</replaceable><literal>)</literal>
    (where <replaceable>xyz</replaceable> is one or more alphabetic characters)
    specifies options affecting the rest of the RE.
    These options override any previously determined options &mdash;
    in particular, they can override the case-sensitivity behavior implied by
    a regex operator, or the <replaceable>flags</replaceable> parameter to a regex
    function.
    The available option letters are
    shown in <xref linkend="posix-embedded-options-table"/>.
    Note that these same option letters are used in the <replaceable>flags</replaceable>
    parameters of regex functions.
   </para>

   <table id="posix-embedded-options-table">
    <title>ARE Embedded-Option Letters</title>

    <tgroup cols="2">
     <thead>
      <row>
       <entry>Option</entry>
       <entry>Description</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>b</literal> </entry>
       <entry> rest of RE is a BRE </entry>
       </row>

       <row>
       <entry> <literal>c</literal> </entry>
       <entry> case-sensitive matching (overrides operator type) </entry>
       </row>

       <row>
       <entry> <literal>e</literal> </entry>
       <entry> rest of RE is an ERE </entry>
       </row>

       <row>
       <entry> <literal>i</literal> </entry>
       <entry> case-insensitive matching (see
       <xref linkend="posix-matching-rules"/>) (overrides operator type) </entry>
       </row>

       <row>
       <entry> <literal>m</literal> </entry>
       <entry> historical synonym for <literal>n</literal> </entry>
       </row>

       <row>
       <entry> <literal>n</literal> </entry>
       <entry> newline-sensitive matching (see
       <xref linkend="posix-matching-rules"/>) </entry>
       </row>

       <row>
       <entry> <literal>p</literal> </entry>
       <entry> partial newline-sensitive matching (see
       <xref linkend="posix-matching-rules"/>) </entry>
       </row>

       <row>
       <entry> <literal>q</literal> </entry>
       <entry> rest of RE is a literal (<quote>quoted</quote>) string, all ordinary
       characters </entry>
       </row>

       <row>
       <entry> <literal>s</literal> </entry>
       <entry> non-newline-sensitive matching (default) </entry>
       </row>

       <row>
       <entry> <literal>t</literal> </entry>
       <entry> tight syntax (default; see below) </entry>
       </row>

       <row>
       <entry> <literal>w</literal> </entry>
       <entry> inverse partial newline-sensitive (<quote>weird</quote>) matching
       (see <xref linkend="posix-matching-rules"/>) </entry>
       </row>

       <row>
       <entry> <literal>x</literal> </entry>
       <entry> expanded syntax (see below) </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    Embedded options take effect at the <literal>)</literal> terminating the sequence.
    They can appear only at the start of an ARE (after the
    <literal>***:</literal> director if any).
   </para>

   <para>
    In addition to the usual (<firstterm>tight</firstterm>) RE syntax, in which all
    characters are significant, there is an <firstterm>expanded</firstterm> syntax,
    available by specifying the embedded <literal>x</literal> option.
    In the expanded syntax,
    white-space characters in the RE are ignored, as are
    all characters between a <literal>#</literal>
    and the following newline (or the end of the RE).  This
    permits paragraphing and commenting a complex RE.
    There are three exceptions to that basic rule:

    <itemizedlist>
     <listitem>
      <para>
       a white-space character or <literal>#</literal> preceded by <literal>\</literal> is
       retained
      </para>
     </listitem>
     <listitem>
      <para>
       white space or <literal>#</literal> within a bracket expression is retained
      </para>
     </listitem>
     <listitem>
      <para>
       white space and comments cannot appear within multi-character symbols,
       such as <literal>(?:</literal>
      </para>
     </listitem>
    </itemizedlist>

    For this purpose, white-space characters are blank, tab, newline, and
    any character that belongs to the <replaceable>space</replaceable> character class.
   </para>

   <para>
    Finally, in an ARE, outside bracket expressions, the sequence
    <literal>(?#</literal><replaceable>ttt</replaceable><literal>)</literal>
    (where <replaceable>ttt</replaceable> is any text not containing a <literal>)</literal>)
    is a comment, completely ignored.
    Again, this is not allowed between the characters of
    multi-character symbols, like <literal>(?:</literal>.
    Such comments are more a historical artifact than a useful facility,
    and their use is deprecated; use the expanded syntax instead.
   </para>

   <para>
    <emphasis>None</emphasis> of these metasyntax extensions is available if
    an initial <literal>***=</literal> director
    has specified that the user's input be treated as a literal string
    rather than as an RE.
   </para>
   </sect3>

   <sect3 id="posix-matching-rules">
    <title>Regular Expression Matching Rules</title>

   <para>
    In the event that an RE could match more than one substring of a given
    string, the RE matches the one starting earliest in the string.
    If the RE could match more than one substring starting at that point,
    either the longest possible match or the shortest possible match will
    be taken, depending on whether the RE is <firstterm>greedy</firstterm> or
    <firstterm>non-greedy</firstterm>.
   </para>

   <para>
    Whether an RE is greedy or not is determined by the following rules:
    <itemizedlist>
     <listitem>
      <para>
       Most atoms, and all constraints, have no greediness attribute (because
       they cannot match variable amounts of text anyway).
      </para>
     </listitem>
     <listitem>
      <para>
       Adding parentheses around an RE does not change its greediness.
      </para>
     </listitem>
     <listitem>
      <para>
       A quantified atom with a fixed-repetition quantifier
       (<literal>{</literal><replaceable>m</replaceable><literal>}</literal>
       or
       <literal>{</literal><replaceable>m</replaceable><literal>}?</literal>)
       has the same greediness (possibly none) as the atom itself.
      </para>
     </listitem>
     <listitem>
      <para>
       A quantified atom with other normal quantifiers (including
       <literal>{</literal><replaceable>m</replaceable><literal>,</literal><replaceable>n</replaceable><literal>}</literal>
       with <replaceable>m</replaceable> equal to <replaceable>n</replaceable>)
       is greedy (prefers longest match).
      </para>
     </listitem>
     <listitem>
      <para>
       A quantified atom with a non-greedy quantifier (including
       <literal>{</literal><replaceable>m</replaceable><literal>,</literal><replaceable>n</replaceable><literal>}?</literal>
       with <replaceable>m</replaceable> equal to <replaceable>n</replaceable>)
       is non-greedy (prefers shortest match).
      </para>
     </listitem>
     <listitem>
      <para>
       A branch &mdash; that is, an RE that has no top-level
       <literal>|</literal> operator &mdash; has the same greediness as the first
       quantified atom in it that has a greediness attribute.
      </para>
     </listitem>
     <listitem>
      <para>
       An RE consisting of two or more branches connected by the
       <literal>|</literal> operator is always greedy.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    The above rules associate greediness attributes not only with individual
    quantified atoms, but with branches and entire REs that contain quantified
    atoms.  What that means is that the matching is done in such a way that
    the branch, or whole RE, matches the longest or shortest possible
    substring <emphasis>as a whole</emphasis>.  Once the length of the entire match
    is determined, the part of it that matches any particular subexpression
    is determined on the basis of the greediness attribute of that
    subexpression, with subexpressions starting earlier in the RE taking
    priority over ones starting later.
   </para>

   <para>
    An example of what this means:
<screen>
SELECT SUBSTRING('XY1234Z', 'Y*([0-9]{1,3})');
<lineannotation>Result: </lineannotation><computeroutput>123</computeroutput>
SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
<lineannotation>Result: </lineannotation><computeroutput>1</computeroutput>
</screen>
    In the first case, the RE as a whole is greedy because <literal>Y*</literal>
    is greedy.  It can match beginning at the <literal>Y</literal>, and it matches
    the longest possible string starting there, i.e., <literal>Y123</literal>.
    The output is the parenthesized part of that, or <literal>123</literal>.
    In the second case, the RE as a whole is non-greedy because <literal>Y*?</literal>
    is non-greedy.  It can match beginning at the <literal>Y</literal>, and it matches
    the shortest possible string starting there, i.e., <literal>Y1</literal>.
    The subexpression <literal>[0-9]{1,3}</literal> is greedy but it cannot change
    the decision as to the overall match length; so it is forced to match
    just <literal>1</literal>.
   </para>

   <para>
    In short, when an RE contains both greedy and non-greedy subexpressions,
    the total match length is either as long as possible or as short as
    possible, according to the attribute assigned to the whole RE.  The
    attributes assigned to the subexpressions only affect how much of that
    match they are allowed to <quote>eat</quote> relative to each other.
   </para>

   <para>
    The quantifiers <literal>{1,1}</literal> and <literal>{1,1}?</literal>
    can be used to force greediness or non-greediness, respectively,
    on a subexpression or a whole RE.
    This is useful when you need the whole RE to have a greediness attribute
    different from what's deduced from its elements.  As an example,
    suppose that we are trying to separate a string containing some digits
    into the digits and the parts before and after them.  We might try to
    do that like this:
<screen>
SELECT regexp_match('abc01234xyz', '(.*)(\d+)(.*)');
<lineannotation>Result: </lineannotation><computeroutput>{abc0123,4,xyz}</computeroutput>
</screen>
    That didn't work: the first <literal>.*</literal> is greedy so
    it <quote>eats</quote> as much as it can, leaving the <literal>\d+</literal> to
    match at the last possible place, the last digit.  We might try to fix
    that by making it non-greedy:
<screen>
SELECT regexp_match('abc01234xyz', '(.*?)(\d+)(.*)');
<lineannotation>Result: </lineannotation><computeroutput>{abc,0,""}</computeroutput>
</screen>
    That didn't work either, because now the RE as a whole is non-greedy
    and so it ends the overall match as soon as possible.  We can get what
    we want by forcing the RE as a whole to be greedy:
<screen>
SELECT regexp_match('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
<lineannotation>Result: </lineannotation><computeroutput>{abc,01234,xyz}</computeroutput>
</screen>
    Controlling the RE's overall greediness separately from its components'
    greediness allows great flexibility in handling variable-length patterns.
   </para>

   <para>
    When deciding what is a longer or shorter match,
    match lengths are measured in characters, not collating elements.
    An empty string is considered longer than no match at all.
    For example:
    <literal>bb*</literal>
    matches the three middle characters of <literal>abbbc</literal>;
    <literal>(week|wee)(night|knights)</literal>
    matches all ten characters of <literal>weeknights</literal>;
    when <literal>(.*).*</literal>
    is matched against <literal>abc</literal> the parenthesized subexpression
    matches all three characters; and when
    <literal>(a*)*</literal> is matched against <literal>bc</literal>
    both the whole RE and the parenthesized
    subexpression match an empty string.
   </para>

   <para>
    If case-independent matching is specified,
    the effect is much as if all case distinctions had vanished from the
    alphabet.
    When an alphabetic that exists in multiple cases appears as an
    ordinary character outside a bracket expression, it is effectively
    transformed into a bracket expression containing both cases,
    e.g., <literal>x</literal> becomes <literal>[xX]</literal>.
    When it appears inside a bracket expression, all case counterparts
    of it are added to the bracket expression, e.g.,
    <literal>[x]</literal> becomes <literal>[xX]</literal>
    and <literal>[^x]</literal> becomes <literal>[^xX]</literal>.
   </para>

   <para>
    If newline-sensitive matching is specified, <literal>.</literal>
    and bracket expressions using <literal>^</literal>
    will never match the newline character
    (so that matches will never cross newlines unless the RE
    explicitly arranges it)
    and <literal>^</literal> and <literal>$</literal>
    will match the empty string after and before a newline
    respectively, in addition to matching at beginning and end of string
    respectively.
    But the ARE escapes <literal>\A</literal> and <literal>\Z</literal>
    continue to match beginning or end of string <emphasis>only</emphasis>.
   </para>

   <para>
    If partial newline-sensitive matching is specified,
    this affects <literal>.</literal> and bracket expressions
    as with newline-sensitive matching, but not <literal>^</literal>
    and <literal>$</literal>.
   </para>

   <para>
    If inverse partial newline-sensitive matching is specified,
    this affects <literal>^</literal> and <literal>$</literal>
    as with newline-sensitive matching, but not <literal>.</literal>
    and bracket expressions.
    This isn't very useful but is provided for symmetry.
   </para>
   </sect3>

   <sect3 id="posix-limits-compatibility">
    <title>Limits and Compatibility</title>

   <para>
    No particular limit is imposed on the length of REs in this
    implementation.  However,
    programs intended to be highly portable should not employ REs longer
    than 256 bytes,
    as a POSIX-compliant implementation can refuse to accept such REs.
   </para>

   <para>
    The only feature of AREs that is actually incompatible with
    POSIX EREs is that <literal>\</literal> does not lose its special
    significance inside bracket expressions.
    All other ARE features use syntax which is illegal or has
    undefined or unspecified effects in POSIX EREs;
    the <literal>***</literal> syntax of directors likewise is outside the POSIX
    syntax for both BREs and EREs.
   </para>

   <para>
    Many of the ARE extensions are borrowed from Perl, but some have
    been changed to clean them up, and a few Perl extensions are not present.
    Incompatibilities of note include <literal>\b</literal>, <literal>\B</literal>,
    the lack of special treatment for a trailing newline,
    the addition of complemented bracket expressions to the things
    affected by newline-sensitive matching,
    the restrictions on parentheses and back references in lookahead/lookbehind
    constraints, and the longest/shortest-match (rather than first-match)
    matching semantics.
   </para>

   <para>
    Two significant incompatibilities exist between AREs and the ERE syntax
    recognized by pre-7.4 releases of <productname>PostgreSQL</productname>:

    <itemizedlist>
     <listitem>
      <para>
       In AREs, <literal>\</literal> followed by an alphanumeric character is either
       an escape or an error, while in previous releases, it was just another
       way of writing the alphanumeric.
       This should not be much of a problem because there was no reason to
       write such a sequence in earlier releases.
      </para>
     </listitem>
     <listitem>
      <para>
       In AREs, <literal>\</literal> remains a special character within
       <literal>[]</literal>, so a literal <literal>\</literal> within a bracket
       expression must be written <literal>\\</literal>.
      </para>
     </listitem>
    </itemizedlist>
   </para>
   </sect3>

   <sect3 id="posix-basic-regexes">
    <title>Basic Regular Expressions</title>

   <para>
    BREs differ from EREs in several respects.
    In BREs, <literal>|</literal>, <literal>+</literal>, and <literal>?</literal>
    are ordinary characters and there is no equivalent
    for their functionality.
    The delimiters for bounds are
    <literal>\{</literal> and <literal>\}</literal>,
    with <literal>{</literal> and <literal>}</literal>
    by themselves ordinary characters.
    The parentheses for nested subexpressions are
    <literal>\(</literal> and <literal>\)</literal>,
    with <literal>(</literal> and <literal>)</literal> by themselves ordinary characters.
    <literal>^</literal> is an ordinary character except at the beginning of the
    RE or the beginning of a parenthesized subexpression,
    <literal>$</literal> is an ordinary character except at the end of the
    RE or the end of a parenthesized subexpression,
    and <literal>*</literal> is an ordinary character if it appears at the beginning
    of the RE or the beginning of a parenthesized subexpression
    (after a possible leading <literal>^</literal>).
    Finally, single-digit back references are available, and
    <literal>\&lt;</literal> and <literal>\&gt;</literal>
    are synonyms for
    <literal>[[:&lt;:]]</literal> and <literal>[[:&gt;:]]</literal>
    respectively; no other escapes are available in BREs.
   </para>
   </sect3>

<!-- end re_syntax.n man page -->

   <sect3 id="posix-vs-xquery">
   <title>Differences from XQuery (<literal>LIKE_REGEX</literal>)</title>

   <indexterm zone="posix-vs-xquery">
    <primary><literal>LIKE_REGEX</literal></primary>
   </indexterm>

   <indexterm zone="posix-vs-xquery">
    <primary>XQuery regular expressions</primary>
   </indexterm>

    <para>
     Since SQL:2008, the SQL standard includes
     a <literal>LIKE_REGEX</literal> operator that performs pattern
     matching according to the XQuery regular expression
     standard.  <productname>PostgreSQL</productname> does not yet
     implement this operator, but you can get very similar behavior using
     the <function>regexp_match()</function> function, since XQuery
     regular expressions are quite close to the ARE syntax described above.
    </para>

    <para>
     Notable differences between the existing POSIX-based
     regular-expression feature and XQuery regular expressions include:

     <itemizedlist>
      <listitem>
       <para>
        XQuery character class subtraction is not supported.  An example of
        this feature is using the following to match only English
        consonants: <literal>[a-z-[aeiou]]</literal>.
       </para>
      </listitem>
      <listitem>
       <para>
        XQuery character class shorthands <literal>\c</literal>,
        <literal>\C</literal>, <literal>\i</literal>,
        and <literal>\I</literal> are not supported.
       </para>
      </listitem>
      <listitem>
       <para>
        XQuery character class elements
        using <literal>\p{UnicodeProperty}</literal> or the
        inverse <literal>\P{UnicodeProperty}</literal> are not supported.
       </para>
      </listitem>
      <listitem>
       <para>
        POSIX interprets character classes such as <literal>\w</literal>
        (see <xref linkend="posix-class-shorthand-escapes-table"/>)
        according to the prevailing locale (which you can control by
        attaching a <literal>COLLATE</literal> clause to the operator or
        function).  XQuery specifies these classes by reference to Unicode
        character properties, so equivalent behavior is obtained only with
        a locale that follows the Unicode rules.
       </para>
      </listitem>
      <listitem>
       <para>
        The SQL standard (not XQuery itself) attempts to cater for more
        variants of <quote>newline</quote> than POSIX does.  The
        newline-sensitive matching options described above consider only
        ASCII NL (<literal>\n</literal>) to be a newline, but SQL would have
        us treat CR (<literal>\r</literal>), CRLF (<literal>\r\n</literal>)
        (a Windows-style newline), and some Unicode-only characters like
        LINE SEPARATOR (U+2028) as newlines as well.
        Notably, <literal>.</literal> and <literal>\s</literal> should
        count <literal>\r\n</literal> as one character not two according to
        SQL.
       </para>
      </listitem>
      <listitem>
       <para>
        Of the character-entry escapes described in
        <xref linkend="posix-character-entry-escapes-table"/>,
        XQuery supports only <literal>\n</literal>, <literal>\r</literal>,
        and <literal>\t</literal>.
       </para>
      </listitem>
      <listitem>
       <para>
        XQuery does not support
        the <literal>[:<replaceable>name</replaceable>:]</literal> syntax
        for character classes within bracket expressions.
       </para>
      </listitem>
      <listitem>
       <para>
        XQuery does not have lookahead or lookbehind constraints,
        nor any of the constraint escapes described in
        <xref linkend="posix-constraint-escapes-table"/>.
       </para>
      </listitem>
      <listitem>
       <para>
        The metasyntax forms described in <xref linkend="posix-metasyntax"/>
        do not exist in XQuery.
       </para>
      </listitem>
      <listitem>
       <para>
        The regular expression flag letters defined by XQuery are
        related to but not the same as the option letters for POSIX
        (<xref linkend="posix-embedded-options-table"/>).  While the
        <literal>i</literal> and <literal>q</literal> options behave the
        same, others do not:
        <itemizedlist>
         <listitem>
          <para>
           XQuery's <literal>s</literal> (allow dot to match newline)
           and <literal>m</literal> (allow <literal>^</literal>
           and <literal>$</literal> to match at newlines) flags provide
           access to the same behaviors as
           POSIX's <literal>n</literal>, <literal>p</literal>
           and <literal>w</literal> flags, but they
           do <emphasis>not</emphasis> match the behavior of
           POSIX's <literal>s</literal> and <literal>m</literal> flags.
           Note in particular that dot-matches-newline is the default
           behavior in POSIX but not XQuery.
          </para>
         </listitem>
         <listitem>
          <para>
           XQuery's <literal>x</literal> (ignore whitespace in pattern) flag
           is noticeably different from POSIX's expanded-mode flag.
           POSIX's <literal>x</literal> flag also
           allows <literal>#</literal> to begin a comment in the pattern,
           and POSIX will not ignore a whitespace character after a
           backslash.
          </para>
         </listitem>
        </itemizedlist>
       </para>
      </listitem>
     </itemizedlist>
    </para>

   </sect3>
  </sect2>
 </sect1>


  <sect1 id="functions-formatting">
   <title>Data Type Formatting Functions</title>

   <indexterm>
    <primary>formatting</primary>
   </indexterm>

   <para>
    The <productname>PostgreSQL</productname> formatting functions
    provide a powerful set of tools for converting various data types
    (date/time, integer, floating point, numeric) to formatted strings
    and for converting from formatted strings to specific data types.
    <xref linkend="functions-formatting-table"/> lists them.
    These functions all follow a common calling convention: the first
    argument is the value to be formatted and the second argument is a
    template that defines the output or input format.
   </para>

   <table id="functions-formatting-table">
    <title>Formatting Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_char</primary>
        </indexterm>
        <function>to_char</function> ( <type>timestamp</type>, <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <function>to_char</function> ( <type>timestamp with time zone</type>, <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts time stamp to string according to the given format.
       </para>
       <para>
        <literal>to_char(timestamp '2002-04-20 17:31:12.66', 'HH12:MI:SS')</literal>
        <returnvalue>05:31:12</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>to_char</function> ( <type>interval</type>, <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts interval to string according to the given format.
       </para>
       <para>
       <literal>to_char(interval '15h 2m 12s', 'HH24:MI:SS')</literal>
       <returnvalue>15:02:12</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>to_char</function> ( <replaceable>numeric_type</replaceable>, <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts number to string according to the given format; available
        for <type>integer</type>, <type>bigint</type>, <type>numeric</type>,
        <type>real</type>, <type>double precision</type>.
       </para>
       <para>
        <literal>to_char(125, '999')</literal>
        <returnvalue>125</returnvalue>
       </para>
       <para>
        <literal>to_char(125.8::real, '999D9')</literal>
        <returnvalue>125.8</returnvalue>
       </para>
       <para>
        <literal>to_char(-125.8, '999D99S')</literal>
        <returnvalue>125.80-</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_date</primary>
        </indexterm>
        <function>to_date</function> ( <type>text</type>, <type>text</type> )
        <returnvalue>date</returnvalue>
       </para>
       <para>
        Converts string to date according to the given format.
       </para>
       <para>
        <literal>to_date('05 Dec 2000', 'DD Mon YYYY')</literal>
        <returnvalue>2000-12-05</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_number</primary>
        </indexterm>
        <function>to_number</function> ( <type>text</type>, <type>text</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para>
        Converts string to numeric according to the given format.
       </para>
       <para>
        <literal>to_number('12,454.8-', '99G999D9S')</literal>
        <returnvalue>-12454.8</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_timestamp</primary>
        </indexterm>
        <function>to_timestamp</function> ( <type>text</type>, <type>text</type> )
        <returnvalue>timestamp with time zone</returnvalue>
       </para>
       <para>
        Converts string to time stamp according to the given format.
        (See also <function>to_timestamp(double precision)</function> in
        <xref linkend="functions-datetime-table"/>.)
       </para>
       <para>
        <literal>to_timestamp('05 Dec 2000', 'DD Mon YYYY')</literal>
        <returnvalue>2000-12-05 00:00:00-05</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <tip>
    <para>
     <function>to_timestamp</function> and <function>to_date</function>
     exist to handle input formats that cannot be converted by
     simple casting.  For most standard date/time formats, simply casting the
     source string to the required data type works, and is much easier.
     Similarly, <function>to_number</function> is unnecessary for standard numeric
     representations.
    </para>
   </tip>

   <para>
    In a <function>to_char</function> output template string, there are certain
    patterns that are recognized and replaced with appropriately-formatted
    data based on the given value.  Any text that is not a template pattern is
    simply copied verbatim.  Similarly, in an input template string (for the
    other functions), template patterns identify the values to be supplied by
    the input data string.  If there are characters in the template string
    that are not template patterns, the corresponding characters in the input
    data string are simply skipped over (whether or not they are equal to the
    template string characters).
   </para>

  <para>
   <xref linkend="functions-formatting-datetime-table"/> shows the
   template patterns available for formatting date and time values.
  </para>

    <table id="functions-formatting-datetime-table">
     <title>Template Patterns for Date/Time Formatting</title>
     <tgroup cols="2">
      <thead>
       <row>
        <entry>Pattern</entry>
        <entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><literal>HH</literal></entry>
        <entry>hour of day (01&ndash;12)</entry>
       </row>
       <row>
        <entry><literal>HH12</literal></entry>
        <entry>hour of day (01&ndash;12)</entry>
       </row>
       <row>
        <entry><literal>HH24</literal></entry>
        <entry>hour of day (00&ndash;23)</entry>
       </row>
       <row>
        <entry><literal>MI</literal></entry>
        <entry>minute (00&ndash;59)</entry>
       </row>
       <row>
        <entry><literal>SS</literal></entry>
        <entry>second (00&ndash;59)</entry>
       </row>
       <row>
        <entry><literal>MS</literal></entry>
        <entry>millisecond (000&ndash;999)</entry>
       </row>
       <row>
        <entry><literal>US</literal></entry>
        <entry>microsecond (000000&ndash;999999)</entry>
       </row>
       <row>
        <entry><literal>FF1</literal></entry>
        <entry>tenth of second (0&ndash;9)</entry>
       </row>
       <row>
        <entry><literal>FF2</literal></entry>
        <entry>hundredth of second (00&ndash;99)</entry>
       </row>
       <row>
        <entry><literal>FF3</literal></entry>
        <entry>millisecond (000&ndash;999)</entry>
       </row>
       <row>
        <entry><literal>FF4</literal></entry>
        <entry>tenth of a millisecond (0000&ndash;9999)</entry>
       </row>
       <row>
        <entry><literal>FF5</literal></entry>
        <entry>hundredth of a millisecond (00000&ndash;99999)</entry>
       </row>
       <row>
        <entry><literal>FF6</literal></entry>
        <entry>microsecond (000000&ndash;999999)</entry>
       </row>
       <row>
        <entry><literal>SSSS</literal>, <literal>SSSSS</literal></entry>
        <entry>seconds past midnight (0&ndash;86399)</entry>
       </row>
       <row>
        <entry><literal>AM</literal>, <literal>am</literal>,
        <literal>PM</literal> or <literal>pm</literal></entry>
        <entry>meridiem indicator (without periods)</entry>
       </row>
       <row>
        <entry><literal>A.M.</literal>, <literal>a.m.</literal>,
        <literal>P.M.</literal> or <literal>p.m.</literal></entry>
        <entry>meridiem indicator (with periods)</entry>
       </row>
       <row>
        <entry><literal>Y,YYY</literal></entry>
        <entry>year (4 or more digits) with comma</entry>
       </row>
       <row>
        <entry><literal>YYYY</literal></entry>
        <entry>year (4 or more digits)</entry>
       </row>
       <row>
        <entry><literal>YYY</literal></entry>
        <entry>last 3 digits of year</entry>
       </row>
       <row>
        <entry><literal>YY</literal></entry>
        <entry>last 2 digits of year</entry>
       </row>
       <row>
        <entry><literal>Y</literal></entry>
        <entry>last digit of year</entry>
       </row>
       <row>
        <entry><literal>IYYY</literal></entry>
        <entry>ISO 8601 week-numbering year (4 or more digits)</entry>
       </row>
       <row>
        <entry><literal>IYY</literal></entry>
        <entry>last 3 digits of ISO 8601 week-numbering year</entry>
       </row>
       <row>
        <entry><literal>IY</literal></entry>
        <entry>last 2 digits of ISO 8601 week-numbering year</entry>
       </row>
       <row>
        <entry><literal>I</literal></entry>
        <entry>last digit of ISO 8601 week-numbering year</entry>
       </row>
       <row>
        <entry><literal>BC</literal>, <literal>bc</literal>,
        <literal>AD</literal> or <literal>ad</literal></entry>
        <entry>era indicator (without periods)</entry>
       </row>
       <row>
        <entry><literal>B.C.</literal>, <literal>b.c.</literal>,
        <literal>A.D.</literal> or <literal>a.d.</literal></entry>
        <entry>era indicator (with periods)</entry>
       </row>
       <row>
        <entry><literal>MONTH</literal></entry>
        <entry>full upper case month name (blank-padded to 9 chars)</entry>
       </row>
       <row>
        <entry><literal>Month</literal></entry>
        <entry>full capitalized month name (blank-padded to 9 chars)</entry>
       </row>
       <row>
        <entry><literal>month</literal></entry>
        <entry>full lower case month name (blank-padded to 9 chars)</entry>
       </row>
       <row>
        <entry><literal>MON</literal></entry>
        <entry>abbreviated upper case month name (3 chars in English, localized lengths vary)</entry>
       </row>
       <row>
        <entry><literal>Mon</literal></entry>
        <entry>abbreviated capitalized month name (3 chars in English, localized lengths vary)</entry>
       </row>
       <row>
        <entry><literal>mon</literal></entry>
        <entry>abbreviated lower case month name (3 chars in English, localized lengths vary)</entry>
       </row>
       <row>
        <entry><literal>MM</literal></entry>
        <entry>month number (01&ndash;12)</entry>
       </row>
       <row>
        <entry><literal>DAY</literal></entry>
        <entry>full upper case day name (blank-padded to 9 chars)</entry>
       </row>
       <row>
        <entry><literal>Day</literal></entry>
        <entry>full capitalized day name (blank-padded to 9 chars)</entry>
       </row>
       <row>
        <entry><literal>day</literal></entry>
        <entry>full lower case day name (blank-padded to 9 chars)</entry>
       </row>
       <row>
        <entry><literal>DY</literal></entry>
        <entry>abbreviated upper case day name (3 chars in English, localized lengths vary)</entry>
       </row>
       <row>
        <entry><literal>Dy</literal></entry>
        <entry>abbreviated capitalized day name (3 chars in English, localized lengths vary)</entry>
       </row>
       <row>
        <entry><literal>dy</literal></entry>
        <entry>abbreviated lower case day name (3 chars in English, localized lengths vary)</entry>
       </row>
       <row>
        <entry><literal>DDD</literal></entry>
        <entry>day of year (001&ndash;366)</entry>
       </row>
       <row>
        <entry><literal>IDDD</literal></entry>
        <entry>day of ISO 8601 week-numbering year (001&ndash;371; day 1 of the year is Monday of the first ISO week)</entry>
       </row>
       <row>
        <entry><literal>DD</literal></entry>
        <entry>day of month (01&ndash;31)</entry>
       </row>
       <row>
        <entry><literal>D</literal></entry>
        <entry>day of the week, Sunday (<literal>1</literal>) to Saturday (<literal>7</literal>)</entry>
       </row>
       <row>
        <entry><literal>ID</literal></entry>
        <entry>ISO 8601 day of the week, Monday (<literal>1</literal>) to Sunday (<literal>7</literal>)</entry>
       </row>
       <row>
        <entry><literal>W</literal></entry>
        <entry>week of month (1&ndash;5) (the first week starts on the first day of the month)</entry>
       </row>
       <row>
        <entry><literal>WW</literal></entry>
        <entry>week number of year (1&ndash;53) (the first week starts on the first day of the year)</entry>
       </row>
       <row>
        <entry><literal>IW</literal></entry>
        <entry>week number of ISO 8601 week-numbering year (01&ndash;53; the first Thursday of the year is in week 1)</entry>
       </row>
       <row>
        <entry><literal>CC</literal></entry>
        <entry>century (2 digits) (the twenty-first century starts on 2001-01-01)</entry>
       </row>
       <row>
        <entry><literal>J</literal></entry>
        <entry>Julian Day (integer days since November 24, 4714 BC at midnight UTC)</entry>
       </row>
       <row>
        <entry><literal>Q</literal></entry>
        <entry>quarter</entry>
       </row>
       <row>
        <entry><literal>RM</literal></entry>
        <entry>month in upper case Roman numerals (I&ndash;XII; I=January)</entry>
       </row>
       <row>
        <entry><literal>rm</literal></entry>
        <entry>month in lower case Roman numerals (i&ndash;xii; i=January)</entry>
       </row>
       <row>
        <entry><literal>TZ</literal></entry>
        <entry>upper case time-zone abbreviation
         (only supported in <function>to_char</function>)</entry>
       </row>
       <row>
        <entry><literal>tz</literal></entry>
        <entry>lower case time-zone abbreviation
         (only supported in <function>to_char</function>)</entry>
       </row>
       <row>
       <entry><literal>TZH</literal></entry>
        <entry>time-zone hours</entry>
       </row>
       <row>
       <entry><literal>TZM</literal></entry>
        <entry>time-zone minutes</entry>
       </row>
       <row>
        <entry><literal>OF</literal></entry>
        <entry>time-zone offset from UTC
         (only supported in <function>to_char</function>)</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    Modifiers can be applied to any template pattern to alter its
    behavior.  For example, <literal>FMMonth</literal>
    is the <literal>Month</literal> pattern with the
    <literal>FM</literal> modifier.
    <xref linkend="functions-formatting-datetimemod-table"/> shows the
    modifier patterns for date/time formatting.
   </para>

    <table id="functions-formatting-datetimemod-table">
     <title>Template Pattern Modifiers for Date/Time Formatting</title>
     <tgroup cols="3">
      <thead>
       <row>
        <entry>Modifier</entry>
        <entry>Description</entry>
        <entry>Example</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><literal>FM</literal> prefix</entry>
        <entry>fill mode (suppress leading zeroes and padding blanks)</entry>
        <entry><literal>FMMonth</literal></entry>
       </row>
       <row>
        <entry><literal>TH</literal> suffix</entry>
        <entry>upper case ordinal number suffix</entry>
        <entry><literal>DDTH</literal>, e.g., <literal>12TH</literal></entry>
       </row>
       <row>
        <entry><literal>th</literal> suffix</entry>
        <entry>lower case ordinal number suffix</entry>
        <entry><literal>DDth</literal>, e.g., <literal>12th</literal></entry>
       </row>
       <row>
        <entry><literal>FX</literal> prefix</entry>
        <entry>fixed format global option (see usage notes)</entry>
        <entry><literal>FX&nbsp;Month&nbsp;DD&nbsp;Day</literal></entry>
       </row>
       <row>
        <entry><literal>TM</literal> prefix</entry>
        <entry>translation mode (use localized day and month names based on
         <xref linkend="guc-lc-time"/>)</entry>
        <entry><literal>TMMonth</literal></entry>
       </row>
       <row>
        <entry><literal>SP</literal> suffix</entry>
        <entry>spell mode (not implemented)</entry>
        <entry><literal>DDSP</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    Usage notes for date/time formatting:

    <itemizedlist>
     <listitem>
      <para>
       <literal>FM</literal> suppresses leading zeroes and trailing blanks
       that would otherwise be added to make the output of a pattern be
       fixed-width.  In <productname>PostgreSQL</productname>,
       <literal>FM</literal> modifies only the next specification, while in
       Oracle <literal>FM</literal> affects all subsequent
       specifications, and repeated <literal>FM</literal> modifiers
       toggle fill mode on and off.
      </para>
     </listitem>

     <listitem>
      <para>
       <literal>TM</literal> suppresses trailing blanks whether or
       not <literal>FM</literal> is specified.
      </para>
     </listitem>

     <listitem>
      <para>
       <function>to_timestamp</function> and <function>to_date</function>
       ignore letter case in the input; so for
       example <literal>MON</literal>, <literal>Mon</literal>,
       and <literal>mon</literal> all accept the same strings.  When using
       the <literal>TM</literal> modifier, case-folding is done according to
       the rules of the function's input collation (see
       <xref linkend="collation"/>).
      </para>
     </listitem>

     <listitem>
      <para>
       <function>to_timestamp</function> and <function>to_date</function>
       skip multiple blank spaces at the beginning of the input string and
       around date and time values unless the <literal>FX</literal> option is used.  For example,
       <literal>to_timestamp('&nbsp;2000&nbsp;&nbsp;&nbsp;&nbsp;JUN', 'YYYY MON')</literal> and
       <literal>to_timestamp('2000 - JUN', 'YYYY-MON')</literal> work, but
       <literal>to_timestamp('2000&nbsp;&nbsp;&nbsp;&nbsp;JUN', 'FXYYYY MON')</literal> returns an error
       because <function>to_timestamp</function> expects only a single space.
       <literal>FX</literal> must be specified as the first item in
       the template.
      </para>
     </listitem>

     <listitem>
      <para>
       A separator (a space or non-letter/non-digit character) in the template string of
       <function>to_timestamp</function> and <function>to_date</function>
       matches any single separator in the input string or is skipped,
       unless the <literal>FX</literal> option is used.
       For example, <literal>to_timestamp('2000JUN', 'YYYY///MON')</literal> and
       <literal>to_timestamp('2000/JUN', 'YYYY MON')</literal> work, but
       <literal>to_timestamp('2000//JUN', 'YYYY/MON')</literal>
       returns an error because the number of separators in the input string
       exceeds the number of separators in the template.
      </para>
      <para>
       If <literal>FX</literal> is specified, a separator in the template string
       matches exactly one character in the input string.  But note that the
       input string character is not required to be the same as the separator from the template string.
       For example, <literal>to_timestamp('2000/JUN', 'FXYYYY MON')</literal>
       works, but <literal>to_timestamp('2000/JUN', 'FXYYYY&nbsp;&nbsp;MON')</literal>
       returns an error because the second space in the template string consumes
       the letter <literal>J</literal> from the input string.
      </para>
     </listitem>

     <listitem>
      <para>
       A <literal>TZH</literal> template pattern can match a signed number.
       Without the <literal>FX</literal> option, minus signs may be ambiguous,
       and could be interpreted as a separator.
       This ambiguity is resolved as follows:  If the number of separators before
       <literal>TZH</literal> in the template string is less than the number of
       separators before the minus sign in the input string, the minus sign
       is interpreted as part of <literal>TZH</literal>.
       Otherwise, the minus sign is considered to be a separator between values.
       For example, <literal>to_timestamp('2000 -10', 'YYYY TZH')</literal> matches
       <literal>-10</literal> to <literal>TZH</literal>, but
       <literal>to_timestamp('2000 -10', 'YYYY&nbsp;&nbsp;TZH')</literal>
       matches <literal>10</literal> to <literal>TZH</literal>.
      </para>
     </listitem>

     <listitem>
      <para>
       Ordinary text is allowed in <function>to_char</function>
       templates and will be output literally.  You can put a substring
       in double quotes to force it to be interpreted as literal text
       even if it contains template patterns.  For example, in
       <literal>'"Hello Year "YYYY'</literal>, the <literal>YYYY</literal>
       will be replaced by the year data, but the single <literal>Y</literal> in <literal>Year</literal>
       will not be.
       In <function>to_date</function>, <function>to_number</function>,
       and <function>to_timestamp</function>, literal text and double-quoted
       strings result in skipping the number of characters contained in the
       string; for example <literal>"XX"</literal> skips two input characters
       (whether or not they are <literal>XX</literal>).
      </para>
      <tip>
        <para>
          Prior to <productname>PostgreSQL</productname> 12, it was possible to
          skip arbitrary text in the input string using non-letter or non-digit
          characters. For example,
          <literal>to_timestamp('2000y6m1d', 'yyyy-MM-DD')</literal> used to
          work.  Now you can only use letter characters for this purpose.  For example,
          <literal>to_timestamp('2000y6m1d', 'yyyytMMtDDt')</literal> and
          <literal>to_timestamp('2000y6m1d', 'yyyy"y"MM"m"DD"d"')</literal>
          skip <literal>y</literal>, <literal>m</literal>, and
          <literal>d</literal>.
        </para>
      </tip>
     </listitem>

     <listitem>
      <para>
       If you want to have a double quote in the output you must
       precede it with a backslash, for example <literal>'\"YYYY
       Month\"'</literal>. <!-- "" font-lock sanity :-) -->
       Backslashes are not otherwise special outside of double-quoted
       strings.  Within a double-quoted string, a backslash causes the
       next character to be taken literally, whatever it is (but this
       has no special effect unless the next character is a double quote
       or another backslash).
      </para>
     </listitem>

     <listitem>
      <para>
       In <function>to_timestamp</function> and <function>to_date</function>,
       if the year format specification is less than four digits, e.g.,
       <literal>YYY</literal>, and the supplied year is less than four digits,
       the year will be adjusted to be nearest to the year 2020, e.g.,
       <literal>95</literal> becomes 1995.
      </para>
     </listitem>

     <listitem>
      <para>
       In <function>to_timestamp</function> and <function>to_date</function>,
       negative years are treated as signifying BC.  If you write both a
       negative year and an explicit <literal>BC</literal> field, you get AD
       again.  An input of year zero is treated as 1 BC.
      </para>
     </listitem>

     <listitem>
      <para>
       In <function>to_timestamp</function> and <function>to_date</function>,
       the <literal>YYYY</literal> conversion has a restriction when
       processing years with more than 4 digits. You must
       use some non-digit character or template after <literal>YYYY</literal>,
       otherwise the year is always interpreted as 4 digits. For example
       (with the year 20000):
       <literal>to_date('200001131', 'YYYYMMDD')</literal> will be
       interpreted as a 4-digit year; instead use a non-digit
       separator after the year, like
       <literal>to_date('20000-1131', 'YYYY-MMDD')</literal> or
       <literal>to_date('20000Nov31', 'YYYYMonDD')</literal>.
      </para>
     </listitem>

     <listitem>
      <para>
       In <function>to_timestamp</function> and <function>to_date</function>,
       the <literal>CC</literal> (century) field is accepted but ignored
       if there is a <literal>YYY</literal>, <literal>YYYY</literal> or
       <literal>Y,YYY</literal> field. If <literal>CC</literal> is used with
       <literal>YY</literal> or <literal>Y</literal> then the result is
       computed as that year in the specified century.  If the century is
       specified but the year is not, the first year of the century
       is assumed.
      </para>
     </listitem>

     <listitem>
      <para>
       In <function>to_timestamp</function> and <function>to_date</function>,
       weekday names or numbers (<literal>DAY</literal>, <literal>D</literal>,
       and related field types) are accepted but are ignored for purposes of
       computing the result.  The same is true for quarter
       (<literal>Q</literal>) fields.
      </para>
     </listitem>

     <listitem>
      <para>
       In <function>to_timestamp</function> and <function>to_date</function>,
       an ISO 8601 week-numbering date (as distinct from a Gregorian date)
       can be specified in one of two ways:
       <itemizedlist>
        <listitem>
         <para>
          Year, week number, and weekday:  for
          example <literal>to_date('2006-42-4', 'IYYY-IW-ID')</literal>
          returns the date <literal>2006-10-19</literal>.
          If you omit the weekday it is assumed to be 1 (Monday).
         </para>
        </listitem>
        <listitem>
         <para>
          Year and day of year:  for example <literal>to_date('2006-291',
          'IYYY-IDDD')</literal> also returns <literal>2006-10-19</literal>.
         </para>
        </listitem>
       </itemizedlist>
      </para>
      <para>
       Attempting to enter a date using a mixture of ISO 8601 week-numbering
       fields and Gregorian date fields is nonsensical, and will cause an
       error.  In the context of an ISO 8601 week-numbering year, the
       concept of a <quote>month</quote> or <quote>day of month</quote> has no
       meaning.  In the context of a Gregorian year, the ISO week has no
       meaning.
      </para>
      <caution>
       <para>
        While <function>to_date</function> will reject a mixture of
        Gregorian and ISO week-numbering date
        fields, <function>to_char</function> will not, since output format
        specifications like <literal>YYYY-MM-DD (IYYY-IDDD)</literal> can be
        useful.  But avoid writing something like <literal>IYYY-MM-DD</literal>;
        that would yield surprising results near the start of the year.
        (See <xref linkend="functions-datetime-extract"/> for more
        information.)
       </para>
      </caution>
     </listitem>

     <listitem>
      <para>
       In <function>to_timestamp</function>, millisecond
       (<literal>MS</literal>) or microsecond (<literal>US</literal>)
       fields are used as the
       seconds digits after the decimal point. For example
       <literal>to_timestamp('12.3', 'SS.MS')</literal> is not 3 milliseconds,
       but 300, because the conversion treats it as 12 + 0.3 seconds.
       So, for the format <literal>SS.MS</literal>, the input values
       <literal>12.3</literal>, <literal>12.30</literal>,
       and <literal>12.300</literal> specify the
       same number of milliseconds. To get three milliseconds, one must write
       <literal>12.003</literal>, which the conversion treats as
       12 + 0.003 = 12.003 seconds.
      </para>

      <para>
       Here is a more
       complex example:
       <literal>to_timestamp('15:12:02.020.001230', 'HH24:MI:SS.MS.US')</literal>
       is 15 hours, 12 minutes, and 2 seconds + 20 milliseconds +
       1230 microseconds = 2.021230 seconds.
      </para>
     </listitem>

     <listitem>
      <para>
        <function>to_char(..., 'ID')</function>'s day of the week numbering
        matches the <function>extract(isodow from ...)</function> function, but
        <function>to_char(..., 'D')</function>'s does not match
        <function>extract(dow from ...)</function>'s day numbering.
      </para>
     </listitem>

     <listitem>
      <para>
        <function>to_char(interval)</function> formats <literal>HH</literal> and
        <literal>HH12</literal> as shown on a 12-hour clock, for example zero hours
        and 36 hours both output as <literal>12</literal>, while <literal>HH24</literal>
        outputs the full hour value, which can exceed 23 in
        an <type>interval</type> value.
      </para>
     </listitem>

    </itemizedlist>
   </para>

  <para>
   <xref linkend="functions-formatting-numeric-table"/> shows the
   template patterns available for formatting numeric values.
  </para>

    <table id="functions-formatting-numeric-table">
     <title>Template Patterns for Numeric Formatting</title>
     <tgroup cols="2">
      <thead>
       <row>
        <entry>Pattern</entry>
        <entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><literal>9</literal></entry>
        <entry>digit position (can be dropped if insignificant)</entry>
       </row>
       <row>
        <entry><literal>0</literal></entry>
        <entry>digit position (will not be dropped, even if insignificant)</entry>
       </row>
       <row>
        <entry><literal>.</literal> (period)</entry>
        <entry>decimal point</entry>
       </row>
       <row>
        <entry><literal>,</literal> (comma)</entry>
        <entry>group (thousands) separator</entry>
       </row>
       <row>
        <entry><literal>PR</literal></entry>
        <entry>negative value in angle brackets</entry>
       </row>
       <row>
        <entry><literal>S</literal></entry>
        <entry>sign anchored to number (uses locale)</entry>
       </row>
       <row>
        <entry><literal>L</literal></entry>
        <entry>currency symbol (uses locale)</entry>
       </row>
       <row>
        <entry><literal>D</literal></entry>
        <entry>decimal point (uses locale)</entry>
       </row>
       <row>
        <entry><literal>G</literal></entry>
        <entry>group separator (uses locale)</entry>
       </row>
       <row>
        <entry><literal>MI</literal></entry>
        <entry>minus sign in specified position (if number &lt; 0)</entry>
       </row>
       <row>
        <entry><literal>PL</literal></entry>
        <entry>plus sign in specified position (if number &gt; 0)</entry>
       </row>
       <row>
        <entry><literal>SG</literal></entry>
        <entry>plus/minus sign in specified position</entry>
       </row>
       <row>
        <entry><literal>RN</literal></entry>
        <entry>Roman numeral (input between 1 and 3999)</entry>
       </row>
       <row>
        <entry><literal>TH</literal> or <literal>th</literal></entry>
        <entry>ordinal number suffix</entry>
       </row>
       <row>
        <entry><literal>V</literal></entry>
        <entry>shift specified number of digits (see notes)</entry>
       </row>
       <row>
        <entry><literal>EEEE</literal></entry>
        <entry>exponent for scientific notation</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    Usage notes for numeric formatting:

    <itemizedlist>
     <listitem>
      <para>
       <literal>0</literal> specifies a digit position that will always be printed,
       even if it contains a leading/trailing zero.  <literal>9</literal> also
       specifies a digit position, but if it is a leading zero then it will
       be replaced by a space, while if it is a trailing zero and fill mode
       is specified then it will be deleted.  (For <function>to_number()</function>,
       these two pattern characters are equivalent.)
      </para>
     </listitem>

     <listitem>
      <para>
       The pattern characters <literal>S</literal>, <literal>L</literal>, <literal>D</literal>,
       and <literal>G</literal> represent the sign, currency symbol, decimal point,
       and thousands separator characters defined by the current locale
       (see <xref linkend="guc-lc-monetary"/>
       and <xref linkend="guc-lc-numeric"/>).  The pattern characters period
       and comma represent those exact characters, with the meanings of
       decimal point and thousands separator, regardless of locale.
      </para>
     </listitem>

     <listitem>
      <para>
       If no explicit provision is made for a sign
       in <function>to_char()</function>'s pattern, one column will be reserved for
       the sign, and it will be anchored to (appear just left of) the
       number.  If <literal>S</literal> appears just left of some <literal>9</literal>'s,
       it will likewise be anchored to the number.
      </para>
     </listitem>

     <listitem>
      <para>
       A sign formatted using <literal>SG</literal>, <literal>PL</literal>, or
       <literal>MI</literal> is not anchored to
       the number; for example,
       <literal>to_char(-12, 'MI9999')</literal> produces <literal>'-&nbsp;&nbsp;12'</literal>
       but <literal>to_char(-12, 'S9999')</literal> produces <literal>'&nbsp;&nbsp;-12'</literal>.
       (The Oracle implementation does not allow the use of
       <literal>MI</literal> before <literal>9</literal>, but rather
       requires that <literal>9</literal> precede
       <literal>MI</literal>.)
      </para>
     </listitem>

     <listitem>
      <para>
       <literal>TH</literal> does not convert values less than zero
       and does not convert fractional numbers.
      </para>
     </listitem>

     <listitem>
      <para>
       <literal>PL</literal>, <literal>SG</literal>, and
       <literal>TH</literal> are <productname>PostgreSQL</productname>
       extensions.
      </para>
     </listitem>

     <listitem>
      <para>
       In <function>to_number</function>, if non-data template patterns such
       as <literal>L</literal> or <literal>TH</literal> are used, the
       corresponding number of input characters are skipped, whether or not
       they match the template pattern, unless they are data characters
       (that is, digits, sign, decimal point, or comma).  For
       example, <literal>TH</literal> would skip two non-data characters.
      </para>
     </listitem>

     <listitem>
      <para>
       <literal>V</literal> with <function>to_char</function>
       multiplies the input values by
       <literal>10^<replaceable>n</replaceable></literal>, where
       <replaceable>n</replaceable> is the number of digits following
       <literal>V</literal>.  <literal>V</literal> with
       <function>to_number</function> divides in a similar manner.
       <function>to_char</function> and <function>to_number</function>
       do not support the use of
       <literal>V</literal> combined with a decimal point
       (e.g., <literal>99.9V99</literal> is not allowed).
      </para>
     </listitem>

     <listitem>
      <para>
       <literal>EEEE</literal> (scientific notation) cannot be used in
       combination with any of the other formatting patterns or
       modifiers other than digit and decimal point patterns, and must be at the end of the format string
       (e.g., <literal>9.99EEEE</literal> is a valid pattern).
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    Certain modifiers can be applied to any template pattern to alter its
    behavior.  For example, <literal>FM99.99</literal>
    is the <literal>99.99</literal> pattern with the
    <literal>FM</literal> modifier.
    <xref linkend="functions-formatting-numericmod-table"/> shows the
    modifier patterns for numeric formatting.
   </para>

    <table id="functions-formatting-numericmod-table">
     <title>Template Pattern Modifiers for Numeric Formatting</title>
     <tgroup cols="3">
      <thead>
       <row>
        <entry>Modifier</entry>
        <entry>Description</entry>
        <entry>Example</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><literal>FM</literal> prefix</entry>
        <entry>fill mode (suppress trailing zeroes and padding blanks)</entry>
        <entry><literal>FM99.99</literal></entry>
       </row>
       <row>
        <entry><literal>TH</literal> suffix</entry>
        <entry>upper case ordinal number suffix</entry>
        <entry><literal>999TH</literal></entry>
       </row>
       <row>
        <entry><literal>th</literal> suffix</entry>
        <entry>lower case ordinal number suffix</entry>
        <entry><literal>999th</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

  <para>
   <xref linkend="functions-formatting-examples-table"/> shows some
   examples of the use of the <function>to_char</function> function.
  </para>

    <table id="functions-formatting-examples-table">
     <title><function>to_char</function> Examples</title>
     <tgroup cols="2">
      <thead>
       <row>
        <entry>Expression</entry>
        <entry>Result</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><literal>to_char(current_timestamp, 'Day,&nbsp;DD&nbsp;&nbsp;HH12:MI:SS')</literal></entry>
        <entry><literal>'Tuesday&nbsp;&nbsp;,&nbsp;06&nbsp;&nbsp;05:39:18'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(current_timestamp, 'FMDay,&nbsp;FMDD&nbsp;&nbsp;HH12:MI:SS')</literal></entry>
        <entry><literal>'Tuesday,&nbsp;6&nbsp;&nbsp;05:39:18'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(-0.1, '99.99')</literal></entry>
        <entry><literal>'&nbsp;&nbsp;-.10'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(-0.1, 'FM9.99')</literal></entry>
        <entry><literal>'-.1'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(-0.1, 'FM90.99')</literal></entry>
        <entry><literal>'-0.1'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(0.1, '0.9')</literal></entry>
        <entry><literal>'&nbsp;0.1'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(12, '9990999.9')</literal></entry>
        <entry><literal>'&nbsp;&nbsp;&nbsp;&nbsp;0012.0'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(12, 'FM9990999.9')</literal></entry>
        <entry><literal>'0012.'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(485, '999')</literal></entry>
        <entry><literal>'&nbsp;485'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(-485, '999')</literal></entry>
        <entry><literal>'-485'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(485, '9&nbsp;9&nbsp;9')</literal></entry>
        <entry><literal>'&nbsp;4&nbsp;8&nbsp;5'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(1485, '9,999')</literal></entry>
        <entry><literal>'&nbsp;1,485'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(1485, '9G999')</literal></entry>
        <entry><literal>'&nbsp;1&nbsp;485'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(148.5, '999.999')</literal></entry>
        <entry><literal>'&nbsp;148.500'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(148.5, 'FM999.999')</literal></entry>
        <entry><literal>'148.5'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(148.5, 'FM999.990')</literal></entry>
        <entry><literal>'148.500'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(148.5, '999D999')</literal></entry>
        <entry><literal>'&nbsp;148,500'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(3148.5, '9G999D999')</literal></entry>
        <entry><literal>'&nbsp;3&nbsp;148,500'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(-485, '999S')</literal></entry>
        <entry><literal>'485-'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(-485, '999MI')</literal></entry>
        <entry><literal>'485-'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(485, '999MI')</literal></entry>
        <entry><literal>'485&nbsp;'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(485, 'FM999MI')</literal></entry>
        <entry><literal>'485'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(485, 'PL999')</literal></entry>
        <entry><literal>'+485'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(485, 'SG999')</literal></entry>
        <entry><literal>'+485'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(-485, 'SG999')</literal></entry>
        <entry><literal>'-485'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(-485, '9SG99')</literal></entry>
        <entry><literal>'4-85'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(-485, '999PR')</literal></entry>
        <entry><literal>'&lt;485&gt;'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(485, 'L999')</literal></entry>
        <entry><literal>'DM&nbsp;485'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(485, 'RN')</literal></entry>
        <entry><literal>'&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;CDLXXXV'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(485, 'FMRN')</literal></entry>
        <entry><literal>'CDLXXXV'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(5.2, 'FMRN')</literal></entry>
        <entry><literal>'V'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(482, '999th')</literal></entry>
        <entry><literal>'&nbsp;482nd'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(485, '"Good&nbsp;number:"999')</literal></entry>
        <entry><literal>'Good&nbsp;number:&nbsp;485'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(485.8, '"Pre:"999"&nbsp;Post:"&nbsp;.999')</literal></entry>
        <entry><literal>'Pre:&nbsp;485&nbsp;Post:&nbsp;.800'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(12, '99V999')</literal></entry>
        <entry><literal>'&nbsp;12000'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(12.4, '99V999')</literal></entry>
        <entry><literal>'&nbsp;12400'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(12.45, '99V9')</literal></entry>
        <entry><literal>'&nbsp;125'</literal></entry>
       </row>
       <row>
        <entry><literal>to_char(0.0004859, '9.99EEEE')</literal></entry>
        <entry><literal>' 4.86e-04'</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

  </sect1>


  <sect1 id="functions-datetime">
   <title>Date/Time Functions and Operators</title>

  <para>
   <xref linkend="functions-datetime-table"/> shows the available
   functions for date/time value processing, with details appearing in
   the following subsections.  <xref
   linkend="operators-datetime-table"/> illustrates the behaviors of
   the basic arithmetic operators (<literal>+</literal>,
   <literal>*</literal>, etc.).  For formatting functions, refer to
   <xref linkend="functions-formatting"/>.  You should be familiar with
   the background information on date/time data types from <xref
   linkend="datatype-datetime"/>.
  </para>

  <para>
   All the functions and operators described below that take <type>time</type> or <type>timestamp</type>
   inputs actually come in two variants: one that takes <type>time with time zone</type> or <type>timestamp
   with time zone</type>, and one that takes <type>time without time zone</type> or <type>timestamp without time zone</type>.
   For brevity, these variants are not shown separately.  Also, the
   <literal>+</literal> and <literal>*</literal> operators come in commutative pairs (for
   example both date + integer and integer + date); we show only one of each
   such pair.
  </para>

    <table id="operators-datetime-table">
     <title>Date/Time Operators</title>

     <tgroup cols="1">
      <thead>
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         Operator
        </para>
        <para>
         Description
        </para>
        <para>
         Example(s)
        </para></entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>date</type> <literal>+</literal> <type>integer</type>
         <returnvalue>date</returnvalue>
        </para>
        <para>
         Add a number of days to a date
        </para>
        <para>
         <literal>date '2001-09-28' + 7</literal>
         <returnvalue>2001-10-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>date</type> <literal>+</literal> <type>interval</type>
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Add an interval to a date
        </para>
        <para>
         <literal>date '2001-09-28' + interval '1 hour'</literal>
         <returnvalue>2001-09-28 01:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>date</type> <literal>+</literal> <type>time</type>
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Add a time-of-day to a date
        </para>
        <para>
         <literal>date '2001-09-28' + time '03:00'</literal>
         <returnvalue>2001-09-28 03:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>interval</type> <literal>+</literal> <type>interval</type>
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Add intervals
        </para>
        <para>
         <literal>interval '1 day' + interval '1 hour'</literal>
         <returnvalue>1 day 01:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>timestamp</type> <literal>+</literal> <type>interval</type>
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Add an interval to a timestamp
        </para>
        <para>
         <literal>timestamp '2001-09-28 01:00' + interval '23 hours'</literal>
         <returnvalue>2001-09-29 00:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>time</type> <literal>+</literal> <type>interval</type>
         <returnvalue>time</returnvalue>
        </para>
        <para>
         Add an interval to a time
        </para>
        <para>
         <literal>time '01:00' + interval '3 hours'</literal>
         <returnvalue>04:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <literal>-</literal> <type>interval</type>
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Negate an interval
        </para>
        <para>
         <literal>- interval '23 hours'</literal>
         <returnvalue>-23:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>date</type> <literal>-</literal> <type>date</type>
         <returnvalue>integer</returnvalue>
        </para>
        <para>
         Subtract dates, producing the number of days elapsed
        </para>
        <para>
         <literal>date '2001-10-01' - date '2001-09-28'</literal>
         <returnvalue>3</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>date</type> <literal>-</literal> <type>integer</type>
         <returnvalue>date</returnvalue>
        </para>
        <para>
         Subtract a number of days from a date
        </para>
        <para>
         <literal>date '2001-10-01' - 7</literal>
         <returnvalue>2001-09-24</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>date</type> <literal>-</literal> <type>interval</type>
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Subtract an interval from a date
        </para>
        <para>
         <literal>date '2001-09-28' - interval '1 hour'</literal>
         <returnvalue>2001-09-27 23:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>time</type> <literal>-</literal> <type>time</type>
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Subtract times
        </para>
        <para>
         <literal>time '05:00' - time '03:00'</literal>
         <returnvalue>02:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>time</type> <literal>-</literal> <type>interval</type>
         <returnvalue>time</returnvalue>
        </para>
        <para>
         Subtract an interval from a time
        </para>
        <para>
         <literal>time '05:00' - interval '2 hours'</literal>
         <returnvalue>03:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>timestamp</type> <literal>-</literal> <type>interval</type>
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Subtract an interval from a timestamp
        </para>
        <para>
         <literal>timestamp '2001-09-28 23:00' - interval '23 hours'</literal>
         <returnvalue>2001-09-28 00:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>interval</type> <literal>-</literal> <type>interval</type>
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Subtract intervals
        </para>
        <para>
         <literal>interval '1 day' - interval '1 hour'</literal>
         <returnvalue>1 day -01:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>timestamp</type> <literal>-</literal> <type>timestamp</type>
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Subtract timestamps (converting 24-hour intervals into days,
         similarly to <function>justify_hours()</function>)
        </para>
        <para>
         <literal>timestamp '2001-09-29 03:00' - timestamp '2001-07-27 12:00'</literal>
         <returnvalue>63 days 15:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>interval</type> <literal>*</literal> <type>double precision</type>
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Multiply an interval by a scalar
        </para>
        <para>
         <literal>interval '1 second' * 900</literal>
         <returnvalue>00:15:00</returnvalue>
        </para>
        <para>
         <literal>interval '1 day' * 21</literal>
         <returnvalue>21 days</returnvalue>
        </para>
        <para>
         <literal>interval '1 hour' * 3.5</literal>
         <returnvalue>03:30:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>interval</type> <literal>/</literal> <type>double precision</type>
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Divide an interval by a scalar
        </para>
        <para>
         <literal>interval '1 hour' / 1.5</literal>
         <returnvalue>00:40:00</returnvalue>
        </para></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <table id="functions-datetime-table">
     <title>Date/Time Functions</title>
     <tgroup cols="1">
      <thead>
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         Function
        </para>
        <para>
         Description
        </para>
        <para>
         Example(s)
        </para></entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>age</primary>
         </indexterm>
         <function>age</function> ( <type>timestamp</type>, <type>timestamp</type> )
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Subtract arguments, producing a <quote>symbolic</quote> result that
         uses years and months, rather than just days
        </para>
        <para>
         <literal>age(timestamp '2001-04-10', timestamp '1957-06-13')</literal>
         <returnvalue>43 years 9 mons 27 days</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>age</function> ( <type>timestamp</type> )
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Subtract argument from <function>current_date</function> (at midnight)
        </para>
        <para>
         <literal>age(timestamp '1957-06-13')</literal>
         <returnvalue>62 years 6 mons 10 days</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>clock_timestamp</primary>
         </indexterm>
         <function>clock_timestamp</function> ( )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Current date and time (changes during statement execution);
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>clock_timestamp()</literal>
         <returnvalue>2019-12-23 14:39:53.662522-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>current_date</primary>
         </indexterm>
         <function>current_date</function>
         <returnvalue>date</returnvalue>
        </para>
        <para>
         Current date; see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>current_date</literal>
         <returnvalue>2019-12-23</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>current_time</primary>
         </indexterm>
         <function>current_time</function>
         <returnvalue>time with time zone</returnvalue>
        </para>
        <para>
         Current time of day; see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>current_time</literal>
         <returnvalue>14:39:53.662522-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>current_time</function> ( <type>integer</type> )
         <returnvalue>time with time zone</returnvalue>
        </para>
        <para>
         Current time of day, with limited precision;
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>current_time(2)</literal>
         <returnvalue>14:39:53.66-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>current_timestamp</primary>
         </indexterm>
         <function>current_timestamp</function>
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Current date and time (start of current transaction);
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>current_timestamp</literal>
         <returnvalue>2019-12-23 14:39:53.662522-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>current_timestamp</function> ( <type>integer</type> )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Current date and time (start of current transaction), with limited precision;
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>current_timestamp(0)</literal>
         <returnvalue>2019-12-23 14:39:53-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>date_part</primary>
         </indexterm>
         <function>date_part</function> ( <type>text</type>, <type>timestamp</type> )
         <returnvalue>double precision</returnvalue>
        </para>
        <para>
         Get timestamp subfield (equivalent to <function>extract</function>);
         see <xref linkend="functions-datetime-extract"/>
        </para>
        <para>
         <literal>date_part('hour', timestamp '2001-02-16 20:38:40')</literal>
         <returnvalue>20</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>date_part</function> ( <type>text</type>, <type>interval</type> )
         <returnvalue>double precision</returnvalue>
        </para>
        <para>
         Get interval subfield (equivalent to <function>extract</function>);
         see <xref linkend="functions-datetime-extract"/>
        </para>
        <para>
         <literal>date_part('month', interval '2 years 3 months')</literal>
         <returnvalue>3</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>date_trunc</primary>
         </indexterm>
         <function>date_trunc</function> ( <type>text</type>, <type>timestamp</type> )
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Truncate to specified precision; see <xref linkend="functions-datetime-trunc"/>
        </para>
        <para>
         <literal>date_trunc('hour', timestamp '2001-02-16 20:38:40')</literal>
         <returnvalue>2001-02-16 20:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>date_trunc</function> ( <type>text</type>, <type>timestamp with time zone</type>, <type>text</type> )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Truncate to specified precision in the specified time zone; see
         <xref linkend="functions-datetime-trunc"/>
        </para>
        <para>
         <literal>date_trunc('day', timestamptz '2001-02-16 20:38:40+00', 'Australia/Sydney')</literal>
         <returnvalue>2001-02-16 13:00:00+00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>date_trunc</function> ( <type>text</type>, <type>interval</type> )
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Truncate to specified precision; see
         <xref linkend="functions-datetime-trunc"/>
        </para>
        <para>
         <literal>date_trunc('hour', interval '2 days 3 hours 40 minutes')</literal>
         <returnvalue>2 days 03:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>extract</primary>
         </indexterm>
         <function>extract</function> ( <parameter>field</parameter> <literal>from</literal> <type>timestamp</type> )
         <returnvalue>double precision</returnvalue>
        </para>
        <para>
         Get timestamp subfield; see <xref linkend="functions-datetime-extract"/>
        </para>
        <para>
         <literal>extract(hour from timestamp '2001-02-16 20:38:40')</literal>
         <returnvalue>20</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>extract</function> ( <parameter>field</parameter> <literal>from</literal> <type>interval</type> )
         <returnvalue>double precision</returnvalue>
        </para>
        <para>
         Get interval subfield; see <xref linkend="functions-datetime-extract"/>
        </para>
        <para>
         <literal>extract(month from interval '2 years 3 months')</literal>
         <returnvalue>3</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>isfinite</primary>
         </indexterm>
         <function>isfinite</function> ( <type>date</type> )
         <returnvalue>boolean</returnvalue>
        </para>
        <para>
         Test for finite date (not +/-infinity)
        </para>
        <para>
         <literal>isfinite(date '2001-02-16')</literal>
         <returnvalue>true</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>isfinite</function> ( <type>timestamp</type> )
         <returnvalue>boolean</returnvalue>
        </para>
        <para>
         Test for finite timestamp (not +/-infinity)
        </para>
        <para>
         <literal>isfinite(timestamp 'infinity')</literal>
         <returnvalue>false</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>isfinite</function> ( <type>interval</type> )
         <returnvalue>boolean</returnvalue>
        </para>
        <para>
         Test for finite interval (currently always true)
        </para>
        <para>
         <literal>isfinite(interval '4 hours')</literal>
         <returnvalue>true</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>justify_days</primary>
         </indexterm>
         <function>justify_days</function> ( <type>interval</type> )
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Adjust interval so 30-day time periods are represented as months
        </para>
        <para>
         <literal>justify_days(interval '35 days')</literal>
         <returnvalue>1 mon 5 days</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>justify_hours</primary>
         </indexterm>
         <function>justify_hours</function> ( <type>interval</type> )
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Adjust interval so 24-hour time periods are represented as days
        </para>
        <para>
         <literal>justify_hours(interval '27 hours')</literal>
         <returnvalue>1 day 03:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>justify_interval</primary>
         </indexterm>
         <function>justify_interval</function> ( <type>interval</type> )
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Adjust interval using <function>justify_days</function>
         and <function>justify_hours</function>, with additional sign
         adjustments
        </para>
        <para>
         <literal>justify_interval(interval '1 mon -1 hour')</literal>
         <returnvalue>29 days 23:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>localtime</primary>
         </indexterm>
         <function>localtime</function>
         <returnvalue>time</returnvalue>
        </para>
        <para>
         Current time of day;
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>localtime</literal>
         <returnvalue>14:39:53.662522</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>localtime</function> ( <type>integer</type> )
         <returnvalue>time</returnvalue>
        </para>
        <para>
         Current time of day, with limited precision;
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>localtime(0)</literal>
         <returnvalue>14:39:53</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>localtimestamp</primary>
         </indexterm>
         <function>localtimestamp</function>
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Current date and time (start of current transaction);
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>localtimestamp</literal>
         <returnvalue>2019-12-23 14:39:53.662522</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>localtimestamp</function> ( <type>integer</type> )
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Current date and time (start of current
         transaction), with limited precision;
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>localtimestamp(2)</literal>
         <returnvalue>2019-12-23 14:39:53.66</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>make_date</primary>
         </indexterm>
         <function>make_date</function> ( <parameter>year</parameter> <type>int</type>,
         <parameter>month</parameter> <type>int</type>,
         <parameter>day</parameter> <type>int</type> )
         <returnvalue>date</returnvalue>
        </para>
        <para>
         Create date from year, month and day fields
         (negative years signify BC)
        </para>
        <para>
         <literal>make_date(2013, 7, 15)</literal>
         <returnvalue>2013-07-15</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature"><indexterm>
          <primary>make_interval</primary>
         </indexterm>
         <function>make_interval</function> ( <optional> <parameter>years</parameter> <type>int</type>
         <optional>, <parameter>months</parameter> <type>int</type>
         <optional>, <parameter>weeks</parameter> <type>int</type>
         <optional>, <parameter>days</parameter> <type>int</type>
         <optional>, <parameter>hours</parameter> <type>int</type>
         <optional>, <parameter>mins</parameter> <type>int</type>
         <optional>, <parameter>secs</parameter> <type>double precision</type>
         </optional></optional></optional></optional></optional></optional></optional> )
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Create interval from years, months, weeks, days, hours, minutes and
         seconds fields, each of which can default to zero
        </para>
        <para>
         <literal>make_interval(days =&gt; 10)</literal>
         <returnvalue>10 days</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>make_time</primary>
         </indexterm>
         <function>make_time</function> ( <parameter>hour</parameter> <type>int</type>,
         <parameter>min</parameter> <type>int</type>,
         <parameter>sec</parameter> <type>double precision</type> )
         <returnvalue>time</returnvalue>
        </para>
        <para>
         Create time from hour, minute and seconds fields
        </para>
        <para>
         <literal>make_time(8, 15, 23.5)</literal>
         <returnvalue>08:15:23.5</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>make_timestamp</primary>
         </indexterm>
         <function>make_timestamp</function> ( <parameter>year</parameter> <type>int</type>,
         <parameter>month</parameter> <type>int</type>,
         <parameter>day</parameter> <type>int</type>,
         <parameter>hour</parameter> <type>int</type>,
         <parameter>min</parameter> <type>int</type>,
         <parameter>sec</parameter> <type>double precision</type> )
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Create timestamp from year, month, day, hour, minute and seconds fields
         (negative years signify BC)
        </para>
        <para>
         <literal>make_timestamp(2013, 7, 15, 8, 15, 23.5)</literal>
         <returnvalue>2013-07-15 08:15:23.5</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>make_timestamptz</primary>
         </indexterm>
         <function>make_timestamptz</function> ( <parameter>year</parameter> <type>int</type>,
         <parameter>month</parameter> <type>int</type>,
         <parameter>day</parameter> <type>int</type>,
         <parameter>hour</parameter> <type>int</type>,
         <parameter>min</parameter> <type>int</type>,
         <parameter>sec</parameter> <type>double precision</type>
         <optional>, <parameter>timezone</parameter> <type>text</type> </optional> )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Create timestamp with time zone from year, month, day, hour, minute
         and seconds fields (negative years signify BC).
         If <parameter>timezone</parameter> is not
         specified, the current time zone is used; the examples assume the
         session time zone is <literal>Europe/London</literal>
        </para>
        <para>
         <literal>make_timestamptz(2013, 7, 15, 8, 15, 23.5)</literal>
         <returnvalue>2013-07-15 08:15:23.5+01</returnvalue>
        </para>
        <para>
         <literal>make_timestamptz(2013, 7, 15, 8, 15, 23.5, 'America/New_York')</literal>
         <returnvalue>2013-07-15 13:15:23.5+01</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>now</primary>
         </indexterm>
         <function>now</function> ( )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Current date and time (start of current transaction);
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>now()</literal>
         <returnvalue>2019-12-23 14:39:53.662522-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>statement_timestamp</primary>
         </indexterm>
         <function>statement_timestamp</function> ( )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Current date and time (start of current statement);
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>statement_timestamp()</literal>
         <returnvalue>2019-12-23 14:39:53.662522-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>timeofday</primary>
         </indexterm>
         <function>timeofday</function> ( )
         <returnvalue>text</returnvalue>
        </para>
        <para>
         Current date and time
         (like <function>clock_timestamp</function>, but as a <type>text</type> string);
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>timeofday()</literal>
         <returnvalue>Mon Dec 23 14:39:53.662522 2019 EST</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>transaction_timestamp</primary>
         </indexterm>
         <function>transaction_timestamp</function> ( )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Current date and time (start of current transaction);
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>transaction_timestamp()</literal>
         <returnvalue>2019-12-23 14:39:53.662522-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>to_timestamp</primary>
         </indexterm>
         <function>to_timestamp</function> ( <type>double precision</type> )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Convert Unix epoch (seconds since 1970-01-01 00:00:00+00) to
         timestamp with time zone
        </para>
        <para>
         <literal>to_timestamp(1284352323)</literal>
         <returnvalue>2010-09-13 04:32:03+00</returnvalue>
        </para></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    <indexterm>
      <primary>OVERLAPS</primary>
    </indexterm>
    In addition to these functions, the SQL <literal>OVERLAPS</literal> operator is
    supported:
<synopsis>
(<replaceable>start1</replaceable>, <replaceable>end1</replaceable>) OVERLAPS (<replaceable>start2</replaceable>, <replaceable>end2</replaceable>)
(<replaceable>start1</replaceable>, <replaceable>length1</replaceable>) OVERLAPS (<replaceable>start2</replaceable>, <replaceable>length2</replaceable>)
</synopsis>
    This expression yields true when two time periods (defined by their
    endpoints) overlap, false when they do not overlap.  The endpoints
    can be specified as pairs of dates, times, or time stamps; or as
    a date, time, or time stamp followed by an interval.  When a pair
    of values is provided, either the start or the end can be written
    first; <literal>OVERLAPS</literal> automatically takes the earlier value
    of the pair as the start.  Each time period is considered to
    represent the half-open interval <replaceable>start</replaceable> <literal>&lt;=</literal>
    <replaceable>time</replaceable> <literal>&lt;</literal> <replaceable>end</replaceable>, unless
    <replaceable>start</replaceable> and <replaceable>end</replaceable> are equal in which case it
    represents that single time instant.  This means for instance that two
    time periods with only an endpoint in common do not overlap.
   </para>

<screen>
SELECT (DATE '2001-02-16', DATE '2001-12-21') OVERLAPS
       (DATE '2001-10-30', DATE '2002-10-30');
<lineannotation>Result: </lineannotation><computeroutput>true</computeroutput>
SELECT (DATE '2001-02-16', INTERVAL '100 days') OVERLAPS
       (DATE '2001-10-30', DATE '2002-10-30');
<lineannotation>Result: </lineannotation><computeroutput>false</computeroutput>
SELECT (DATE '2001-10-29', DATE '2001-10-30') OVERLAPS
       (DATE '2001-10-30', DATE '2001-10-31');
<lineannotation>Result: </lineannotation><computeroutput>false</computeroutput>
SELECT (DATE '2001-10-30', DATE '2001-10-30') OVERLAPS
       (DATE '2001-10-30', DATE '2001-10-31');
<lineannotation>Result: </lineannotation><computeroutput>true</computeroutput>
</screen>

  <para>
   When adding an <type>interval</type> value to (or subtracting an
   <type>interval</type> value from) a <type>timestamp with time zone</type>
   value, the days component advances or decrements the date of the
   <type>timestamp with time zone</type> by the indicated number of days,
   keeping the time of day the same.
   Across daylight saving time changes (when the session time zone is set to a
   time zone that recognizes DST), this means <literal>interval '1 day'</literal>
   does not necessarily equal <literal>interval '24 hours'</literal>.
   For example, with the session time zone set
   to <literal>America/Denver</literal>:
<screen>
SELECT timestamp with time zone '2005-04-02 12:00:00-07' + interval '1 day';
<lineannotation>Result: </lineannotation><computeroutput>2005-04-03 12:00:00-06</computeroutput>
SELECT timestamp with time zone '2005-04-02 12:00:00-07' + interval '24 hours';
<lineannotation>Result: </lineannotation><computeroutput>2005-04-03 13:00:00-06</computeroutput>
</screen>
   This happens because an hour was skipped due to a change in daylight saving
   time at <literal>2005-04-03 02:00:00</literal> in time zone
   <literal>America/Denver</literal>.
  </para>

  <para>
   Note there can be ambiguity in the <literal>months</literal> field returned by
   <function>age</function> because different months have different numbers of
   days.  <productname>PostgreSQL</productname>'s approach uses the month from the
   earlier of the two dates when calculating partial months.  For example,
   <literal>age('2004-06-01', '2004-04-30')</literal> uses April to yield
   <literal>1 mon 1 day</literal>, while using May would yield <literal>1 mon 2
   days</literal> because May has 31 days, while April has only 30.
  </para>

  <para>
   Subtraction of dates and timestamps can also be complex.  One conceptually
   simple way to perform subtraction is to convert each value to a number
   of seconds using <literal>EXTRACT(EPOCH FROM ...)</literal>, then subtract the
   results; this produces the
   number of <emphasis>seconds</emphasis> between the two values.  This will adjust
   for the number of days in each month, timezone changes, and daylight
   saving time adjustments.  Subtraction of date or timestamp
   values with the <quote><literal>-</literal></quote> operator
   returns the number of days (24-hours) and hours/minutes/seconds
   between the values, making the same adjustments.  The <function>age</function>
   function returns years, months, days, and hours/minutes/seconds,
   performing field-by-field subtraction and then adjusting for negative
   field values.  The following queries illustrate the differences in these
   approaches.  The sample results were produced with <literal>timezone
   = 'US/Eastern'</literal>; there is a daylight saving time change between the
   two dates used:
  </para>

<screen>
SELECT EXTRACT(EPOCH FROM timestamptz '2013-07-01 12:00:00') -
       EXTRACT(EPOCH FROM timestamptz '2013-03-01 12:00:00');
<lineannotation>Result: </lineannotation><computeroutput>10537200</computeroutput>
SELECT (EXTRACT(EPOCH FROM timestamptz '2013-07-01 12:00:00') -
        EXTRACT(EPOCH FROM timestamptz '2013-03-01 12:00:00'))
        / 60 / 60 / 24;
<lineannotation>Result: </lineannotation><computeroutput>121.958333333333</computeroutput>
SELECT timestamptz '2013-07-01 12:00:00' - timestamptz '2013-03-01 12:00:00';
<lineannotation>Result: </lineannotation><computeroutput>121 days 23:00:00</computeroutput>
SELECT age(timestamptz '2013-07-01 12:00:00', timestamptz '2013-03-01 12:00:00');
<lineannotation>Result: </lineannotation><computeroutput>4 mons</computeroutput>
</screen>

  <sect2 id="functions-datetime-extract">
   <title><function>EXTRACT</function>, <function>date_part</function></title>

   <indexterm>
    <primary>date_part</primary>
   </indexterm>
   <indexterm>
    <primary>extract</primary>
   </indexterm>

<synopsis>
EXTRACT(<replaceable>field</replaceable> FROM <replaceable>source</replaceable>)
</synopsis>

   <para>
    The <function>extract</function> function retrieves subfields
    such as year or hour from date/time values.
    <replaceable>source</replaceable> must be a value expression of
    type <type>timestamp</type>, <type>time</type>, or <type>interval</type>.
    (Expressions of type <type>date</type> are
    cast to <type>timestamp</type> and can therefore be used as
    well.)  <replaceable>field</replaceable> is an identifier or
    string that selects what field to extract from the source value.
    The <function>extract</function> function returns values of type
    <type>double precision</type>.
    The following are valid field names:

    <!-- alphabetical -->
    <variablelist>
     <varlistentry>
      <term><literal>century</literal></term>
      <listitem>
       <para>
        The century
       </para>

<screen>
SELECT EXTRACT(CENTURY FROM TIMESTAMP '2000-12-16 12:21:13');
<lineannotation>Result: </lineannotation><computeroutput>20</computeroutput>
SELECT EXTRACT(CENTURY FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>21</computeroutput>
</screen>

       <para>
        The first century starts at 0001-01-01 00:00:00 AD, although
        they did not know it at the time. This definition applies to all
        Gregorian calendar countries. There is no century number 0,
        you go from -1 century to 1 century.

        If you disagree with this, please write your complaint to:
        Pope, Cathedral Saint-Peter of Roma, Vatican.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>day</literal></term>
      <listitem>
       <para>
        For <type>timestamp</type> values, the day (of the month) field
        (1&ndash;31) ; for <type>interval</type> values, the number of days
       </para>

<screen>
SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>16</computeroutput>

SELECT EXTRACT(DAY FROM INTERVAL '40 days 1 minute');
<lineannotation>Result: </lineannotation><computeroutput>40</computeroutput>
</screen>

      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>decade</literal></term>
      <listitem>
       <para>
        The year field divided by 10
       </para>

<screen>
SELECT EXTRACT(DECADE FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>200</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>dow</literal></term>
      <listitem>
       <para>
        The day of the week as Sunday (<literal>0</literal>) to
        Saturday (<literal>6</literal>)
       </para>

<screen>
SELECT EXTRACT(DOW FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>5</computeroutput>
</screen>
       <para>
        Note that <function>extract</function>'s day of the week numbering
        differs from that of the <function>to_char(...,
        'D')</function> function.
       </para>

      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>doy</literal></term>
      <listitem>
       <para>
        The day of the year (1&ndash;365/366)
       </para>

<screen>
SELECT EXTRACT(DOY FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>47</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>epoch</literal></term>
      <listitem>
       <para>
        For <type>timestamp with time zone</type> values, the
        number of seconds since 1970-01-01 00:00:00 UTC (can be negative);
        for <type>date</type> and <type>timestamp</type> values, the
        number of seconds since 1970-01-01 00:00:00 local time;
        for <type>interval</type> values, the total number
        of seconds in the interval
       </para>

<screen>
SELECT EXTRACT(EPOCH FROM TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40.12-08');
<lineannotation>Result: </lineannotation><computeroutput>982384720.12</computeroutput>

SELECT EXTRACT(EPOCH FROM INTERVAL '5 days 3 hours');
<lineannotation>Result: </lineannotation><computeroutput>442800</computeroutput>
</screen>

       <para>
        You can convert an epoch value back to a time stamp
        with <function>to_timestamp</function>:
       </para>
<screen>
SELECT to_timestamp(982384720.12);
<lineannotation>Result: </lineannotation><computeroutput>2001-02-17 04:38:40.12+00</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>hour</literal></term>
      <listitem>
       <para>
        The hour field (0&ndash;23)
       </para>

<screen>
SELECT EXTRACT(HOUR FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>20</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>isodow</literal></term>
      <listitem>
       <para>
        The day of the week as Monday (<literal>1</literal>) to
        Sunday (<literal>7</literal>)
       </para>

<screen>
SELECT EXTRACT(ISODOW FROM TIMESTAMP '2001-02-18 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>7</computeroutput>
</screen>
       <para>
        This is identical to <literal>dow</literal> except for Sunday.  This
        matches the <acronym>ISO</acronym> 8601 day of the week numbering.
       </para>

      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>isoyear</literal></term>
      <listitem>
       <para>
        The <acronym>ISO</acronym> 8601 week-numbering year that the date
        falls in (not applicable to intervals)
       </para>

<screen>
SELECT EXTRACT(ISOYEAR FROM DATE '2006-01-01');
<lineannotation>Result: </lineannotation><computeroutput>2005</computeroutput>
SELECT EXTRACT(ISOYEAR FROM DATE '2006-01-02');
<lineannotation>Result: </lineannotation><computeroutput>2006</computeroutput>
</screen>

       <para>
        Each <acronym>ISO</acronym> 8601 week-numbering year begins with the
        Monday of the week containing the 4th of January, so in early
        January or late December the <acronym>ISO</acronym> year may be
        different from the Gregorian year.  See the <literal>week</literal>
        field for more information.
       </para>
       <para>
        This field is not available in PostgreSQL releases prior to 8.3.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>microseconds</literal></term>
      <listitem>
       <para>
        The seconds field, including fractional parts, multiplied by 1
        000 000;  note that this includes full seconds
       </para>

<screen>
SELECT EXTRACT(MICROSECONDS FROM TIME '17:12:28.5');
<lineannotation>Result: </lineannotation><computeroutput>28500000</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>millennium</literal></term>
      <listitem>
       <para>
        The millennium
       </para>

<screen>
SELECT EXTRACT(MILLENNIUM FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>3</computeroutput>
</screen>

       <para>
        Years in the 1900s are in the second millennium.
        The third millennium started January 1, 2001.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>milliseconds</literal></term>
      <listitem>
       <para>
        The seconds field, including fractional parts, multiplied by
        1000.  Note that this includes full seconds.
       </para>

<screen>
SELECT EXTRACT(MILLISECONDS FROM TIME '17:12:28.5');
<lineannotation>Result: </lineannotation><computeroutput>28500</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>minute</literal></term>
      <listitem>
       <para>
        The minutes field (0&ndash;59)
       </para>

<screen>
SELECT EXTRACT(MINUTE FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>38</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>month</literal></term>
      <listitem>
       <para>
        For <type>timestamp</type> values, the number of the month
        within the year (1&ndash;12) ; for <type>interval</type> values,
        the number of months, modulo 12 (0&ndash;11)
       </para>

<screen>
SELECT EXTRACT(MONTH FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>2</computeroutput>

SELECT EXTRACT(MONTH FROM INTERVAL '2 years 3 months');
<lineannotation>Result: </lineannotation><computeroutput>3</computeroutput>

SELECT EXTRACT(MONTH FROM INTERVAL '2 years 13 months');
<lineannotation>Result: </lineannotation><computeroutput>1</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>quarter</literal></term>
      <listitem>
       <para>
        The quarter of the year (1&ndash;4) that the date is in
       </para>

<screen>
SELECT EXTRACT(QUARTER FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>1</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>second</literal></term>
      <listitem>
       <para>
        The seconds field, including any fractional seconds
       </para>

<screen>
SELECT EXTRACT(SECOND FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>40</computeroutput>

SELECT EXTRACT(SECOND FROM TIME '17:12:28.5');
<lineannotation>Result: </lineannotation><computeroutput>28.5</computeroutput>
</screen>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><literal>timezone</literal></term>
      <listitem>
       <para>
        The time zone offset from UTC, measured in seconds.  Positive values
        correspond to time zones east of UTC, negative values to
        zones west of UTC.  (Technically,
        <productname>PostgreSQL</productname> does not use UTC because
        leap seconds are not handled.)
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>timezone_hour</literal></term>
      <listitem>
       <para>
        The hour component of the time zone offset
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>timezone_minute</literal></term>
      <listitem>
       <para>
        The minute component of the time zone offset
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>week</literal></term>
      <listitem>
       <para>
        The number of the <acronym>ISO</acronym> 8601 week-numbering week of
        the year.  By definition, ISO weeks start on Mondays and the first
        week of a year contains January 4 of that year.  In other words, the
        first Thursday of a year is in week 1 of that year.
       </para>
       <para>
        In the ISO week-numbering system, it is possible for early-January
        dates to be part of the 52nd or 53rd week of the previous year, and for
        late-December dates to be part of the first week of the next year.
        For example, <literal>2005-01-01</literal> is part of the 53rd week of year
        2004, and <literal>2006-01-01</literal> is part of the 52nd week of year
        2005, while <literal>2012-12-31</literal> is part of the first week of 2013.
        It's recommended to use the <literal>isoyear</literal> field together with
        <literal>week</literal> to get consistent results.
       </para>

<screen>
SELECT EXTRACT(WEEK FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>7</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>year</literal></term>
      <listitem>
       <para>
        The year field.  Keep in mind there is no <literal>0 AD</literal>, so subtracting
        <literal>BC</literal> years from <literal>AD</literal> years should be done with care.
       </para>

<screen>
SELECT EXTRACT(YEAR FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>2001</computeroutput>
</screen>
      </listitem>
     </varlistentry>

    </variablelist>
   </para>

   <note>
    <para>
     When the input value is +/-Infinity, <function>extract</function> returns
     +/-Infinity for monotonically-increasing fields (<literal>epoch</literal>,
     <literal>julian</literal>, <literal>year</literal>, <literal>isoyear</literal>,
     <literal>decade</literal>, <literal>century</literal>, and <literal>millennium</literal>).
     For other fields, NULL is returned.  <productname>PostgreSQL</productname>
     versions before 9.6 returned zero for all cases of infinite input.
    </para>
   </note>

   <para>
    The <function>extract</function> function is primarily intended
    for computational processing.  For formatting date/time values for
    display, see <xref linkend="functions-formatting"/>.
   </para>

   <para>
    The <function>date_part</function> function is modeled on the traditional
    <productname>Ingres</productname> equivalent to the
    <acronym>SQL</acronym>-standard function <function>extract</function>:
<synopsis>
date_part('<replaceable>field</replaceable>', <replaceable>source</replaceable>)
</synopsis>
    Note that here the <replaceable>field</replaceable> parameter needs to
    be a string value, not a name.  The valid field names for
    <function>date_part</function> are the same as for
    <function>extract</function>.
   </para>

<screen>
SELECT date_part('day', TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>16</computeroutput>

SELECT date_part('hour', INTERVAL '4 hours 3 minutes');
<lineannotation>Result: </lineannotation><computeroutput>4</computeroutput>
</screen>

  </sect2>

  <sect2 id="functions-datetime-trunc">
   <title><function>date_trunc</function></title>

   <indexterm>
    <primary>date_trunc</primary>
   </indexterm>

   <para>
    The function <function>date_trunc</function> is conceptually
    similar to the <function>trunc</function> function for numbers.
   </para>

   <para>
<synopsis>
date_trunc(<replaceable>field</replaceable>, <replaceable>source</replaceable> [, <replaceable>time_zone</replaceable> ])
</synopsis>
    <replaceable>source</replaceable> is a value expression of type
    <type>timestamp</type>, <type>timestamp with time zone</type>,
    or <type>interval</type>.
    (Values of type <type>date</type> and
    <type>time</type> are cast automatically to <type>timestamp</type> or
    <type>interval</type>, respectively.)
    <replaceable>field</replaceable> selects to which precision to
    truncate the input value.  The return value is likewise of type
    <type>timestamp</type>, <type>timestamp with time zone</type>,
    or <type>interval</type>,
    and it has all fields that are less significant than the
    selected one set to zero (or one, for day and month).
   </para>

   <para>
    Valid values for <replaceable>field</replaceable> are:
    <simplelist>
     <member><literal>microseconds</literal></member>
     <member><literal>milliseconds</literal></member>
     <member><literal>second</literal></member>
     <member><literal>minute</literal></member>
     <member><literal>hour</literal></member>
     <member><literal>day</literal></member>
     <member><literal>week</literal></member>
     <member><literal>month</literal></member>
     <member><literal>quarter</literal></member>
     <member><literal>year</literal></member>
     <member><literal>decade</literal></member>
     <member><literal>century</literal></member>
     <member><literal>millennium</literal></member>
    </simplelist>
   </para>

   <para>
    When the input value is of type <type>timestamp with time zone</type>,
    the truncation is performed with respect to a particular time zone;
    for example, truncation to <literal>day</literal> produces a value that
    is midnight in that zone.  By default, truncation is done with respect
    to the current <xref linkend="guc-timezone"/> setting, but the
    optional <replaceable>time_zone</replaceable> argument can be provided
    to specify a different time zone.  The time zone name can be specified
    in any of the ways described in <xref linkend="datatype-timezones"/>.
   </para>

   <para>
    A time zone cannot be specified when processing <type>timestamp without
    time zone</type> or <type>interval</type> inputs.  These are always
    taken at face value.
   </para>

   <para>
    Examples (assuming the local time zone is <literal>America/New_York</literal>):
<screen>
SELECT date_trunc('hour', TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 20:00:00</computeroutput>

SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>2001-01-01 00:00:00</computeroutput>

SELECT date_trunc('day', TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40+00');
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 00:00:00-05</computeroutput>

SELECT date_trunc('day', TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40+00', 'Australia/Sydney');
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 08:00:00-05</computeroutput>

SELECT date_trunc('hour', INTERVAL '3 days 02:47:33');
<lineannotation>Result: </lineannotation><computeroutput>3 days 02:00:00</computeroutput>
</screen>
   </para>
  </sect2>

  <sect2 id="functions-datetime-zoneconvert">
   <title><literal>AT TIME ZONE</literal></title>

   <indexterm>
    <primary>time zone</primary>
    <secondary>conversion</secondary>
   </indexterm>

   <indexterm>
    <primary>AT TIME ZONE</primary>
   </indexterm>

   <para>
    The <literal>AT TIME ZONE</literal> operator converts time
    stamp <emphasis>without</emphasis> time zone to/from
    time stamp <emphasis>with</emphasis> time zone, and
    <type>time with time zone</type> values to different time
    zones. <xref linkend="functions-datetime-zoneconvert-table"/> shows its
    variants.
   </para>

    <table id="functions-datetime-zoneconvert-table">
     <title><literal>AT TIME ZONE</literal> Variants</title>
     <tgroup cols="1">
      <thead>
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         Operator
        </para>
        <para>
         Description
        </para>
        <para>
         Example(s)
        </para></entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>timestamp without time zone</type> <literal>AT TIME ZONE</literal> <replaceable>zone</replaceable>
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Converts given time stamp <emphasis>without</emphasis> time zone to
         time stamp <emphasis>with</emphasis> time zone, assuming the given
         value is in the named time zone.
        </para>
        <para>
         <literal>timestamp '2001-02-16 20:38:40' at time zone 'America/Denver'</literal>
         <returnvalue>2001-02-17 03:38:40+00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>timestamp with time zone</type> <literal>AT TIME ZONE</literal> <replaceable>zone</replaceable>
         <returnvalue>timestamp without time zone</returnvalue>
        </para>
        <para>
         Converts given time stamp <emphasis>with</emphasis> time zone to
         time stamp <emphasis>without</emphasis> time zone, as the time would
         appear in that zone.
        </para>
        <para>
         <literal>timestamp with time zone '2001-02-16 20:38:40-05' at time zone 'America/Denver'</literal>
         <returnvalue>2001-02-16 18:38:40</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>time with time zone</type> <literal>AT TIME ZONE</literal> <replaceable>zone</replaceable>
         <returnvalue>time with time zone</returnvalue>
        </para>
        <para>
         Converts given time <emphasis>with</emphasis> time zone to a new time
         zone.  Since no date is supplied, this uses the currently active UTC
         offset for the named destination zone.
        </para>
        <para>
         <literal>time with time zone '05:34:17-05' at time zone 'UTC'</literal>
         <returnvalue>10:34:17+00</returnvalue>
        </para></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    In these expressions, the desired time zone <replaceable>zone</replaceable> can be
    specified either as a text value (e.g., <literal>'America/Los_Angeles'</literal>)
    or as an interval (e.g., <literal>INTERVAL '-08:00'</literal>).
    In the text case, a time zone name can be specified in any of the ways
    described in <xref linkend="datatype-timezones"/>.
    The interval case is only useful for zones that have fixed offsets from
    UTC, so it is not very common in practice.
   </para>

   <para>
    Examples (assuming the current <xref linkend="guc-timezone"/> setting
    is <literal>America/Los_Angeles</literal>):
<screen>
SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'America/Denver';
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 19:38:40-08</computeroutput>

SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'America/Denver';
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 18:38:40</computeroutput>

SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'Asia/Tokyo' AT TIME ZONE 'America/Chicago';
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 05:38:40</computeroutput>
</screen>
    The first example adds a time zone to a value that lacks it, and
    displays the value using the current <varname>TimeZone</varname>
    setting.  The second example shifts the time stamp with time zone value
    to the specified time zone, and returns the value without a time zone.
    This allows storage and display of values different from the current
    <varname>TimeZone</varname> setting.  The third example converts
    Tokyo time to Chicago time.
   </para>

   <para>
    The function <literal><function>timezone</function>(<replaceable>zone</replaceable>,
    <replaceable>timestamp</replaceable>)</literal> is equivalent to the SQL-conforming construct
    <literal><replaceable>timestamp</replaceable> AT TIME ZONE
    <replaceable>zone</replaceable></literal>.
   </para>
  </sect2>

  <sect2 id="functions-datetime-current">
   <title>Current Date/Time</title>

   <indexterm>
    <primary>date</primary>
    <secondary>current</secondary>
   </indexterm>

   <indexterm>
    <primary>time</primary>
    <secondary>current</secondary>
   </indexterm>

   <para>
    <productname>PostgreSQL</productname> provides a number of functions
    that return values related to the current date and time.  These
    SQL-standard functions all return values based on the start time of
    the current transaction:
<synopsis>
CURRENT_DATE
CURRENT_TIME
CURRENT_TIMESTAMP
CURRENT_TIME(<replaceable>precision</replaceable>)
CURRENT_TIMESTAMP(<replaceable>precision</replaceable>)
LOCALTIME
LOCALTIMESTAMP
LOCALTIME(<replaceable>precision</replaceable>)
LOCALTIMESTAMP(<replaceable>precision</replaceable>)
</synopsis>
    </para>

    <para>
     <function>CURRENT_TIME</function> and
     <function>CURRENT_TIMESTAMP</function> deliver values with time zone;
     <function>LOCALTIME</function> and
     <function>LOCALTIMESTAMP</function> deliver values without time zone.
    </para>

    <para>
     <function>CURRENT_TIME</function>,
     <function>CURRENT_TIMESTAMP</function>,
     <function>LOCALTIME</function>, and
     <function>LOCALTIMESTAMP</function>
     can optionally take
     a precision parameter, which causes the result to be rounded
     to that many fractional digits in the seconds field.  Without a precision parameter,
     the result is given to the full available precision.
    </para>

   <para>
    Some examples:
<screen>
SELECT CURRENT_TIME;
<lineannotation>Result: </lineannotation><computeroutput>14:39:53.662522-05</computeroutput>

SELECT CURRENT_DATE;
<lineannotation>Result: </lineannotation><computeroutput>2019-12-23</computeroutput>

SELECT CURRENT_TIMESTAMP;
<lineannotation>Result: </lineannotation><computeroutput>2019-12-23 14:39:53.662522-05</computeroutput>

SELECT CURRENT_TIMESTAMP(2);
<lineannotation>Result: </lineannotation><computeroutput>2019-12-23 14:39:53.66-05</computeroutput>

SELECT LOCALTIMESTAMP;
<lineannotation>Result: </lineannotation><computeroutput>2019-12-23 14:39:53.662522</computeroutput>
</screen>
   </para>

   <para>
    Since these functions return
    the start time of the current transaction, their values do not
    change during the transaction. This is considered a feature:
    the intent is to allow a single transaction to have a consistent
    notion of the <quote>current</quote> time, so that multiple
    modifications within the same transaction bear the same
    time stamp.
   </para>

   <note>
    <para>
     Other database systems might advance these values more
     frequently.
    </para>
   </note>

   <para>
    <productname>PostgreSQL</productname> also provides functions that
    return the start time of the current statement, as well as the actual
    current time at the instant the function is called.  The complete list
    of non-SQL-standard time functions is:
<synopsis>
transaction_timestamp()
statement_timestamp()
clock_timestamp()
timeofday()
now()
</synopsis>
   </para>

   <para>
    <function>transaction_timestamp()</function> is equivalent to
    <function>CURRENT_TIMESTAMP</function>, but is named to clearly reflect
    what it returns.
    <function>statement_timestamp()</function> returns the start time of the current
    statement (more specifically, the time of receipt of the latest command
    message from the client).
    <function>statement_timestamp()</function> and <function>transaction_timestamp()</function>
    return the same value during the first command of a transaction, but might
    differ during subsequent commands.
    <function>clock_timestamp()</function> returns the actual current time, and
    therefore its value changes even within a single SQL command.
    <function>timeofday()</function> is a historical
    <productname>PostgreSQL</productname> function.  Like
    <function>clock_timestamp()</function>, it returns the actual current time,
    but as a formatted <type>text</type> string rather than a <type>timestamp
    with time zone</type> value.
    <function>now()</function> is a traditional <productname>PostgreSQL</productname>
    equivalent to <function>transaction_timestamp()</function>.
   </para>

   <para>
    All the date/time data types also accept the special literal value
    <literal>now</literal> to specify the current date and time (again,
    interpreted as the transaction start time).  Thus,
    the following three all return the same result:
<programlisting>
SELECT CURRENT_TIMESTAMP;
SELECT now();
SELECT TIMESTAMP 'now';  -- but see tip below
</programlisting>
   </para>

    <tip>
     <para>
      Do not use the third form when specifying a value to be evaluated later,
      for example in a <literal>DEFAULT</literal> clause for a table column.
      The system will convert <literal>now</literal>
      to a <type>timestamp</type> as soon as the constant is parsed, so that when
      the default value is needed,
      the time of the table creation would be used!  The first two
      forms will not be evaluated until the default value is used,
      because they are function calls.  Thus they will give the desired
      behavior of defaulting to the time of row insertion.
      (See also <xref linkend="datatype-datetime-special-values"/>.)
     </para>
    </tip>
  </sect2>

  <sect2 id="functions-datetime-delay">
   <title>Delaying Execution</title>

   <indexterm>
    <primary>pg_sleep</primary>
   </indexterm>
   <indexterm>
    <primary>pg_sleep_for</primary>
   </indexterm>
   <indexterm>
    <primary>pg_sleep_until</primary>
   </indexterm>
   <indexterm>
    <primary>sleep</primary>
   </indexterm>
   <indexterm>
    <primary>delay</primary>
   </indexterm>

   <para>
    The following functions are available to delay execution of the server
    process:
<synopsis>
pg_sleep ( <type>double precision</type> )
pg_sleep_for ( <type>interval</type> )
pg_sleep_until ( <type>timestamp with time zone</type> )
</synopsis>

    <function>pg_sleep</function> makes the current session's process
    sleep until the given number of seconds have
    elapsed.  Fractional-second delays can be specified.
    <function>pg_sleep_for</function> is a convenience function to
    allow the sleep time to be specified as an <type>interval</type>.
    <function>pg_sleep_until</function> is a convenience function for when
    a specific wake-up time is desired.
    For example:

<programlisting>
SELECT pg_sleep(1.5);
SELECT pg_sleep_for('5 minutes');
SELECT pg_sleep_until('tomorrow 03:00');
</programlisting>
   </para>

   <note>
     <para>
      The effective resolution of the sleep interval is platform-specific;
      0.01 seconds is a common value.  The sleep delay will be at least as long
      as specified. It might be longer depending on factors such as server load.
      In particular, <function>pg_sleep_until</function> is not guaranteed to
      wake up exactly at the specified time, but it will not wake up any earlier.
     </para>
   </note>

   <warning>
     <para>
      Make sure that your session does not hold more locks than necessary
      when calling <function>pg_sleep</function> or its variants.  Otherwise
      other sessions might have to wait for your sleeping process, slowing down
      the entire system.
     </para>
   </warning>
  </sect2>

 </sect1>


 <sect1 id="functions-enum">
  <title>Enum Support Functions</title>

  <para>
   For enum types (described in <xref linkend="datatype-enum"/>),
   there are several functions that allow cleaner programming without
   hard-coding particular values of an enum type.
   These are listed in <xref linkend="functions-enum-table"/>. The examples
   assume an enum type created as:

<programlisting>
CREATE TYPE rainbow AS ENUM ('red', 'orange', 'yellow', 'green', 'blue', 'purple');
</programlisting>

  </para>

  <table id="functions-enum-table">
    <title>Enum Support Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>enum_first</primary>
        </indexterm>
        <function>enum_first</function> ( <type>anyenum</type> )
        <returnvalue>anyenum</returnvalue>
       </para>
       <para>
        Returns the first value of the input enum type.
       </para>
       <para>
        <literal>enum_first(null::rainbow)</literal>
        <returnvalue>red</returnvalue>
       </para></entry>
      </row>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>enum_last</primary>
        </indexterm>
        <function>enum_last</function> ( <type>anyenum</type> )
        <returnvalue>anyenum</returnvalue>
       </para>
       <para>
        Returns the last value of the input enum type.
       </para>
       <para>
        <literal>enum_last(null::rainbow)</literal>
        <returnvalue>purple</returnvalue>
       </para></entry>
      </row>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>enum_range</primary>
        </indexterm>
        <function>enum_range</function> ( <type>anyenum</type> )
        <returnvalue>anyarray</returnvalue>
       </para>
       <para>
        Returns all values of the input enum type in an ordered array.
       </para>
       <para>
        <literal>enum_range(null::rainbow)</literal>
        <returnvalue>{red,orange,yellow,&zwsp;green,blue,purple}</returnvalue>
       </para></entry>
      </row>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>enum_range</function> ( <type>anyenum</type>, <type>anyenum</type> )
        <returnvalue>anyarray</returnvalue>
       </para>
       <para>
        Returns the range between the two given enum values, as an ordered
        array. The values must be from the same enum type. If the first
        parameter is null, the result will start with the first value of
        the enum type.
        If the second parameter is null, the result will end with the last
        value of the enum type.
       </para>
       <para>
        <literal>enum_range('orange'::rainbow, 'green'::rainbow)</literal>
        <returnvalue>{orange,yellow,green}</returnvalue>
       </para>
       <para>
        <literal>enum_range(NULL, 'green'::rainbow)</literal>
        <returnvalue>{red,orange,&zwsp;yellow,green}</returnvalue>
       </para>
       <para>
        <literal>enum_range('orange'::rainbow, NULL)</literal>
        <returnvalue>{orange,yellow,green,&zwsp;blue,purple}</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    Notice that except for the two-argument form of <function>enum_range</function>,
    these functions disregard the specific value passed to them; they care
    only about its declared data type.  Either null or a specific value of
    the type can be passed, with the same result.  It is more common to
    apply these functions to a table column or function argument than to
    a hardwired type name as used in the examples.
   </para>
 </sect1>

 <sect1 id="functions-geometry">
  <title>Geometric Functions and Operators</title>

   <para>
    The geometric types <type>point</type>, <type>box</type>,
    <type>lseg</type>, <type>line</type>, <type>path</type>,
    <type>polygon</type>, and <type>circle</type> have a large set of
    native support functions and operators, shown in <xref
    linkend="functions-geometry-op-table"/>, <xref
    linkend="functions-geometry-func-table"/>, and <xref
    linkend="functions-geometry-conv-table"/>.
   </para>

   <table id="functions-geometry-op-table">
    <title>Geometric Operators</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Operator
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>+</literal> <type>point</type>
        <returnvalue><replaceable>geometric_type</replaceable></returnvalue>
       </para>
       <para>
        Adds the coordinates of the second <type>point</type> to those of each
        point of the first argument, thus performing translation.
        Available for <type>point</type>, <type>box</type>, <type>path</type>,
        <type>circle</type>.
       </para>
       <para>
        <literal>box '(1,1),(0,0)' + point '(2,0)'</literal>
        <returnvalue>(3,1),(2,0)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>path</type> <literal>+</literal> <type>path</type>
        <returnvalue>path</returnvalue>
       </para>
       <para>
        Concatenates two open paths (returns NULL if either path is closed).
       </para>
       <para>
        <literal>path '[(0,0),(1,1)]' + path '[(2,2),(3,3),(4,4)]'</literal>
        <returnvalue>[(0,0),(1,1),(2,2),(3,3),(4,4)]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>-</literal> <type>point</type>
        <returnvalue><replaceable>geometric_type</replaceable></returnvalue>
       </para>
       <para>
        Subtracts the coordinates of the second <type>point</type> from those
        of each point of the first argument, thus performing translation.
        Available for <type>point</type>, <type>box</type>, <type>path</type>,
        <type>circle</type>.
       </para>
       <para>
        <literal>box '(1,1),(0,0)' - point '(2,0)'</literal>
        <returnvalue>(-1,1),(-2,0)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>*</literal> <type>point</type>
        <returnvalue><replaceable>geometric_type</replaceable></returnvalue>
       </para>
       <para>
        Multiplies each point of the first argument by the second
        <type>point</type> (treating a point as being a complex number
        represented by real and imaginary parts, and performing standard
        complex multiplication).  If one interprets
        the second <type>point</type> as a vector, this is equivalent to
        scaling the object's size and distance from the origin by the length
        of the vector, and rotating it counterclockwise around the origin by
        the vector's angle from the <replaceable>x</replaceable> axis.
        Available for <type>point</type>, <type>box</type>,<footnote
        id="functions-geometry-rotation-fn"><para><quote>Rotating</quote> a
        box with these operators only moves its corner points: the box is
        still considered to have sides parallel to the axes.  Hence the box's
        size is not preserved, as a true rotation would do.</para></footnote>
        <type>path</type>, <type>circle</type>.
       </para>
       <para>
        <literal>path '((0,0),(1,0),(1,1))' * point '(3.0,0)'</literal>
        <returnvalue>((0,0),(3,0),(3,3))</returnvalue>
       </para>
       <para>
        <literal>path '((0,0),(1,0),(1,1))' * point(cosd(45), sind(45))</literal>
        <returnvalue>((0,0),&zwsp;(0.7071067811865475,0.7071067811865475),&zwsp;(0,1.414213562373095))</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>/</literal> <type>point</type>
        <returnvalue><replaceable>geometric_type</replaceable></returnvalue>
       </para>
       <para>
        Divides each point of the first argument by the second
        <type>point</type> (treating a point as being a complex number
        represented by real and imaginary parts, and performing standard
        complex division).  If one interprets
        the second <type>point</type> as a vector, this is equivalent to
        scaling the object's size and distance from the origin down by the
        length of the vector, and rotating it clockwise around the origin by
        the vector's angle from the <replaceable>x</replaceable> axis.
        Available for <type>point</type>, <type>box</type>,<footnoteref
        linkend="functions-geometry-rotation-fn"/> <type>path</type>,
        <type>circle</type>.
       </para>
       <para>
        <literal>path '((0,0),(1,0),(1,1))' / point '(2.0,0)'</literal>
        <returnvalue>((0,0),(0.5,0),(0.5,0.5))</returnvalue>
       </para>
       <para>
        <literal>path '((0,0),(1,0),(1,1))' / point(cosd(45), sind(45))</literal>
        <returnvalue>((0,0),&zwsp;(0.7071067811865476,-0.7071067811865476),&zwsp;(1.4142135623730951,0))</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>@-@</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes the total length.
        Available for <type>lseg</type>, <type>path</type>.
       </para>
       <para>
        <literal>@-@ path '[(0,0),(1,0),(1,1)]'</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>@@</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>point</returnvalue>
       </para>
       <para>
        Computes the center point.
        Available for <type>box</type>, <type>lseg</type>, <type>path</type>,
        <type>polygon</type>, <type>circle</type>.
       </para>
       <para>
        <literal>@@ box '(2,2),(0,0)'</literal>
        <returnvalue>(1,1)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>#</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the number of points.
        Available for <type>path</type>, <type>polygon</type>.
       </para>
       <para>
        <literal># path '((1,0),(0,1),(-1,0))'</literal>
        <returnvalue>3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>#</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>point</returnvalue>
       </para>
       <para>
        Computes the point of intersection, or NULL if there is none.
        Available for <type>lseg</type>, <type>line</type>.
       </para>
       <para>
        <literal>lseg '[(0,0),(1,1)]' # lseg '[(1,0),(0,1)]'</literal>
        <returnvalue>(0.5,0.5)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>box</type> <literal>#</literal> <type>box</type>
        <returnvalue>box</returnvalue>
       </para>
       <para>
        Computes the intersection of two boxes, or NULL if there is none.
       </para>
       <para>
        <literal>box '(2,2),(-1,-1)' # box '(1,1),(-2,-2)'</literal>
        <returnvalue>(1,1),(-1,-1)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>##</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>point</returnvalue>
       </para>
       <para>
        Computes the closest point to the first object on the second object.
        Available for these pairs of types:
        (<type>point</type>, <type>box</type>),
        (<type>point</type>, <type>lseg</type>),
        (<type>point</type>, <type>line</type>),
        (<type>lseg</type>, <type>box</type>),
        (<type>lseg</type>, <type>lseg</type>),
        (<type>lseg</type>, <type>line</type>),
        (<type>line</type>, <type>box</type>),
        (<type>line</type>, <type>lseg</type>).
       </para>
       <para>
        <literal>point '(0,0)' ## lseg '[(2,0),(0,2)]'</literal>
        <returnvalue>(1,1)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>&lt;-&gt;</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes the distance between the objects.
        Available for all seven geometric types, for all combinations
        of <type>point</type> with another geometric type, and for
        these additional pairs of types:
        (<type>box</type>, <type>lseg</type>),
        (<type>box</type>, <type>line</type>),
        (<type>lseg</type>, <type>line</type>),
        (<type>polygon</type>, <type>circle</type>)
        (and the commutator cases).
       </para>
       <para>
        <literal>circle '&lt;(0,0),1&gt;' &lt;-&gt; circle '&lt;(5,0),1&gt;'</literal>
        <returnvalue>3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>@&gt;</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does first object contain second?
        Available for these pairs of types:
        (<literal>box</literal>, <literal>point</literal>),
        (<literal>box</literal>, <literal>box</literal>),
        (<literal>path</literal>, <literal>point</literal>),
        (<literal>polygon</literal>, <literal>point</literal>),
        (<literal>polygon</literal>, <literal>polygon</literal>),
        (<literal>circle</literal>, <literal>point</literal>),
        (<literal>circle</literal>, <literal>circle</literal>).
       </para>
       <para>
        <literal>circle '&lt;(0,0),2&gt;' @&gt; point '(1,1)'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>&lt;@</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is first object contained in or on second?
        Available for these pairs of types:
        (<literal>point</literal>, <literal>box</literal>),
        (<literal>point</literal>, <literal>lseg</literal>),
        (<literal>point</literal>, <literal>line</literal>),
        (<literal>point</literal>, <literal>path</literal>),
        (<literal>point</literal>, <literal>polygon</literal>),
        (<literal>point</literal>, <literal>circle</literal>),
        (<literal>box</literal>, <literal>box</literal>),
        (<literal>lseg</literal>, <literal>box</literal>),
        (<literal>lseg</literal>, <literal>line</literal>),
        (<literal>polygon</literal>, <literal>polygon</literal>),
        (<literal>circle</literal>, <literal>circle</literal>).
       </para>
       <para>
        <literal>point '(1,1)' &lt;@ circle '&lt;(0,0),2&gt;'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>&amp;&amp;</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Do these objects overlap?  (One point in common makes this true.)
        Available for <type>box</type>, <type>polygon</type>,
        <type>circle</type>.
       </para>
       <para>
        <literal>box '(1,1),(0,0)' &amp;&amp; box '(2,2),(0,0)'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>&lt;&lt;</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is first object strictly left of second?
        Available for <type>point</type>, <type>box</type>,
        <type>polygon</type>, <type>circle</type>.
       </para>
       <para>
        <literal>circle '&lt;(0,0),1&gt;' &lt;&lt; circle '&lt;(5,0),1&gt;'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>&gt;&gt;</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is first object strictly right of second?
        Available for <type>point</type>, <type>box</type>,
        <type>polygon</type>, <type>circle</type>.
       </para>
       <para>
        <literal>circle '&lt;(5,0),1&gt;' &gt;&gt; circle '&lt;(0,0),1&gt;'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>&amp;&lt;</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does first object not extend to the right of second?
        Available for <type>box</type>, <type>polygon</type>,
        <type>circle</type>.
       </para>
       <para>
        <literal>box '(1,1),(0,0)' &amp;&lt; box '(2,2),(0,0)'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>&amp;&gt;</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does first object not extend to the left of second?
        Available for <type>box</type>, <type>polygon</type>,
        <type>circle</type>.
       </para>
       <para>
        <literal>box '(3,3),(0,0)' &amp;&gt; box '(2,2),(0,0)'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>&lt;&lt;|</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is first object strictly below second?
        Available for <type>box</type>, <type>polygon</type>,
        <type>circle</type>.
       </para>
       <para>
        <literal>box '(3,3),(0,0)' &lt;&lt;| box '(5,5),(3,4)'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>|&gt;&gt;</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is first object strictly above second?
        Available for <type>box</type>, <type>polygon</type>,
        <type>circle</type>.
       </para>
       <para>
        <literal>box '(5,5),(3,4)' |&gt;&gt; box '(3,3),(0,0)'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>&amp;&lt;|</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does first object not extend above second?
        Available for <type>box</type>, <type>polygon</type>,
        <type>circle</type>.
       </para>
       <para>
        <literal>box '(1,1),(0,0)' &amp;&lt;| box '(2,2),(0,0)'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>|&amp;&gt;</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does first object not extend below second?
        Available for <type>box</type>, <type>polygon</type>,
        <type>circle</type>.
       </para>
       <para>
        <literal>box '(3,3),(0,0)' |&amp;&gt; box '(2,2),(0,0)'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>box</type> <literal>&lt;^</literal> <type>box</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is first object below second (allows edges to touch)?
       </para>
       <para>
        <literal>box '((1,1),(0,0))' &lt;^ box '((2,2),(1,1))'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>point</type> <literal>&lt;^</literal> <type>point</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is first object strictly below second?
        (This operator is misnamed; it should be <literal>&lt;&lt;|</literal>.)
       </para>
       <para>
        <literal>point '(1,0)' &lt;^ point '(1,1)'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>box</type> <literal>&gt;^</literal> <type>box</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is first object above second (allows edges to touch)?
       </para>
       <para>
        <literal>box '((2,2),(1,1))' &gt;^ box '((1,1),(0,0))'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>point</type> <literal>&gt;^</literal> <type>point</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is first object strictly above second?
        (This operator is misnamed; it should be <literal>|&gt;&gt;</literal>.)
       </para>
       <para>
        <literal>point '(1,1)' &gt;^ point '(1,0)'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>?#</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Do these objects intersect?
        Available for these pairs of types:
        (<type>box</type>, <type>box</type>),
        (<type>lseg</type>, <type>box</type>),
        (<type>lseg</type>, <type>lseg</type>),
        (<type>lseg</type>, <type>line</type>),
        (<type>line</type>, <type>box</type>),
        (<type>line</type>, <type>line</type>),
        (<type>path</type>, <type>path</type>).
       </para>
       <para>
        <literal>lseg '[(-1,0),(1,0)]' ?# box '(2,2),(-2,-2)'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>?-</literal> <type>line</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para role="func_signature">
        <literal>?-</literal> <type>lseg</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is line horizontal?
       </para>
       <para>
        <literal>?- lseg '[(-1,0),(1,0)]'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>point</type> <literal>?-</literal> <type>point</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Are points horizontally aligned (that is, have same y coordinate)?
       </para>
       <para>
        <literal>point '(1,0)' ?- point '(0,0)'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>?|</literal> <type>line</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para role="func_signature">
        <literal>?|</literal> <type>lseg</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is line vertical?
       </para>
       <para>
        <literal>?| lseg '[(-1,0),(1,0)]'</literal>
        <returnvalue>f</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>point</type> <literal>?|</literal> <type>point</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Are points vertically aligned (that is, have same x coordinate)?
       </para>
       <para>
        <literal>point '(0,1)' ?| point '(0,0)'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>line</type> <literal>?-|</literal> <type>line</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para role="func_signature">
        <type>lseg</type> <literal>?-|</literal> <type>lseg</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Are lines perpendicular?
       </para>
       <para>
        <literal>lseg '[(0,0),(0,1)]' ?-| lseg '[(0,0),(1,0)]'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>line</type> <literal>?||</literal> <type>line</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para role="func_signature">
        <type>lseg</type> <literal>?||</literal> <type>lseg</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Are lines parallel?
       </para>
       <para>
        <literal>lseg '[(-1,0),(1,0)]' ?|| lseg '[(-1,2),(1,2)]'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>geometric_type</replaceable> <literal>~=</literal> <replaceable>geometric_type</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Are these objects the same?
        Available for <type>point</type>, <type>box</type>,
        <type>polygon</type>, <type>circle</type>.
       </para>
       <para>
        <literal>polygon '((0,0),(1,1))' ~= polygon '((1,1),(0,0))'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <caution>
    <para>
     Note that the <quote>same as</quote> operator, <literal>~=</literal>,
     represents the usual notion of equality for the <type>point</type>,
     <type>box</type>, <type>polygon</type>, and <type>circle</type> types.
     Some of the geometric types also have an <literal>=</literal> operator, but
     <literal>=</literal> compares for equal <emphasis>areas</emphasis> only.
     The other scalar comparison operators (<literal>&lt;=</literal> and so
     on), where available for these types, likewise compare areas.
    </para>
   </caution>

   <note>
    <para>
     Before <productname>PostgreSQL</productname> 8.2, the containment
     operators <literal>@&gt;</literal> and <literal>&lt;@</literal> were respectively
     called <literal>~</literal> and <literal>@</literal>.  These names are still
     available, but are deprecated and will eventually be removed.
    </para>
   </note>

   <table id="functions-geometry-func-table">
    <title>Geometric Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>area</primary>
        </indexterm>
        <function>area</function> ( <replaceable>geometric_type</replaceable> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes area.
        Available for <type>box</type>, <type>path</type>, <type>circle</type>.
        A <type>path</type> input must be closed, else NULL is returned.
        Also, if the <type>path</type> is self-intersecting, the result may be
        meaningless.
       </para>
       <para>
        <literal>area(box '(2,2),(0,0)')</literal>
        <returnvalue>4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>center</primary>
        </indexterm>
        <function>center</function> ( <replaceable>geometric_type</replaceable> )
        <returnvalue>point</returnvalue>
       </para>
       <para>
        Computes center point.
        Available for <type>box</type>, <type>circle</type>.
       </para>
       <para>
        <literal>center(box '(1,2),(0,0)')</literal>
        <returnvalue>(0.5,1)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>diagonal</primary>
        </indexterm>
        <function>diagonal</function> ( <type>box</type> )
        <returnvalue>lseg</returnvalue>
       </para>
       <para>
        Extracts box's diagonal as a line segment
        (same as <function>lseg(box)</function>).
       </para>
       <para>
        <literal>diagonal(box '(1,2),(0,0)')</literal>
        <returnvalue>[(1,2),(0,0)]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>diameter</primary>
        </indexterm>
        <function>diameter</function> ( <type>circle</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes diameter of circle.
       </para>
       <para>
        <literal>diameter(circle '&lt;(0,0),2&gt;')</literal>
        <returnvalue>4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>height</primary>
        </indexterm>
        <function>height</function> ( <type>box</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes vertical size of box.
       </para>
       <para>
        <literal>height(box '(1,2),(0,0)')</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>isclosed</primary>
        </indexterm>
        <function>isclosed</function> ( <type>path</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is path closed?
       </para>
       <para>
        <literal>isclosed(path '((0,0),(1,1),(2,0))')</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>isopen</primary>
        </indexterm>
        <function>isopen</function> ( <type>path</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is path open?
       </para>
       <para>
        <literal>isopen(path '[(0,0),(1,1),(2,0)]')</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>length</primary>
        </indexterm>
        <function>length</function> ( <replaceable>geometric_type</replaceable> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes the total length.
        Available for <type>lseg</type>, <type>path</type>.
       </para>
       <para>
        <literal>length(path '((-1,0),(1,0))')</literal>
        <returnvalue>4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>npoints</primary>
        </indexterm>
        <function>npoints</function> ( <replaceable>geometric_type</replaceable> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the number of points.
        Available for <type>path</type>, <type>polygon</type>.
       </para>
       <para>
        <literal>npoints(path '[(0,0),(1,1),(2,0)]')</literal>
        <returnvalue>3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pclose</primary>
        </indexterm>
        <function>pclose</function> ( <type>path</type> )
        <returnvalue>path</returnvalue>
       </para>
       <para>
        Converts path to closed form.
       </para>
       <para>
        <literal>pclose(path '[(0,0),(1,1),(2,0)]')</literal>
        <returnvalue>((0,0),(1,1),(2,0))</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>popen</primary>
        </indexterm>
        <function>popen</function> ( <type>path</type> )
        <returnvalue>path</returnvalue>
       </para>
       <para>
        Converts path to open form.
       </para>
       <para>
        <literal>popen(path '((0,0),(1,1),(2,0))')</literal>
        <returnvalue>[(0,0),(1,1),(2,0)]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>radius</primary>
        </indexterm>
        <function>radius</function> ( <type>circle</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes radius of circle.
       </para>
       <para>
        <literal>radius(circle '&lt;(0,0),2&gt;')</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>slope</primary>
        </indexterm>
        <function>slope</function> ( <type>point</type>, <type>point</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes slope of a line drawn through the two points.
       </para>
       <para>
        <literal>slope(point '(0,0)', point '(2,1)')</literal>
        <returnvalue>0.5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>width</primary>
        </indexterm>
        <function>width</function> ( <type>box</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes horizontal size of box.
       </para>
       <para>
        <literal>width(box '(1,2),(0,0)')</literal>
        <returnvalue>1</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <table id="functions-geometry-conv-table">
    <title>Geometric Type Conversion Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>
     <tbody>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>box</primary>
        </indexterm>
        <function>box</function> ( <type>circle</type> )
        <returnvalue>box</returnvalue>
       </para>
       <para>
        Computes box inscribed within the circle.
       </para>
       <para>
        <literal>box(circle '&lt;(0,0),2&gt;')</literal>
        <returnvalue>(1.414213562373095,1.414213562373095),&zwsp;(-1.414213562373095,-1.414213562373095)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>box</function> ( <type>point</type> )
        <returnvalue>box</returnvalue>
       </para>
       <para>
        Converts point to empty box.
       </para>
       <para>
        <literal>box(point '(1,0)')</literal>
        <returnvalue>(1,0),(1,0)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>box</function> ( <type>point</type>, <type>point</type> )
        <returnvalue>box</returnvalue>
       </para>
       <para>
        Converts any two corner points to box.
       </para>
       <para>
        <literal>box(point '(0,1)', point '(1,0)')</literal>
        <returnvalue>(1,1),(0,0)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>box</function> ( <type>polygon</type> )
        <returnvalue>box</returnvalue>
       </para>
       <para>
        Computes bounding box of polygon.
       </para>
       <para>
        <literal>box(polygon '((0,0),(1,1),(2,0))')</literal>
        <returnvalue>(2,1),(0,0)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>bound_box</primary>
        </indexterm>
        <function>bound_box</function> ( <type>box</type>, <type>box</type> )
        <returnvalue>box</returnvalue>
       </para>
       <para>
        Computes bounding box of two boxes.
       </para>
       <para>
        <literal>bound_box(box '(1,1),(0,0)', box '(4,4),(3,3)')</literal>
        <returnvalue>(4,4),(0,0)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>circle</primary>
        </indexterm>
        <function>circle</function> ( <type>box</type> )
        <returnvalue>circle</returnvalue>
       </para>
       <para>
        Computes smallest circle enclosing box.
       </para>
       <para>
        <literal>circle(box '(1,1),(0,0)')</literal>
        <returnvalue>&lt;(0.5,0.5),0.7071067811865476&gt;</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>circle</function> ( <type>point</type>, <type>double precision</type> )
        <returnvalue>circle</returnvalue>
       </para>
       <para>
        Constructs circle from center and radius.
       </para>
       <para>
        <literal>circle(point '(0,0)', 2.0)</literal>
        <returnvalue>&lt;(0,0),2&gt;</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>circle</function> ( <type>polygon</type> )
        <returnvalue>circle</returnvalue>
       </para>
       <para>
        Converts polygon to circle.  The circle's center is the mean of the
        positions of the polygon's points, and the radius is the average
        distance of the polygon's points from that center.
       </para>
       <para>
        <literal>circle(polygon '((0,0),(1,3),(2,0))')</literal>
        <returnvalue>&lt;(1,1),1.6094757082487299&gt;</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>line</primary>
        </indexterm>
        <function>line</function> ( <type>point</type>, <type>point</type> )
        <returnvalue>line</returnvalue>
       </para>
       <para>
        Converts two points to the line through them.
       </para>
       <para>
        <literal>line(point '(-1,0)', point '(1,0)')</literal>
        <returnvalue>{0,-1,0}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>lseg</primary>
        </indexterm>
        <function>lseg</function> ( <type>box</type> )
        <returnvalue>lseg</returnvalue>
       </para>
       <para>
        Extracts box's diagonal as a line segment.
       </para>
       <para>
        <literal>lseg(box '(1,0),(-1,0)')</literal>
        <returnvalue>[(1,0),(-1,0)]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>lseg</function> ( <type>point</type>, <type>point</type> )
        <returnvalue>lseg</returnvalue>
       </para>
       <para>
        Constructs line segment from two endpoints.
       </para>
       <para>
        <literal>lseg(point '(-1,0)', point '(1,0)')</literal>
        <returnvalue>[(-1,0),(1,0)]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>path</primary>
        </indexterm>
        <function>path</function> ( <type>polygon</type> )
        <returnvalue>path</returnvalue>
       </para>
       <para>
        Converts polygon to a closed path with the same list of points.
       </para>
       <para>
        <literal>path(polygon '((0,0),(1,1),(2,0))')</literal>
        <returnvalue>((0,0),(1,1),(2,0))</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>point</primary>
        </indexterm>
        <function>point</function> ( <type>double precision</type>, <type>double precision</type> )
        <returnvalue>point</returnvalue>
       </para>
       <para>
        Constructs point from its coordinates.
       </para>
       <para>
        <literal>point(23.4, -44.5)</literal>
        <returnvalue>(23.4,-44.5)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>point</function> ( <type>box</type> )
        <returnvalue>point</returnvalue>
       </para>
       <para>
        Computes center of box.
       </para>
       <para>
        <literal>point(box '(1,0),(-1,0)')</literal>
        <returnvalue>(0,0)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>point</function> ( <type>circle</type> )
        <returnvalue>point</returnvalue>
       </para>
       <para>
        Computes center of circle.
       </para>
       <para>
        <literal>point(circle '&lt;(0,0),2&gt;')</literal>
        <returnvalue>(0,0)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>point</function> ( <type>lseg</type> )
        <returnvalue>point</returnvalue>
       </para>
       <para>
        Computes center of line segment.
       </para>
       <para>
        <literal>point(lseg '[(-1,0),(1,0)]')</literal>
        <returnvalue>(0,0)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>point</function> ( <type>polygon</type> )
        <returnvalue>point</returnvalue>
       </para>
       <para>
        Computes center of polygon (the mean of the
        positions of the polygon's points).
       </para>
       <para>
        <literal>point(polygon '((0,0),(1,1),(2,0))')</literal>
        <returnvalue>(1,0.3333333333333333)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>polygon</primary>
        </indexterm>
        <function>polygon</function> ( <type>box</type> )
        <returnvalue>polygon</returnvalue>
       </para>
       <para>
        Converts box to a 4-point polygon.
       </para>
       <para>
        <literal>polygon(box '(1,1),(0,0)')</literal>
        <returnvalue>((0,0),(0,1),(1,1),(1,0))</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>polygon</function> ( <type>circle</type> )
        <returnvalue>polygon</returnvalue>
       </para>
       <para>
        Converts circle to a 12-point polygon.
       </para>
       <para>
        <literal>polygon(circle '&lt;(0,0),2&gt;')</literal>
        <returnvalue>((-2,0),&zwsp;(-1.7320508075688774,0.9999999999999999),&zwsp;(-1.0000000000000002,1.7320508075688772),&zwsp;(-1.2246063538223773e-16,2),&zwsp;(0.9999999999999996,1.7320508075688774),&zwsp;(1.732050807568877,1.0000000000000007),&zwsp;(2,2.4492127076447545e-16),&zwsp;(1.7320508075688776,-0.9999999999999994),&zwsp;(1.0000000000000009,-1.7320508075688767),&zwsp;(3.673819061467132e-16,-2),&zwsp;(-0.9999999999999987,-1.732050807568878),&zwsp;(-1.7320508075688767,-1.0000000000000009))</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>polygon</function> ( <type>integer</type>, <type>circle</type> )
        <returnvalue>polygon</returnvalue>
       </para>
       <para>
        Converts circle to an <replaceable>n</replaceable>-point polygon.
       </para>
       <para>
        <literal>polygon(4, circle '&lt;(3,0),1&gt;')</literal>
        <returnvalue>((2,0),&zwsp;(3,1),&zwsp;(4,1.2246063538223773e-16),&zwsp;(3,-1))</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>polygon</function> ( <type>path</type> )
        <returnvalue>polygon</returnvalue>
       </para>
       <para>
        Converts closed path to a polygon with the same list of points.
       </para>
       <para>
        <literal>polygon(path '((0,0),(1,1),(2,0))')</literal>
        <returnvalue>((0,0),(1,1),(2,0))</returnvalue>
       </para></entry>
      </row>

     </tbody>
    </tgroup>
   </table>

    <para>
     It is possible to access the two component numbers of a <type>point</type>
     as though the point were an array with indexes 0 and 1.  For example, if
     <literal>t.p</literal> is a <type>point</type> column then
     <literal>SELECT p[0] FROM t</literal> retrieves the X coordinate and
     <literal>UPDATE t SET p[1] = ...</literal> changes the Y coordinate.
     In the same way, a value of type <type>box</type> or <type>lseg</type> can be treated
     as an array of two <type>point</type> values.
    </para>

  </sect1>


 <sect1 id="functions-net">
  <title>Network Address Functions and Operators</title>

  <para>
   The IP network address types, <type>cidr</type> and <type>inet</type>,
   support the usual comparison operators shown in
   <xref linkend="functions-comparison-op-table"/>
   as well as the specialized operators and functions shown in
   <xref linkend="cidr-inet-operators-table"/> and
   <xref linkend="cidr-inet-functions-table"/>.
  </para>

  <para>
   Any <type>cidr</type> value can be cast to <type>inet</type> implicitly;
   therefore, the operators and functions shown below as operating on
   <type>inet</type> also work on <type>cidr</type> values.  (Where there are
   separate functions for <type>inet</type> and <type>cidr</type>, it is
   because the behavior should be different for the two cases.)
   Also, it is permitted to cast an <type>inet</type> value
   to <type>cidr</type>.  When this is done, any bits to the right of the
   netmask are silently zeroed to create a valid <type>cidr</type> value.
  </para>

   <table id="cidr-inet-operators-table">
    <title>IP Address Operators</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Operator
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>inet</type> <literal>&lt;&lt;</literal> <type>inet</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is subnet strictly contained by subnet?
        This operator, and the next four, test for subnet inclusion.  They
        consider only the network parts of the two addresses (ignoring any
        bits to the right of the netmasks) and determine whether one network
        is identical to or a subnet of the other.
       </para>
       <para>
        <literal>inet '192.168.1.5' &lt;&lt; inet '192.168.1/24'</literal>
        <returnvalue>t</returnvalue>
       </para>
       <para>
        <literal>inet '192.168.0.5' &lt;&lt; inet '192.168.1/24'</literal>
        <returnvalue>f</returnvalue>
       </para>
       <para>
        <literal>inet '192.168.1/24' &lt;&lt; inet '192.168.1/24'</literal>
        <returnvalue>f</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>inet</type> <literal>&lt;&lt;=</literal> <type>inet</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is subnet contained by or equal to subnet?
       </para>
       <para>
        <literal>inet '192.168.1/24' &lt;&lt;= inet '192.168.1/24'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>inet</type> <literal>&gt;&gt;</literal> <type>inet</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does subnet strictly contain subnet?
       </para>
       <para>
        <literal>inet '192.168.1/24' &gt;&gt; inet '192.168.1.5'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>inet</type> <literal>&gt;&gt;=</literal> <type>inet</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does subnet contain or equal subnet?
       </para>
       <para>
        <literal>inet '192.168.1/24' &gt;&gt;= inet '192.168.1/24'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>inet</type> <literal>&amp;&amp;</literal> <type>inet</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does either subnet contain or equal the other?
       </para>
       <para>
        <literal>inet '192.168.1/24' &amp;&amp; inet '192.168.1.80/28'</literal>
        <returnvalue>t</returnvalue>
       </para>
       <para>
        <literal>inet '192.168.1/24' &amp;&amp; inet '192.168.2.0/28'</literal>
        <returnvalue>f</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>~</literal> <type>inet</type>
        <returnvalue>inet</returnvalue>
       </para>
       <para>
        Computes bitwise NOT.
       </para>
       <para>
        <literal>~ inet '192.168.1.6'</literal>
        <returnvalue>63.87.254.249</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>inet</type> <literal>&amp;</literal> <type>inet</type>
        <returnvalue>inet</returnvalue>
       </para>
       <para>
        Computes bitwise AND.
       </para>
       <para>
        <literal>inet '192.168.1.6' &amp; inet '0.0.0.255'</literal>
        <returnvalue>0.0.0.6</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>inet</type> <literal>|</literal> <type>inet</type>
        <returnvalue>inet</returnvalue>
       </para>
       <para>
        Computes bitwise OR.
       </para>
       <para>
        <literal>inet '192.168.1.6' | inet '0.0.0.255'</literal>
        <returnvalue>192.168.1.255</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>inet</type> <literal>+</literal> <type>bigint</type>
        <returnvalue>inet</returnvalue>
       </para>
       <para>
        Adds an offset to an address.
       </para>
       <para>
        <literal>inet '192.168.1.6' + 25</literal>
        <returnvalue>192.168.1.31</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>bigint</type> <literal>+</literal> <type>inet</type>
        <returnvalue>inet</returnvalue>
       </para>
       <para>
        Adds an offset to an address.
       </para>
       <para>
        <literal>200 + inet '::ffff:fff0:1'</literal>
        <returnvalue>::ffff:255.240.0.201</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>inet</type> <literal>-</literal> <type>bigint</type>
        <returnvalue>inet</returnvalue>
       </para>
       <para>
        Subtracts an offset from an address.
       </para>
       <para>
        <literal>inet '192.168.1.43' - 36</literal>
        <returnvalue>192.168.1.7</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>inet</type> <literal>-</literal> <type>inet</type>
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Computes the difference of two addresses.
       </para>
       <para>
        <literal>inet '192.168.1.43' - inet '192.168.1.19'</literal>
        <returnvalue>24</returnvalue>
       </para>
       <para>
        <literal>inet '::1' - inet '::ffff:1'</literal>
        <returnvalue>-4294901760</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <table id="cidr-inet-functions-table">
    <title>IP Address Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>abbrev</primary>
        </indexterm>
        <function>abbrev</function> ( <type>inet</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Creates an abbreviated display format as text.
        (The result is the same as the <type>inet</type> output function
        produces; it is <quote>abbreviated</quote> only in comparison to the
        result of an explicit cast to <type>text</type>, which for historical
        reasons will never suppress the netmask part.)
       </para>
       <para>
        <literal>abbrev(inet '10.1.0.0/32')</literal>
        <returnvalue>10.1.0.0</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>abbrev</function> ( <type>cidr</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Creates an abbreviated display format as text.
        (The abbreviation consists of dropping all-zero octets to the right
        of the netmask; more examples are in
        <xref linkend="datatype-net-cidr-table"/>.)
       </para>
       <para>
        <literal>abbrev(cidr '10.1.0.0/16')</literal>
        <returnvalue>10.1/16</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>broadcast</primary>
        </indexterm>
        <function>broadcast</function> ( <type>inet</type> )
        <returnvalue>inet</returnvalue>
       </para>
       <para>
        Computes the broadcast address for the address's network.
       </para>
       <para>
        <literal>broadcast(inet '192.168.1.5/24')</literal>
        <returnvalue>192.168.1.255/24</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>family</primary>
        </indexterm>
        <function>family</function> ( <type>inet</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the address's family: <literal>4</literal> for IPv4,
        <literal>6</literal> for IPv6.
       </para>
       <para>
        <literal>family(inet '::1')</literal>
        <returnvalue>6</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>host</primary>
        </indexterm>
        <function>host</function> ( <type>inet</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the IP address as text, ignoring the netmask.
       </para>
       <para>
        <literal>host(inet '192.168.1.0/24')</literal>
        <returnvalue>192.168.1.0</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>hostmask</primary>
        </indexterm>
        <function>hostmask</function> ( <type>inet</type> )
        <returnvalue>inet</returnvalue>
       </para>
       <para>
        Computes the host mask for the address's network.
       </para>
       <para>
        <literal>hostmask(inet '192.168.23.20/30')</literal>
        <returnvalue>0.0.0.3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>inet_merge</primary>
        </indexterm>
        <function>inet_merge</function> ( <type>inet</type>, <type>inet</type> )
        <returnvalue>cidr</returnvalue>
       </para>
       <para>
        Computes the smallest network that includes both of the given networks.
       </para>
       <para>
        <literal>inet_merge(inet '192.168.1.5/24', inet '192.168.2.5/24')</literal>
        <returnvalue>192.168.0.0/22</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>inet_same_family</primary>
        </indexterm>
        <function>inet_same_family</function> ( <type>inet</type>, <type>inet</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Tests whether the addresses belong to the same IP family.
       </para>
       <para>
        <literal>inet_same_family(inet '192.168.1.5/24', inet '::1')</literal>
        <returnvalue>f</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>masklen</primary>
        </indexterm>
        <function>masklen</function> ( <type>inet</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the netmask length in bits.
       </para>
       <para>
        <literal>masklen(inet '192.168.1.5/24')</literal>
        <returnvalue>24</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>netmask</primary>
        </indexterm>
        <function>netmask</function> ( <type>inet</type> )
        <returnvalue>inet</returnvalue>
       </para>
       <para>
        Computes the network mask for the address's network.
       </para>
       <para>
        <literal>netmask(inet '192.168.1.5/24')</literal>
        <returnvalue>255.255.255.0</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>network</primary>
        </indexterm>
        <function>network</function> ( <type>inet</type> )
        <returnvalue>cidr</returnvalue>
       </para>
       <para>
        Returns the network part of the address, zeroing out
        whatever is to the right of the netmask.
        (This is equivalent to casting the value to <type>cidr</type>.)
       </para>
       <para>
        <literal>network(inet '192.168.1.5/24')</literal>
        <returnvalue>192.168.1.0/24</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>set_masklen</primary>
        </indexterm>
        <function>set_masklen</function> ( <type>inet</type>, <type>integer</type> )
        <returnvalue>inet</returnvalue>
       </para>
       <para>
        Sets the netmask length for an <type>inet</type> value.
        The address part does not change.
       </para>
       <para>
        <literal>set_masklen(inet '192.168.1.5/24', 16)</literal>
        <returnvalue>192.168.1.5/16</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>set_masklen</function> ( <type>cidr</type>, <type>integer</type> )
        <returnvalue>cidr</returnvalue>
       </para>
       <para>
        Sets the netmask length for a <type>cidr</type> value.
        Address bits to the right of the new netmask are set to zero.
       </para>
       <para>
        <literal>set_masklen(cidr '192.168.1.0/24', 16)</literal>
        <returnvalue>192.168.0.0/16</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>text</primary>
        </indexterm>
        <function>text</function> ( <type>inet</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the unabbreviated IP address and netmask length as text.
        (This has the same result as an explicit cast to <type>text</type>.)
       </para>
       <para>
        <literal>text(inet '192.168.1.5')</literal>
        <returnvalue>192.168.1.5/32</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <tip>
   <para>
    The <function>abbrev</function>, <function>host</function>,
    and <function>text</function> functions are primarily intended to offer
    alternative display formats for IP addresses.
   </para>
  </tip>

  <para>
   The MAC address types, <type>macaddr</type> and <type>macaddr8</type>,
   support the usual comparison operators shown in
   <xref linkend="functions-comparison-op-table"/>
   as well as the specialized functions shown in
   <xref linkend="macaddr-functions-table"/>.
   In addition, they support the bitwise logical operators
   <literal>~</literal>, <literal>&amp;</literal> and <literal>|</literal>
   (NOT, AND and OR), just as shown above for IP addresses.
  </para>

   <table id="macaddr-functions-table">
    <title>MAC Address Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>trunc</primary>
        </indexterm>
        <function>trunc</function> ( <type>macaddr</type> )
        <returnvalue>macaddr</returnvalue>
       </para>
       <para>
        Sets the last 3 bytes of the address to zero.  The remaining prefix
        can be associated with a particular manufacturer (using data not
        included in <productname>PostgreSQL</productname>).
       </para>
       <para>
        <literal>trunc(macaddr '12:34:56:78:90:ab')</literal>
        <returnvalue>12:34:56:00:00:00</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>trunc</function> ( <type>macaddr8</type> )
        <returnvalue>macaddr8</returnvalue>
       </para>
       <para>
        Sets the last 5 bytes of the address to zero.  The remaining prefix
        can be associated with a particular manufacturer (using data not
        included in <productname>PostgreSQL</productname>).
       </para>
       <para>
        <literal>trunc(macaddr8 '12:34:56:78:90:ab:cd:ef')</literal>
        <returnvalue>12:34:56:00:00:00:00:00</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>macaddr8_set7bit</primary>
        </indexterm>
        <function>macaddr8_set7bit</function> ( <type>macaddr8</type> )
        <returnvalue>macaddr8</returnvalue>
       </para>
       <para>
        Sets the 7th bit of the address to one, creating what is known as
        modified EUI-64, for inclusion in an IPv6 address.
       </para>
       <para>
        <literal>macaddr8_set7bit(macaddr8 '00:34:56:ab:cd:ef')</literal>
        <returnvalue>02:34:56:ff:fe:ab:cd:ef</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  </sect1>


 <sect1 id="functions-textsearch">
  <title>Text Search Functions and Operators</title>

   <indexterm zone="datatype-textsearch">
    <primary>full text search</primary>
    <secondary>functions and operators</secondary>
   </indexterm>

   <indexterm zone="datatype-textsearch">
    <primary>text search</primary>
    <secondary>functions and operators</secondary>
   </indexterm>

  <para>
   <xref linkend="textsearch-operators-table"/>,
   <xref linkend="textsearch-functions-table"/> and
   <xref linkend="textsearch-functions-debug-table"/>
   summarize the functions and operators that are provided
   for full text searching.  See <xref linkend="textsearch"/> for a detailed
   explanation of <productname>PostgreSQL</productname>'s text search
   facility.
  </para>

   <table id="textsearch-operators-table">
    <title>Text Search Operators</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Operator
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>tsvector</type> <literal>@@</literal> <type>tsquery</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para role="func_signature">
        <type>tsquery</type> <literal>@@</literal> <type>tsvector</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does <type>tsvector</type> match <type>tsquery</type>?
        (The arguments can be given in either order.)
       </para>
       <para>
        <literal>to_tsvector('fat cats ate rats') @@ to_tsquery('cat &amp; rat')</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>text</type> <literal>@@</literal> <type>tsquery</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does text string, after implicit invocation
        of <function>to_tsvector()</function>, match <type>tsquery</type>?
       </para>
       <para>
        <literal>'fat cats ate rats' @@ to_tsquery('cat &amp; rat')</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>tsvector</type> <literal>@@@</literal> <type>tsquery</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para role="func_signature">
        <type>tsquery</type> <literal>@@@</literal> <type>tsvector</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        This is a deprecated synonym for <literal>@@</literal>.
       </para>
       <para>
        <literal>to_tsvector('fat cats ate rats') @@@ to_tsquery('cat &amp; rat')</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>tsvector</type> <literal>||</literal> <type>tsvector</type>
        <returnvalue>tsvector</returnvalue>
       </para>
       <para>
        Concatenates two <type>tsvector</type>s.  If both inputs contain
        lexeme positions, the second input's positions are adjusted
        accordingly.
       </para>
       <para>
        <literal>'a:1 b:2'::tsvector || 'c:1 d:2 b:3'::tsvector</literal>
        <returnvalue>'a':1 'b':2,5 'c':3 'd':4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>tsquery</type> <literal>&amp;&amp;</literal> <type>tsquery</type>
        <returnvalue>tsquery</returnvalue>
       </para>
       <para>
        ANDs two <type>tsquery</type>s together, producing a query that
        matches documents that match both input queries.
       </para>
       <para>
        <literal>'fat | rat'::tsquery &amp;&amp; 'cat'::tsquery</literal>
        <returnvalue>( 'fat' | 'rat' ) &amp; 'cat'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>tsquery</type> <literal>||</literal> <type>tsquery</type>
        <returnvalue>tsquery</returnvalue>
       </para>
       <para>
        ORs two <type>tsquery</type>s together, producing a query that
        matches documents that match either input query.
       </para>
       <para>
        <literal>'fat | rat'::tsquery || 'cat'::tsquery</literal>
        <returnvalue>'fat' | 'rat' | 'cat'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>!!</literal> <type>tsquery</type>
        <returnvalue>tsquery</returnvalue>
       </para>
       <para>
        Negates a <type>tsquery</type>, producing a query that matches
        documents that do not match the input query.
       </para>
       <para>
        <literal>!! 'cat'::tsquery</literal>
        <returnvalue>!'cat'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>tsquery</type> <literal>&lt;-&gt;</literal> <type>tsquery</type>
        <returnvalue>tsquery</returnvalue>
       </para>
       <para>
        Constructs a phrase query, which matches if the two input queries
        match at successive lexemes.
       </para>
       <para>
        <literal>to_tsquery('fat') &lt;-&gt; to_tsquery('rat')</literal>
        <returnvalue>'fat' &lt;-&gt; 'rat'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>tsquery</type> <literal>@&gt;</literal> <type>tsquery</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does first <type>tsquery</type> contain the second?  (This considers
        only whether all the lexemes appearing in one query appear in the
        other, ignoring the combining operators.)
       </para>
       <para>
        <literal>'cat'::tsquery @&gt; 'cat &amp; rat'::tsquery</literal>
        <returnvalue>f</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>tsquery</type> <literal>&lt;@</literal> <type>tsquery</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is first <type>tsquery</type> contained in the second?  (This
        considers only whether all the lexemes appearing in one query appear
        in the other, ignoring the combining operators.)
       </para>
       <para>
        <literal>'cat'::tsquery &lt;@ 'cat &amp; rat'::tsquery</literal>
        <returnvalue>t</returnvalue>
       </para>
       <para>
        <literal>'cat'::tsquery &lt;@ '!cat &amp; rat'::tsquery</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

    <para>
     In addition to these specialized operators, the usual comparison
     operators shown in <xref linkend="functions-comparison-op-table"/> are
     available for types <type>tsvector</type> and <type>tsquery</type>.
     These are not very
     useful for text searching but allow, for example, unique indexes to be
     built on columns of these types.
    </para>

   <table id="textsearch-functions-table">
    <title>Text Search Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>array_to_tsvector</primary>
        </indexterm>
        <function>array_to_tsvector</function> ( <type>text[]</type> )
        <returnvalue>tsvector</returnvalue>
       </para>
       <para>
        Converts an array of lexemes to a <type>tsvector</type>.
        The given strings are used as-is without further processing.
       </para>
       <para>
        <literal>array_to_tsvector('{fat,cat,rat}'::text[])</literal>
        <returnvalue>'cat' 'fat' 'rat'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>get_current_ts_config</primary>
        </indexterm>
        <function>get_current_ts_config</function> ( )
        <returnvalue>regconfig</returnvalue>
       </para>
       <para>
        Returns the OID of the current default text search configuration
        (as set by <xref linkend="guc-default-text-search-config"/>).
       </para>
       <para>
        <literal>get_current_ts_config()</literal>
        <returnvalue>english</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>length</primary>
        </indexterm>
        <function>length</function> ( <type>tsvector</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the number of lexemes in the <type>tsvector</type>.
       </para>
       <para>
        <literal>length('fat:2,4 cat:3 rat:5A'::tsvector)</literal>
        <returnvalue>3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>numnode</primary>
        </indexterm>
        <function>numnode</function> ( <type>tsquery</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the number of lexemes plus operators in
        the <type>tsquery</type>.
       </para>
       <para>
        <literal>numnode('(fat &amp; rat) | cat'::tsquery)</literal>
        <returnvalue>5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>plainto_tsquery</primary>
        </indexterm>
        <function>plainto_tsquery</function> (
        <optional> <parameter>config</parameter> <type>regconfig</type>, </optional>
        <parameter>query</parameter> <type>text</type> )
        <returnvalue>tsquery</returnvalue>
       </para>
       <para>
        Converts text to a <type>tsquery</type>, normalizing words according to
        the specified or default configuration.  Any punctuation in the string
        is ignored (it does not determine query operators).  The resulting
        query matches documents containing all non-stopwords in the text.
       </para>
       <para>
        <literal>plainto_tsquery('english', 'The Fat Rats')</literal>
        <returnvalue>'fat' &amp; 'rat'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>phraseto_tsquery</primary>
        </indexterm>
        <function>phraseto_tsquery</function> (
        <optional> <parameter>config</parameter> <type>regconfig</type>, </optional>
        <parameter>query</parameter> <type>text</type> )
        <returnvalue>tsquery</returnvalue>
       </para>
       <para>
        Converts text to a <type>tsquery</type>, normalizing words according to
        the specified or default configuration.  Any punctuation in the string
        is ignored (it does not determine query operators).  The resulting
        query matches phrases containing all non-stopwords in the text.
       </para>
       <para>
        <literal>phraseto_tsquery('english', 'The Fat Rats')</literal>
        <returnvalue>'fat' &lt;-&gt; 'rat'</returnvalue>
       </para>
       <para>
        <literal>phraseto_tsquery('english', 'The Cat and Rats')</literal>
        <returnvalue>'cat' &lt;2&gt; 'rat'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>websearch_to_tsquery</primary>
        </indexterm>
        <function>websearch_to_tsquery</function> (
        <optional> <parameter>config</parameter> <type>regconfig</type>, </optional>
        <parameter>query</parameter> <type>text</type> )
        <returnvalue>tsquery</returnvalue>
       </para>
       <para>
        Converts text to a <type>tsquery</type>, normalizing words according
        to the specified or default configuration.  Quoted word sequences are
        converted to phrase tests.  The word <quote>or</quote> is understood
        as producing an OR operator, and a dash produces a NOT operator;
        other punctuation is ignored.
        This approximates the behavior of some common web search tools.
       </para>
       <para>
        <literal>websearch_to_tsquery('english', '"fat rat" or cat dog')</literal>
        <returnvalue>'fat' &lt;-&gt; 'rat' | 'cat' &amp; 'dog'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>querytree</primary>
        </indexterm>
        <function>querytree</function> ( <type>tsquery</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Produces a representation of the indexable portion of
        a <type>tsquery</type>.  A result that is empty or
        just <literal>T</literal> indicates a non-indexable query.
       </para>
       <para>
        <literal>querytree('foo &amp; ! bar'::tsquery)</literal>
        <returnvalue>'foo'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>setweight</primary>
        </indexterm>
        <function>setweight</function> ( <parameter>vector</parameter> <type>tsvector</type>, <parameter>weight</parameter> <type>"char"</type> )
        <returnvalue>tsvector</returnvalue>
       </para>
       <para>
        Assigns the specified <parameter>weight</parameter> to each element
        of the <parameter>vector</parameter>.
       </para>
       <para>
        <literal>setweight('fat:2,4 cat:3 rat:5B'::tsvector, 'A')</literal>
        <returnvalue>'cat':3A 'fat':2A,4A 'rat':5A</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>setweight</primary>
         <secondary>setweight for specific lexeme(s)</secondary>
        </indexterm>
        <function>setweight</function> ( <parameter>vector</parameter> <type>tsvector</type>, <parameter>weight</parameter> <type>"char"</type>, <parameter>lexemes</parameter> <type>text[]</type> )
        <returnvalue>tsvector</returnvalue>
       </para>
       <para>
        Assigns the specified <parameter>weight</parameter> to elements
        of the <parameter>vector</parameter> that are listed
        in <parameter>lexemes</parameter>.
       </para>
       <para>
        <literal>setweight('fat:2,4 cat:3 rat:5,6B'::tsvector, 'A', '{cat,rat}')</literal>
        <returnvalue>'cat':3A 'fat':2,4 'rat':5A,6A</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>strip</primary>
        </indexterm>
        <function>strip</function> ( <type>tsvector</type> )
        <returnvalue>tsvector</returnvalue>
       </para>
       <para>
        Removes positions and weights from the <type>tsvector</type>.
       </para>
       <para>
        <literal>strip('fat:2,4 cat:3 rat:5A'::tsvector)</literal>
        <returnvalue>'cat' 'fat' 'rat'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_tsquery</primary>
        </indexterm>
        <function>to_tsquery</function> (
        <optional> <parameter>config</parameter> <type>regconfig</type>, </optional>
        <parameter>query</parameter> <type>text</type> )
        <returnvalue>tsquery</returnvalue>
       </para>
       <para>
        Converts text to a <type>tsquery</type>, normalizing words according to
        the specified or default configuration.  The words must be combined
        by valid <type>tsquery</type> operators.
       </para>
       <para>
        <literal>to_tsquery('english', 'The &amp; Fat &amp; Rats')</literal>
        <returnvalue>'fat' &amp; 'rat'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_tsvector</primary>
        </indexterm>
        <function>to_tsvector</function> (
        <optional> <parameter>config</parameter> <type>regconfig</type>, </optional>
         <parameter>document</parameter> <type>text</type> )
        <returnvalue>tsvector</returnvalue>
       </para>
       <para>
        Converts text to a <type>tsvector</type>, normalizing words according
        to the specified or default configuration.  Position information is
        included in the result.
       </para>
       <para>
        <literal>to_tsvector('english', 'The Fat Rats')</literal>
        <returnvalue>'fat':2 'rat':3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>to_tsvector</function> (
        <optional> <parameter>config</parameter> <type>regconfig</type>, </optional>
        <parameter>document</parameter> <type>json</type> )
        <returnvalue>tsvector</returnvalue>
       </para>
       <para role="func_signature">
        <function>to_tsvector</function> (
        <optional> <parameter>config</parameter> <type>regconfig</type>, </optional>
        <parameter>document</parameter> <type>jsonb</type> )
        <returnvalue>tsvector</returnvalue>
       </para>
       <para>
        Converts each string value in the JSON document to
        a <type>tsvector</type>, normalizing words according to the specified
        or default configuration.  The results are then concatenated in
        document order to produce the output.  Position information is
        generated as though one stopword exists between each pair of string
        values.  (Beware that <quote>document order</quote> of the fields of a
        JSON object is implementation-dependent when the input
        is <type>jsonb</type>; observe the difference in the examples.)
       </para>
       <para>
        <literal>to_tsvector('english', '{"aa": "The Fat Rats", "b": "dog"}'::json)</literal>
        <returnvalue>'dog':5 'fat':2 'rat':3</returnvalue>
       </para>
       <para>
        <literal>to_tsvector('english', '{"aa": "The Fat Rats", "b": "dog"}'::jsonb)</literal>
        <returnvalue>'dog':1 'fat':4 'rat':5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_to_tsvector</primary>
        </indexterm>
        <function>json_to_tsvector</function> (
        <optional> <parameter>config</parameter> <type>regconfig</type>, </optional>
        <parameter>document</parameter> <type>json</type>,
        <parameter>filter</parameter> <type>jsonb</type> )
        <returnvalue>tsvector</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_to_tsvector</primary>
        </indexterm>
        <function>jsonb_to_tsvector</function> (
        <optional> <parameter>config</parameter> <type>regconfig</type>, </optional>
        <parameter>document</parameter> <type>jsonb</type>,
        <parameter>filter</parameter> <type>jsonb</type> )
        <returnvalue>tsvector</returnvalue>
       </para>
       <para>
        Selects each item in the JSON document that is requested by
        the <parameter>filter</parameter> and converts each one to
        a <type>tsvector</type>, normalizing words according to the specified
        or default configuration.  The results are then concatenated in
        document order to produce the output.  Position information is
        generated as though one stopword exists between each pair of selected
        items.  (Beware that <quote>document order</quote> of the fields of a
        JSON object is implementation-dependent when the input
        is <type>jsonb</type>.)
        The <parameter>filter</parameter> must be a <type>jsonb</type>
        array containing zero or more of these keywords:
        <literal>"string"</literal> (to include all string values),
        <literal>"numeric"</literal> (to include all numeric values),
        <literal>"boolean"</literal> (to include all boolean values),
        <literal>"key"</literal> (to include all keys), or
        <literal>"all"</literal> (to include all the above).
        As a special case, the <parameter>filter</parameter> can also be a
        simple JSON value that is one of these keywords.
       </para>
       <para>
        <literal>json_to_tsvector('english', '{"a": "The Fat Rats", "b": 123}'::json, '["string", "numeric"]')</literal>
        <returnvalue>'123':5 'fat':2 'rat':3</returnvalue>
       </para>
       <para>
        <literal>json_to_tsvector('english', '{"cat": "The Fat Rats", "dog": 123}'::json, '"all"')</literal>
        <returnvalue>'123':9 'cat':1 'dog':7 'fat':4 'rat':5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ts_delete</primary>
        </indexterm>
        <function>ts_delete</function> ( <parameter>vector</parameter> <type>tsvector</type>, <parameter>lexeme</parameter> <type>text</type> )
        <returnvalue>tsvector</returnvalue>
       </para>
       <para>
        Removes any occurrence of the given <parameter>lexeme</parameter>
        from the <parameter>vector</parameter>.
       </para>
       <para>
        <literal>ts_delete('fat:2,4 cat:3 rat:5A'::tsvector, 'fat')</literal>
        <returnvalue>'cat':3 'rat':5A</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>ts_delete</function> ( <parameter>vector</parameter> <type>tsvector</type>, <parameter>lexemes</parameter> <type>text[]</type> )
        <returnvalue>tsvector</returnvalue>
       </para>
       <para>
        Removes any occurrences of the lexemes
        in <parameter>lexemes</parameter>
        from the <parameter>vector</parameter>.
       </para>
       <para>
        <literal>ts_delete('fat:2,4 cat:3 rat:5A'::tsvector, ARRAY['fat','rat'])</literal>
        <returnvalue>'cat':3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ts_filter</primary>
        </indexterm>
        <function>ts_filter</function> ( <parameter>vector</parameter> <type>tsvector</type>, <parameter>weights</parameter> <type>"char"[]</type> )
        <returnvalue>tsvector</returnvalue>
       </para>
       <para>
        Selects only elements with the given <parameter>weights</parameter>
        from the <parameter>vector</parameter>.
       </para>
       <para>
        <literal>ts_filter('fat:2,4 cat:3b,7c rat:5A'::tsvector, '{a,b}')</literal>
        <returnvalue>'cat':3B 'rat':5A</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ts_headline</primary>
        </indexterm>
        <function>ts_headline</function> (
        <optional> <parameter>config</parameter> <type>regconfig</type>, </optional>
        <parameter>document</parameter> <type>text</type>,
        <parameter>query</parameter> <type>tsquery</type>
        <optional>, <parameter>options</parameter> <type>text</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Displays, in an abbreviated form, the match(es) for
        the <parameter>query</parameter> in
        the <parameter>document</parameter>, which must be raw text not
        a <type>tsvector</type>.  Words in the document are normalized
        according to the specified or default configuration before matching to
        the query.  Use of this function is discussed in
        <xref linkend="textsearch-headline"/>, which also describes the
        available <parameter>options</parameter>.
       </para>
       <para>
        <literal>ts_headline('The fat cat ate the rat.', 'cat')</literal>
        <returnvalue>The fat &lt;b&gt;cat&lt;/b&gt; ate the rat.</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>ts_headline</function> (
        <optional> <parameter>config</parameter> <type>regconfig</type>, </optional>
        <parameter>document</parameter> <type>json</type>,
        <parameter>query</parameter> <type>tsquery</type>
        <optional>, <parameter>options</parameter> <type>text</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <function>ts_headline</function> (
        <optional> <parameter>config</parameter> <type>regconfig</type>, </optional>
        <parameter>document</parameter> <type>jsonb</type>,
        <parameter>query</parameter> <type>tsquery</type>
        <optional>, <parameter>options</parameter> <type>text</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Displays, in an abbreviated form, match(es) for
        the <parameter>query</parameter> that occur in string values
        within the JSON <parameter>document</parameter>.
        See <xref linkend="textsearch-headline"/> for more details.
       </para>
       <para>
        <literal>ts_headline('{"cat":"raining cats and dogs"}'::jsonb, 'cat')</literal>
        <returnvalue>{"cat": "raining &lt;b&gt;cats&lt;/b&gt; and dogs"}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ts_rank</primary>
        </indexterm>
        <function>ts_rank</function> (
        <optional> <parameter>weights</parameter> <type>real[]</type>, </optional>
        <parameter>vector</parameter> <type>tsvector</type>,
        <parameter>query</parameter> <type>tsquery</type>
        <optional>, <parameter>normalization</parameter> <type>integer</type> </optional> )
        <returnvalue>real</returnvalue>
       </para>
       <para>
        Computes a score showing how well
        the <parameter>vector</parameter> matches
        the <parameter>query</parameter>.  See
        <xref linkend="textsearch-ranking"/> for details.
       </para>
       <para>
        <literal>ts_rank(to_tsvector('raining cats and dogs'), 'cat')</literal>
        <returnvalue>0.06079271</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ts_rank_cd</primary>
        </indexterm>
        <function>ts_rank_cd</function> (
        <optional> <parameter>weights</parameter> <type>real[]</type>, </optional>
        <parameter>vector</parameter> <type>tsvector</type>,
        <parameter>query</parameter> <type>tsquery</type>
        <optional>, <parameter>normalization</parameter> <type>integer</type> </optional> )
        <returnvalue>real</returnvalue>
       </para>
       <para>
        Computes a score showing how well
        the <parameter>vector</parameter> matches
        the <parameter>query</parameter>, using a cover density
        algorithm.  See <xref linkend="textsearch-ranking"/> for details.
       </para>
       <para>
        <literal>ts_rank_cd(to_tsvector('raining cats and dogs'), 'cat')</literal>
        <returnvalue>0.1</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ts_rewrite</primary>
        </indexterm>
        <function>ts_rewrite</function> ( <parameter>query</parameter> <type>tsquery</type>,
        <parameter>target</parameter> <type>tsquery</type>,
        <parameter>substitute</parameter> <type>tsquery</type> )
        <returnvalue>tsquery</returnvalue>
       </para>
       <para>
        Replaces occurrences of <parameter>target</parameter>
        with <parameter>substitute</parameter>
        within the <parameter>query</parameter>.
        See <xref linkend="textsearch-query-rewriting"/> for details.
       </para>
       <para>
        <literal>ts_rewrite('a &amp; b'::tsquery, 'a'::tsquery, 'foo|bar'::tsquery)</literal>
        <returnvalue>'b' &amp; ( 'foo' | 'bar' )</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>ts_rewrite</function> ( <parameter>query</parameter> <type>tsquery</type>,
        <parameter>select</parameter> <type>text</type> )
        <returnvalue>tsquery</returnvalue>
       </para>
       <para>
        Replaces portions of the <parameter>query</parameter> according to
        target(s) and substitute(s) obtained by executing
        a <command>SELECT</command> command.
        See <xref linkend="textsearch-query-rewriting"/> for details.
       </para>
       <para>
        <literal>SELECT ts_rewrite('a &amp; b'::tsquery, 'SELECT t,s FROM aliases')</literal>
        <returnvalue>'b' &amp; ( 'foo' | 'bar' )</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>tsquery_phrase</primary>
        </indexterm>
        <function>tsquery_phrase</function> ( <parameter>query1</parameter> <type>tsquery</type>, <parameter>query2</parameter> <type>tsquery</type> )
        <returnvalue>tsquery</returnvalue>
       </para>
       <para>
        Constructs a phrase query that searches
        for matches of <parameter>query1</parameter>
        and <parameter>query2</parameter> at successive lexemes (same
        as <literal>&lt;-&gt;</literal> operator).
       </para>
       <para>
        <literal>tsquery_phrase(to_tsquery('fat'), to_tsquery('cat'))</literal>
        <returnvalue>'fat' &lt;-&gt; 'cat'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>tsquery_phrase</function> ( <parameter>query1</parameter> <type>tsquery</type>, <parameter>query2</parameter> <type>tsquery</type>, <parameter>distance</parameter> <type>integer</type> )
        <returnvalue>tsquery</returnvalue>
       </para>
       <para>
        Constructs a phrase query that searches
        for matches of <parameter>query1</parameter> and
        <parameter>query2</parameter> that occur exactly
        <parameter>distance</parameter> lexemes apart.
       </para>
       <para>
        <literal>tsquery_phrase(to_tsquery('fat'), to_tsquery('cat'), 10)</literal>
        <returnvalue>'fat' &lt;10&gt; 'cat'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>tsvector_to_array</primary>
        </indexterm>
        <function>tsvector_to_array</function> ( <type>tsvector</type> )
        <returnvalue>text[]</returnvalue>
       </para>
       <para>
        Converts a <type>tsvector</type> to an array of lexemes.
       </para>
       <para>
        <literal>tsvector_to_array('fat:2,4 cat:3 rat:5A'::tsvector)</literal>
        <returnvalue>{cat,fat,rat}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>unnest</primary>
         <secondary>for tsvector</secondary>
        </indexterm>
        <function>unnest</function> ( <type>tsvector</type> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>lexeme</parameter> <type>text</type>,
        <parameter>positions</parameter> <type>smallint[]</type>,
        <parameter>weights</parameter> <type>text</type> )
       </para>
       <para>
        Expands a <type>tsvector</type> into a set of rows, one per lexeme.
       </para>
       <para>
        <literal>select * from unnest('cat:3 fat:2,4 rat:5A'::tsvector)</literal>
        <returnvalue></returnvalue>
<programlisting>
 lexeme | positions | weights
--------+-----------+---------
 cat    | {3}       | {D}
 fat    | {2,4}     | {D,D}
 rat    | {5}       | {A}
</programlisting>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <note>
   <para>
    All the text search functions that accept an optional <type>regconfig</type>
    argument will use the configuration specified by
    <xref linkend="guc-default-text-search-config"/>
    when that argument is omitted.
   </para>
  </note>

  <para>
   The functions in
   <xref linkend="textsearch-functions-debug-table"/>
   are listed separately because they are not usually used in everyday text
   searching operations.  They are primarily helpful for development and
   debugging of new text search configurations.
  </para>

   <table id="textsearch-functions-debug-table">
    <title>Text Search Debugging Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ts_debug</primary>
        </indexterm>
        <function>ts_debug</function> (
        <optional> <parameter>config</parameter> <type>regconfig</type>, </optional>
        <parameter>document</parameter> <type>text</type> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>alias</parameter> <type>text</type>,
        <parameter>description</parameter> <type>text</type>,
        <parameter>token</parameter> <type>text</type>,
        <parameter>dictionaries</parameter> <type>regdictionary[]</type>,
        <parameter>dictionary</parameter> <type>regdictionary</type>,
        <parameter>lexemes</parameter> <type>text[]</type> )
       </para>
       <para>
        Extracts and normalizes tokens from
        the <parameter>document</parameter> according to the specified or
        default text search configuration, and returns information about how
        each token was processed.
        See <xref linkend="textsearch-configuration-testing"/> for details.
       </para>
       <para>
        <literal>ts_debug('english', 'The Brightest supernovaes')</literal>
        <returnvalue>(asciiword,"Word, all ASCII",The,{english_stem},english_stem,{}) ...</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ts_lexize</primary>
        </indexterm>
        <function>ts_lexize</function> ( <parameter>dict</parameter> <type>regdictionary</type>, <parameter>token</parameter> <type>text</type> )
        <returnvalue>text[]</returnvalue>
       </para>
       <para>
        Returns an array of replacement lexemes if the input token is known to
        the dictionary, or an empty array if the token is known to the
        dictionary but it is a stop word, or NULL if it is not a known word.
        See <xref linkend="textsearch-dictionary-testing"/> for details.
       </para>
       <para>
        <literal>ts_lexize('english_stem', 'stars')</literal>
        <returnvalue>{star}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ts_parse</primary>
        </indexterm>
        <function>ts_parse</function> ( <parameter>parser_name</parameter> <type>text</type>,
        <parameter>document</parameter> <type>text</type> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>tokid</parameter> <type>integer</type>,
        <parameter>token</parameter> <type>text</type> )
       </para>
       <para>
        Extracts tokens from the <parameter>document</parameter> using the
        named parser.
        See <xref linkend="textsearch-parser-testing"/> for details.
       </para>
       <para>
        <literal>ts_parse('default', 'foo - bar')</literal>
        <returnvalue>(1,foo) ...</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>ts_parse</function> ( <parameter>parser_oid</parameter> <type>oid</type>,
        <parameter>document</parameter> <type>text</type> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>tokid</parameter> <type>integer</type>,
        <parameter>token</parameter> <type>text</type> )
       </para>
       <para>
        Extracts tokens from the <parameter>document</parameter> using a
        parser specified by OID.
        See <xref linkend="textsearch-parser-testing"/> for details.
       </para>
       <para>
        <literal>ts_parse(3722, 'foo - bar')</literal>
        <returnvalue>(1,foo) ...</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ts_token_type</primary>
        </indexterm>
        <function>ts_token_type</function> ( <parameter>parser_name</parameter> <type>text</type> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>tokid</parameter> <type>integer</type>,
        <parameter>alias</parameter> <type>text</type>,
        <parameter>description</parameter> <type>text</type> )
       </para>
       <para>
        Returns a table that describes each type of token the named parser can
        recognize.
        See <xref linkend="textsearch-parser-testing"/> for details.
       </para>
       <para>
        <literal>ts_token_type('default')</literal>
        <returnvalue>(1,asciiword,"Word, all ASCII") ...</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>ts_token_type</function> ( <parameter>parser_oid</parameter> <type>oid</type> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>tokid</parameter> <type>integer</type>,
        <parameter>alias</parameter> <type>text</type>,
        <parameter>description</parameter> <type>text</type> )
       </para>
       <para>
        Returns a table that describes each type of token a parser specified
        by OID can recognize.
        See <xref linkend="textsearch-parser-testing"/> for details.
       </para>
       <para>
        <literal>ts_token_type(3722)</literal>
        <returnvalue>(1,asciiword,"Word, all ASCII") ...</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ts_stat</primary>
        </indexterm>
        <function>ts_stat</function> ( <parameter>sqlquery</parameter> <type>text</type>
        <optional>, <parameter>weights</parameter> <type>text</type> </optional> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>word</parameter> <type>text</type>,
        <parameter>ndoc</parameter> <type>integer</type>,
        <parameter>nentry</parameter> <type>integer</type> )
       </para>
       <para>
        Executes the <parameter>sqlquery</parameter>, which must return a
        single <type>tsvector</type> column, and returns statistics about each
        distinct lexeme contained in the data.
        See <xref linkend="textsearch-statistics"/> for details.
       </para>
       <para>
        <literal>ts_stat('SELECT vector FROM apod')</literal>
        <returnvalue>(foo,10,15) ...</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

 </sect1>

 <sect1 id="functions-uuid">
  <title>UUID Functions</title>

  <indexterm zone="datatype-uuid">
   <primary>UUID</primary>
   <secondary>generating</secondary>
  </indexterm>

  <indexterm>
   <primary>gen_random_uuid</primary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname> includes one function to generate a UUID:
<synopsis>
<function>gen_random_uuid</function> () <returnvalue>uuid</returnvalue>
</synopsis>
   This function returns a version 4 (random) UUID.  This is the most commonly
   used type of UUID and is appropriate for most applications.
  </para>

  <para>
   The <xref linkend="uuid-ossp"/> module provides additional functions that
   implement other standard algorithms for generating UUIDs.
  </para>

  <para>
   <productname>PostgreSQL</productname> also provides the usual comparison
   operators shown in <xref linkend="functions-comparison-op-table"/> for
   UUIDs.
  </para>
 </sect1>

 <sect1 id="functions-xml">

  <title>XML Functions</title>

  <indexterm>
   <primary>XML Functions</primary>
  </indexterm>

  <para>
   The functions and function-like expressions described in this
   section operate on values of type <type>xml</type>.  See <xref
   linkend="datatype-xml"/> for information about the <type>xml</type>
   type.  The function-like expressions <function>xmlparse</function>
   and <function>xmlserialize</function> for converting to and from
   type <type>xml</type> are documented there, not in this section.
  </para>

  <para>
   Use of most of these functions
   requires <productname>PostgreSQL</productname> to have been built
   with <command>configure --with-libxml</command>.
  </para>

  <sect2 id="functions-producing-xml">
   <title>Producing XML Content</title>

   <para>
    A set of functions and function-like expressions is available for
    producing XML content from SQL data.  As such, they are
    particularly suitable for formatting query results into XML
    documents for processing in client applications.
   </para>

   <sect3>
    <title><literal>xmlcomment</literal></title>

    <indexterm>
     <primary>xmlcomment</primary>
    </indexterm>

<synopsis>
<function>xmlcomment</function> ( <type>text</type> ) <returnvalue>xml</returnvalue>
</synopsis>

    <para>
     The function <function>xmlcomment</function> creates an XML value
     containing an XML comment with the specified text as content.
     The text cannot contain <quote><literal>--</literal></quote> or end with a
     <quote><literal>-</literal></quote>, otherwise the resulting construct
     would not be a valid XML comment.
     If the argument is null, the result is null.
    </para>

    <para>
     Example:
<screen><![CDATA[
SELECT xmlcomment('hello');

  xmlcomment
--------------
 <!--hello-->
]]></screen>
    </para>
   </sect3>

   <sect3>
    <title><literal>xmlconcat</literal></title>

    <indexterm>
     <primary>xmlconcat</primary>
    </indexterm>

<synopsis>
<function>xmlconcat</function> ( <type>xml</type> <optional>, ...</optional> ) <returnvalue>xml</returnvalue>
</synopsis>

    <para>
     The function <function>xmlconcat</function> concatenates a list
     of individual XML values to create a single value containing an
     XML content fragment.  Null values are omitted; the result is
     only null if there are no nonnull arguments.
    </para>

    <para>
     Example:
<screen><![CDATA[
SELECT xmlconcat('<abc/>', '<bar>foo</bar>');

      xmlconcat
----------------------
 <abc/><bar>foo</bar>
]]></screen>
    </para>

    <para>
     XML declarations, if present, are combined as follows.  If all
     argument values have the same XML version declaration, that
     version is used in the result, else no version is used.  If all
     argument values have the standalone declaration value
     <quote>yes</quote>, then that value is used in the result.  If
     all argument values have a standalone declaration value and at
     least one is <quote>no</quote>, then that is used in the result.
     Else the result will have no standalone declaration.  If the
     result is determined to require a standalone declaration but no
     version declaration, a version declaration with version 1.0 will
     be used because XML requires an XML declaration to contain a
     version declaration.  Encoding declarations are ignored and
     removed in all cases.
    </para>

    <para>
     Example:
<screen><![CDATA[
SELECT xmlconcat('<?xml version="1.1"?><foo/>', '<?xml version="1.1" standalone="no"?><bar/>');

             xmlconcat
-----------------------------------
 <?xml version="1.1"?><foo/><bar/>
]]></screen>
    </para>
   </sect3>

   <sect3>
    <title><literal>xmlelement</literal></title>

   <indexterm>
    <primary>xmlelement</primary>
   </indexterm>

<synopsis>
<function>xmlelement</function> ( <literal>NAME</literal> <replaceable>name</replaceable> <optional>, <literal>XMLATTRIBUTES</literal> ( <replaceable>attvalue</replaceable> <optional> <literal>AS</literal> <replaceable>attname</replaceable> </optional> <optional>, ...</optional> ) </optional> <optional>, <replaceable>content</replaceable> <optional>, ...</optional></optional> ) <returnvalue>xml</returnvalue>
</synopsis>

    <para>
     The <function>xmlelement</function> expression produces an XML
     element with the given name, attributes, and content.
     The <replaceable>name</replaceable>
     and <replaceable>attname</replaceable> items shown in the syntax are
     simple identifiers, not values.  The <replaceable>attvalue</replaceable>
     and <replaceable>content</replaceable> items are expressions, which can
     yield any <productname>PostgreSQL</productname> data type.  The
     argument(s) within <literal>XMLATTRIBUTES</literal> generate attributes
     of the XML element; the <replaceable>content</replaceable> value(s) are
     concatenated to form its content.
    </para>

    <para>
     Examples:
<screen><![CDATA[
SELECT xmlelement(name foo);

 xmlelement
------------
 <foo/>

SELECT xmlelement(name foo, xmlattributes('xyz' as bar));

    xmlelement
------------------
 <foo bar="xyz"/>

SELECT xmlelement(name foo, xmlattributes(current_date as bar), 'cont', 'ent');

             xmlelement
-------------------------------------
 <foo bar="2007-01-26">content</foo>
]]></screen>
    </para>

    <para>
     Element and attribute names that are not valid XML names are
     escaped by replacing the offending characters by the sequence
     <literal>_x<replaceable>HHHH</replaceable>_</literal>, where
     <replaceable>HHHH</replaceable> is the character's Unicode
     codepoint in hexadecimal notation.  For example:
<screen><![CDATA[
SELECT xmlelement(name "foo$bar", xmlattributes('xyz' as "a&b"));

            xmlelement
----------------------------------
 <foo_x0024_bar a_x0026_b="xyz"/>
]]></screen>
    </para>

    <para>
     An explicit attribute name need not be specified if the attribute
     value is a column reference, in which case the column's name will
     be used as the attribute name by default.  In other cases, the
     attribute must be given an explicit name.  So this example is
     valid:
<screen>
CREATE TABLE test (a xml, b xml);
SELECT xmlelement(name test, xmlattributes(a, b)) FROM test;
</screen>
     But these are not:
<screen>
SELECT xmlelement(name test, xmlattributes('constant'), a, b) FROM test;
SELECT xmlelement(name test, xmlattributes(func(a, b))) FROM test;
</screen>
    </para>

    <para>
     Element content, if specified, will be formatted according to
     its data type.  If the content is itself of type <type>xml</type>,
     complex XML documents can be constructed.  For example:
<screen><![CDATA[
SELECT xmlelement(name foo, xmlattributes('xyz' as bar),
                            xmlelement(name abc),
                            xmlcomment('test'),
                            xmlelement(name xyz));

                  xmlelement
----------------------------------------------
 <foo bar="xyz"><abc/><!--test--><xyz/></foo>
]]></screen>

     Content of other types will be formatted into valid XML character
     data.  This means in particular that the characters &lt;, &gt;,
     and &amp; will be converted to entities.  Binary data (data type
     <type>bytea</type>) will be represented in base64 or hex
     encoding, depending on the setting of the configuration parameter
     <xref linkend="guc-xmlbinary"/>.  The particular behavior for
     individual data types is expected to evolve in order to align the
     PostgreSQL mappings with those specified in SQL:2006 and later,
     as discussed in <xref linkend="functions-xml-limits-casts"/>.
    </para>
   </sect3>

   <sect3>
    <title><literal>xmlforest</literal></title>

   <indexterm>
    <primary>xmlforest</primary>
   </indexterm>

<synopsis>
<function>xmlforest</function> ( <replaceable>content</replaceable> <optional> <literal>AS</literal> <replaceable>name</replaceable> </optional> <optional>, ...</optional> ) <returnvalue>xml</returnvalue>
</synopsis>

    <para>
     The <function>xmlforest</function> expression produces an XML
     forest (sequence) of elements using the given names and content.
     As for <function>xmlelement</function>,
     each <replaceable>name</replaceable> must be a simple identifier, while
     the <replaceable>content</replaceable> expressions can have any data
     type.
    </para>

    <para>
     Examples:
<screen>
SELECT xmlforest('abc' AS foo, 123 AS bar);

          xmlforest
------------------------------
 &lt;foo&gt;abc&lt;/foo&gt;&lt;bar&gt;123&lt;/bar&gt;


SELECT xmlforest(table_name, column_name)
FROM information_schema.columns
WHERE table_schema = 'pg_catalog';

                                xmlforest
------------------------------------&zwsp;-----------------------------------
 &lt;table_name&gt;pg_authid&lt;/table_name&gt;&zwsp;&lt;column_name&gt;rolname&lt;/column_name&gt;
 &lt;table_name&gt;pg_authid&lt;/table_name&gt;&zwsp;&lt;column_name&gt;rolsuper&lt;/column_name&gt;
 ...
</screen>

     As seen in the second example, the element name can be omitted if
     the content value is a column reference, in which case the column
     name is used by default.  Otherwise, a name must be specified.
    </para>

    <para>
     Element names that are not valid XML names are escaped as shown
     for <function>xmlelement</function> above.  Similarly, content
     data is escaped to make valid XML content, unless it is already
     of type <type>xml</type>.
    </para>

    <para>
     Note that XML forests are not valid XML documents if they consist
     of more than one element, so it might be useful to wrap
     <function>xmlforest</function> expressions in
     <function>xmlelement</function>.
    </para>
   </sect3>

   <sect3>
    <title><literal>xmlpi</literal></title>

   <indexterm>
    <primary>xmlpi</primary>
   </indexterm>

<synopsis>
<function>xmlpi</function> ( <literal>NAME</literal> <replaceable>name</replaceable> <optional>, <replaceable>content</replaceable> </optional> ) <returnvalue>xml</returnvalue>
</synopsis>

    <para>
     The <function>xmlpi</function> expression creates an XML
     processing instruction.
     As for <function>xmlelement</function>,
     the <replaceable>name</replaceable> must be a simple identifier, while
     the <replaceable>content</replaceable> expression can have any data type.
     The <replaceable>content</replaceable>, if present, must not contain the
     character sequence <literal>?&gt;</literal>.
    </para>

    <para>
     Example:
<screen><![CDATA[
SELECT xmlpi(name php, 'echo "hello world";');

            xmlpi
-----------------------------
 <?php echo "hello world";?>
]]></screen>
    </para>
   </sect3>

   <sect3>
    <title><literal>xmlroot</literal></title>

   <indexterm>
    <primary>xmlroot</primary>
   </indexterm>

<synopsis>
<function>xmlroot</function> ( <type>xml</type>, <literal>VERSION</literal> {<type>text</type>|<literal>NO VALUE</literal>} <optional>, <literal>STANDALONE</literal> {<literal>YES</literal>|<literal>NO</literal>|<literal>NO VALUE</literal>} </optional> ) <returnvalue>xml</returnvalue>
</synopsis>

    <para>
     The <function>xmlroot</function> expression alters the properties
     of the root node of an XML value.  If a version is specified,
     it replaces the value in the root node's version declaration; if a
     standalone setting is specified, it replaces the value in the
     root node's standalone declaration.
    </para>

    <para>
<screen><![CDATA[
SELECT xmlroot(xmlparse(document '<?xml version="1.1"?><content>abc</content>'),
               version '1.0', standalone yes);

                xmlroot
----------------------------------------
 <?xml version="1.0" standalone="yes"?>
 <content>abc</content>
]]></screen>
    </para>
   </sect3>

   <sect3 id="functions-xml-xmlagg">
    <title><literal>xmlagg</literal></title>

    <indexterm>
     <primary>xmlagg</primary>
    </indexterm>

<synopsis>
<function>xmlagg</function> ( <type>xml</type> ) <returnvalue>xml</returnvalue>
</synopsis>

    <para>
     The function <function>xmlagg</function> is, unlike the other
     functions described here, an aggregate function.  It concatenates the
     input values to the aggregate function call,
     much like <function>xmlconcat</function> does, except that concatenation
     occurs across rows rather than across expressions in a single row.
     See <xref linkend="functions-aggregate"/> for additional information
     about aggregate functions.
    </para>

    <para>
     Example:
<screen><![CDATA[
CREATE TABLE test (y int, x xml);
INSERT INTO test VALUES (1, '<foo>abc</foo>');
INSERT INTO test VALUES (2, '<bar/>');
SELECT xmlagg(x) FROM test;
        xmlagg
----------------------
 <foo>abc</foo><bar/>
]]></screen>
    </para>

    <para>
     To determine the order of the concatenation, an <literal>ORDER BY</literal>
     clause may be added to the aggregate call as described in
     <xref linkend="syntax-aggregates"/>. For example:

<screen><![CDATA[
SELECT xmlagg(x ORDER BY y DESC) FROM test;
        xmlagg
----------------------
 <bar/><foo>abc</foo>
]]></screen>
    </para>

    <para>
     The following non-standard approach used to be recommended
     in previous versions, and may still be useful in specific
     cases:

<screen><![CDATA[
SELECT xmlagg(x) FROM (SELECT * FROM test ORDER BY y DESC) AS tab;
        xmlagg
----------------------
 <bar/><foo>abc</foo>
]]></screen>
    </para>
   </sect3>
   </sect2>

   <sect2 id="functions-xml-predicates">
    <title>XML Predicates</title>

    <para>
     The expressions described in this section check properties
     of <type>xml</type> values.
    </para>

   <sect3>
    <title><literal>IS DOCUMENT</literal></title>

    <indexterm>
     <primary>IS DOCUMENT</primary>
    </indexterm>

<synopsis>
<type>xml</type> <literal>IS DOCUMENT</literal> <returnvalue>boolean</returnvalue>
</synopsis>

    <para>
     The expression <literal>IS DOCUMENT</literal> returns true if the
     argument XML value is a proper XML document, false if it is not
     (that is, it is a content fragment), or null if the argument is
     null.  See <xref linkend="datatype-xml"/> about the difference
     between documents and content fragments.
    </para>
   </sect3>

   <sect3>
    <title><literal>IS NOT DOCUMENT</literal></title>

    <indexterm>
     <primary>IS NOT DOCUMENT</primary>
    </indexterm>

<synopsis>
<type>xml</type> <literal>IS NOT DOCUMENT</literal> <returnvalue>boolean</returnvalue>
</synopsis>

    <para>
     The expression <literal>IS NOT DOCUMENT</literal> returns false if the
     argument XML value is a proper XML document, true if it is not (that is,
     it is a content fragment), or null if the argument is null.
    </para>
   </sect3>

   <sect3 id="xml-exists">
    <title><literal>XMLEXISTS</literal></title>

    <indexterm>
     <primary>XMLEXISTS</primary>
    </indexterm>

<synopsis>
<function>XMLEXISTS</function> ( <type>text</type> <literal>PASSING</literal> <optional><literal>BY</literal> {<literal>REF</literal>|<literal>VALUE</literal>}</optional> <type>xml</type> <optional><literal>BY</literal> {<literal>REF</literal>|<literal>VALUE</literal>}</optional> ) <returnvalue>boolean</returnvalue>
</synopsis>

    <para>
     The function <function>xmlexists</function> evaluates an XPath 1.0
     expression (the first argument), with the passed XML value as its context
     item.  The function returns false if the result of that evaluation
     yields an empty node-set, true if it yields any other value.  The
     function returns null if any argument is null.  A nonnull value
     passed as the context item must be an XML document, not a content
     fragment or any non-XML value.
    </para>

    <para>
     Example:
     <screen><![CDATA[
SELECT xmlexists('//town[text() = ''Toronto'']' PASSING BY VALUE '<towns><town>Toronto</town><town>Ottawa</town></towns>');

 xmlexists
------------
 t
(1 row)
]]></screen>
    </para>

    <para>
     The <literal>BY REF</literal> and <literal>BY VALUE</literal> clauses
     are accepted in <productname>PostgreSQL</productname>, but are ignored,
     as discussed in <xref linkend="functions-xml-limits-postgresql"/>.
    </para>

    <para>
     In the SQL standard, the <function>xmlexists</function> function
     evaluates an expression in the XML Query language,
     but <productname>PostgreSQL</productname> allows only an XPath 1.0
     expression, as discussed in
     <xref linkend="functions-xml-limits-xpath1"/>.
    </para>
   </sect3>

   <sect3 id="xml-is-well-formed">
    <title><literal>xml_is_well_formed</literal></title>

    <indexterm>
     <primary>xml_is_well_formed</primary>
    </indexterm>

    <indexterm>
     <primary>xml_is_well_formed_document</primary>
    </indexterm>

    <indexterm>
     <primary>xml_is_well_formed_content</primary>
    </indexterm>

<synopsis>
<function>xml_is_well_formed</function> ( <type>text</type> ) <returnvalue>boolean</returnvalue>
<function>xml_is_well_formed_document</function> ( <type>text</type> ) <returnvalue>boolean</returnvalue>
<function>xml_is_well_formed_content</function> ( <type>text</type> ) <returnvalue>boolean</returnvalue>
</synopsis>

    <para>
     These functions check whether a <type>text</type> string represents
     well-formed XML, returning a Boolean result.
     <function>xml_is_well_formed_document</function> checks for a well-formed
     document, while <function>xml_is_well_formed_content</function> checks
     for well-formed content.  <function>xml_is_well_formed</function> does
     the former if the <xref linkend="guc-xmloption"/> configuration
     parameter is set to <literal>DOCUMENT</literal>, or the latter if it is set to
     <literal>CONTENT</literal>.  This means that
     <function>xml_is_well_formed</function> is useful for seeing whether
     a simple cast to type <type>xml</type> will succeed, whereas the other two
     functions are useful for seeing whether the corresponding variants of
     <function>XMLPARSE</function> will succeed.
    </para>

    <para>
     Examples:

<screen><![CDATA[
SET xmloption TO DOCUMENT;
SELECT xml_is_well_formed('<>');
 xml_is_well_formed 
--------------------
 f
(1 row)

SELECT xml_is_well_formed('<abc/>');
 xml_is_well_formed 
--------------------
 t
(1 row)

SET xmloption TO CONTENT;
SELECT xml_is_well_formed('abc');
 xml_is_well_formed 
--------------------
 t
(1 row)

SELECT xml_is_well_formed_document('<pg:foo xmlns:pg="http://postgresql.org/stuff">bar</pg:foo>');
 xml_is_well_formed_document 
-----------------------------
 t
(1 row)

SELECT xml_is_well_formed_document('<pg:foo xmlns:pg="http://postgresql.org/stuff">bar</my:foo>');
 xml_is_well_formed_document 
-----------------------------
 f
(1 row)
]]></screen>

     The last example shows that the checks include whether
     namespaces are correctly matched.
    </para>
   </sect3>
  </sect2>

  <sect2 id="functions-xml-processing">
   <title>Processing XML</title>

   <para>
    To process values of data type <type>xml</type>, PostgreSQL offers
    the functions <function>xpath</function> and
    <function>xpath_exists</function>, which evaluate XPath 1.0
    expressions, and the <function>XMLTABLE</function>
    table function.
   </para>

   <sect3 id="functions-xml-processing-xpath">
    <title><literal>xpath</literal></title>

    <indexterm>
     <primary>XPath</primary>
    </indexterm>

<synopsis>
<function>xpath</function> ( <parameter>xpath</parameter> <type>text</type>, <parameter>xml</parameter> <type>xml</type> <optional>, <parameter>nsarray</parameter> <type>text[]</type> </optional> ) <returnvalue>xml[]</returnvalue>
</synopsis>

    <para>
     The function <function>xpath</function> evaluates the XPath 1.0
     expression <parameter>xpath</parameter> (given as text)
     against the XML value
     <parameter>xml</parameter>.  It returns an array of XML values
     corresponding to the node-set produced by the XPath expression.
     If the XPath expression returns a scalar value rather than a node-set,
     a single-element array is returned.
    </para>

    <para>
     The second argument must be a well formed XML document. In particular,
     it must have a single root node element.
    </para>

    <para>
     The optional third argument of the function is an array of namespace
     mappings.  This array should be a two-dimensional <type>text</type> array with
     the length of the second axis being equal to 2 (i.e., it should be an
     array of arrays, each of which consists of exactly 2 elements).
     The first element of each array entry is the namespace name (alias), the
     second the namespace URI. It is not required that aliases provided in
     this array be the same as those being used in the XML document itself (in
     other words, both in the XML document and in the <function>xpath</function>
     function context, aliases are <emphasis>local</emphasis>).
    </para>

    <para>
     Example:
<screen><![CDATA[
SELECT xpath('/my:a/text()', '<my:a xmlns:my="http://example.com">test</my:a>',
             ARRAY[ARRAY['my', 'http://example.com']]);

 xpath  
--------
 {test}
(1 row)
]]></screen>
    </para>

    <para>
     To deal with default (anonymous) namespaces, do something like this:
<screen><![CDATA[
SELECT xpath('//mydefns:b/text()', '<a xmlns="http://example.com"><b>test</b></a>',
             ARRAY[ARRAY['mydefns', 'http://example.com']]);

 xpath
--------
 {test}
(1 row)
]]></screen>
    </para>
   </sect3>

   <sect3 id="functions-xml-processing-xpath-exists">
    <title><literal>xpath_exists</literal></title>

    <indexterm>
     <primary>xpath_exists</primary>
    </indexterm>

<synopsis>
<function>xpath_exists</function> ( <parameter>xpath</parameter> <type>text</type>, <parameter>xml</parameter> <type>xml</type> <optional>, <parameter>nsarray</parameter> <type>text[]</type> </optional> ) <returnvalue>boolean</returnvalue>
</synopsis>

    <para>
     The function <function>xpath_exists</function> is a specialized form
     of the <function>xpath</function> function.  Instead of returning the
     individual XML values that satisfy the XPath 1.0 expression, this function
     returns a Boolean indicating whether the query was satisfied or not
     (specifically, whether it produced any value other than an empty node-set).
     This function is equivalent to the <literal>XMLEXISTS</literal> predicate,
     except that it also offers support for a namespace mapping argument.
    </para>

    <para>
     Example:
<screen><![CDATA[
SELECT xpath_exists('/my:a/text()', '<my:a xmlns:my="http://example.com">test</my:a>',
                     ARRAY[ARRAY['my', 'http://example.com']]);

 xpath_exists  
--------------
 t
(1 row)
]]></screen>
    </para>
   </sect3>

   <sect3 id="functions-xml-processing-xmltable">
    <title><literal>xmltable</literal></title>

    <indexterm>
     <primary>xmltable</primary>
    </indexterm>

    <indexterm zone="functions-xml-processing-xmltable">
     <primary>table function</primary>
     <secondary>XMLTABLE</secondary>
    </indexterm>

<synopsis>
<function>XMLTABLE</function> (
    <optional> <literal>XMLNAMESPACES</literal> ( <replaceable>namespace_uri</replaceable> <literal>AS</literal> <replaceable>namespace_name</replaceable> <optional>, ...</optional> ), </optional>
    <replaceable>row_expression</replaceable> <literal>PASSING</literal> <optional><literal>BY</literal> {<literal>REF</literal>|<literal>VALUE</literal>}</optional> <replaceable>document_expression</replaceable> <optional><literal>BY</literal> {<literal>REF</literal>|<literal>VALUE</literal>}</optional>
    <literal>COLUMNS</literal> <replaceable>name</replaceable> { <replaceable>type</replaceable> <optional><literal>PATH</literal> <replaceable>column_expression</replaceable></optional> <optional><literal>DEFAULT</literal> <replaceable>default_expression</replaceable></optional> <optional><literal>NOT NULL</literal> | <literal>NULL</literal></optional>
                  | <literal>FOR ORDINALITY</literal> }
            <optional>, ...</optional>
) <returnvalue>setof record</returnvalue>
</synopsis>

    <para>
     The <function>xmltable</function> expression produces a table based
     on an XML value, an XPath filter to extract rows, and a
     set of column definitions.
     Although it syntactically resembles a function, it can only appear
     as a table in a query's <literal>FROM</literal> clause.
    </para>

    <para>
     The optional <literal>XMLNAMESPACES</literal> clause gives a
     comma-separated list of namespace definitions, where
     each <replaceable>namespace_uri</replaceable> is a <type>text</type>
     expression and each <replaceable>namespace_name</replaceable> is a simple
     identifier.  It specifies the XML namespaces used in the document and
     their aliases. A default namespace specification is not currently
     supported.
    </para>

    <para>
     The required <replaceable>row_expression</replaceable> argument is an
     XPath 1.0 expression (given as <type>text</type>) that is evaluated,
     passing the XML value <replaceable>document_expression</replaceable> as
     its context item, to obtain a set of XML nodes. These nodes are what
     <function>xmltable</function> transforms into output rows. No rows
     will be produced if the <replaceable>document_expression</replaceable>
     is null, nor if the <replaceable>row_expression</replaceable> produces
     an empty node-set or any value other than a node-set.
    </para>

    <para>
     <replaceable>document_expression</replaceable> provides the context
     item for the <replaceable>row_expression</replaceable>. It must be a
     well-formed XML document; fragments/forests are not accepted.
     The <literal>BY REF</literal> and <literal>BY VALUE</literal> clauses
     are accepted but ignored, as discussed in
     <xref linkend="functions-xml-limits-postgresql"/>.
    </para>

    <para>
     In the SQL standard, the <function>xmltable</function> function
     evaluates expressions in the XML Query language,
     but <productname>PostgreSQL</productname> allows only XPath 1.0
     expressions, as discussed in
     <xref linkend="functions-xml-limits-xpath1"/>.
    </para>

    <para>
     The required <literal>COLUMNS</literal> clause specifies the
     column(s) that will be produced in the output table.
     See the syntax summary above for the format.
     A name is required for each column, as is a data type
     (unless <literal>FOR ORDINALITY</literal> is specified, in which case
     type <type>integer</type> is implicit).  The path, default and
     nullability clauses are optional.
    </para>

    <para>
     A column marked <literal>FOR ORDINALITY</literal> will be populated
     with row numbers, starting with 1, in the order of nodes retrieved from
     the <replaceable>row_expression</replaceable>'s result node-set.
     At most one column may be marked <literal>FOR ORDINALITY</literal>.
    </para>

    <note>
     <para>
      XPath 1.0 does not specify an order for nodes in a node-set, so code
      that relies on a particular order of the results will be
      implementation-dependent.  Details can be found in
      <xref linkend="xml-xpath-1-specifics"/>.
     </para>
    </note>

    <para>
     The <replaceable>column_expression</replaceable> for a column is an
     XPath 1.0 expression that is evaluated for each row, with the current
     node from the <replaceable>row_expression</replaceable> result as its
     context item, to find the value of the column.  If
     no <replaceable>column_expression</replaceable> is given, then the
     column name is used as an implicit path.
    </para>

    <para>
     If a column's XPath expression returns a non-XML value (which is limited
     to string, boolean, or double in XPath 1.0) and the column has a
     PostgreSQL type other than <type>xml</type>, the column will be set
     as if by assigning the value's string representation to the PostgreSQL
     type.  (If the value is a boolean, its string representation is taken
     to be <literal>1</literal> or <literal>0</literal> if the output
     column's type category is numeric, otherwise <literal>true</literal> or
     <literal>false</literal>.)
    </para>

    <para>
     If a column's XPath expression returns a non-empty set of XML nodes
     and the column's PostgreSQL type is <type>xml</type>, the column will
     be assigned the expression result exactly, if it is of document or
     content form.
     <footnote>
      <para>
       A result containing more than one element node at the top level, or
       non-whitespace text outside of an element, is an example of content form.
       An XPath result can be of neither form, for example if it returns an
       attribute node selected from the element that contains it. Such a result
       will be put into content form with each such disallowed node replaced by
       its string value, as defined for the XPath 1.0
       <function>string</function> function.
      </para>
     </footnote>
    </para>

    <para>
     A non-XML result assigned to an <type>xml</type> output column produces
     content, a single text node with the string value of the result.
     An XML result assigned to a column of any other type may not have more than
     one node, or an error is raised. If there is exactly one node, the column
     will be set as if by assigning the node's string
     value (as defined for the XPath 1.0 <function>string</function> function)
     to the PostgreSQL type.
    </para>

    <para>
     The string value of an XML element is the concatenation, in document order,
     of all text nodes contained in that element and its descendants. The string
     value of an element with no descendant text nodes is an
     empty string (not <literal>NULL</literal>).
     Any <literal>xsi:nil</literal> attributes are ignored.
     Note that the whitespace-only <literal>text()</literal> node between two non-text
     elements is preserved, and that leading whitespace on a <literal>text()</literal>
     node is not flattened.
     The XPath 1.0 <function>string</function> function may be consulted for the
     rules defining the string value of other XML node types and non-XML values.
    </para>

    <para>
     The conversion rules presented here are not exactly those of the SQL
     standard, as discussed in <xref linkend="functions-xml-limits-casts"/>.
    </para>

    <para>
     If the path expression returns an empty node-set
     (typically, when it does not match)
     for a given row, the column will be set to <literal>NULL</literal>, unless
     a <replaceable>default_expression</replaceable> is specified; then the
     value resulting from evaluating that expression is used.
    </para>

    <para>
     A <replaceable>default_expression</replaceable>, rather than being
     evaluated immediately when <function>xmltable</function> is called,
     is evaluated each time a default is needed for the column.
     If the expression qualifies as stable or immutable, the repeat
     evaluation may be skipped.
     This means that you can usefully use volatile functions like
     <function>nextval</function> in
     <replaceable>default_expression</replaceable>.
    </para>

    <para>
     Columns may be marked <literal>NOT NULL</literal>. If the
     <replaceable>column_expression</replaceable> for a <literal>NOT
     NULL</literal> column does not match anything and there is
     no <literal>DEFAULT</literal> or
     the <replaceable>default_expression</replaceable> also evaluates to null,
     an error is reported.
    </para>

    <para>
     Examples:
  <screen><![CDATA[
CREATE TABLE xmldata AS SELECT
xml $$
<ROWS>
  <ROW id="1">
    <COUNTRY_ID>AU</COUNTRY_ID>
    <COUNTRY_NAME>Australia</COUNTRY_NAME>
  </ROW>
  <ROW id="5">
    <COUNTRY_ID>JP</COUNTRY_ID>
    <COUNTRY_NAME>Japan</COUNTRY_NAME>
    <PREMIER_NAME>Shinzo Abe</PREMIER_NAME>
    <SIZE unit="sq_mi">145935</SIZE>
  </ROW>
  <ROW id="6">
    <COUNTRY_ID>SG</COUNTRY_ID>
    <COUNTRY_NAME>Singapore</COUNTRY_NAME>
    <SIZE unit="sq_km">697</SIZE>
  </ROW>
</ROWS>
$$ AS data;

SELECT xmltable.*
  FROM xmldata,
       XMLTABLE('//ROWS/ROW'
                PASSING data
                COLUMNS id int PATH '@id',
                        ordinality FOR ORDINALITY,
                        "COUNTRY_NAME" text,
                        country_id text PATH 'COUNTRY_ID',
                        size_sq_km float PATH 'SIZE[@unit = "sq_km"]',
                        size_other text PATH
                             'concat(SIZE[@unit!="sq_km"], " ", SIZE[@unit!="sq_km"]/@unit)',
                        premier_name text PATH 'PREMIER_NAME' DEFAULT 'not specified');

 id | ordinality | COUNTRY_NAME | country_id | size_sq_km |  size_other  | premier_name  
----+------------+--------------+------------+------------+--------------+---------------
  1 |          1 | Australia    | AU         |            |              | not specified
  5 |          2 | Japan        | JP         |            | 145935 sq_mi | Shinzo Abe
  6 |          3 | Singapore    | SG         |        697 |              | not specified
]]></screen>

     The following example shows concatenation of multiple text() nodes,
     usage of the column name as XPath filter, and the treatment of whitespace,
     XML comments and processing instructions:

  <screen><![CDATA[
CREATE TABLE xmlelements AS SELECT
xml $$
  <root>
   <element>  Hello<!-- xyxxz -->2a2<?aaaaa?> <!--x-->  bbb<x>xxx</x>CC  </element>
  </root>
$$ AS data;

SELECT xmltable.*
  FROM xmlelements, XMLTABLE('/root' PASSING data COLUMNS element text);
         element         
-------------------------
   Hello2a2   bbbxxxCC  
]]></screen>
    </para>

    <para>
     The following example illustrates how
     the <literal>XMLNAMESPACES</literal> clause can be used to specify
     a list of namespaces
     used in the XML document as well as in the XPath expressions:

  <screen><![CDATA[
WITH xmldata(data) AS (VALUES ('
<example xmlns="http://example.com/myns" xmlns:B="http://example.com/b">
 <item foo="1" B:bar="2"/>
 <item foo="3" B:bar="4"/>
 <item foo="4" B:bar="5"/>
</example>'::xml)
)
SELECT xmltable.*
  FROM XMLTABLE(XMLNAMESPACES('http://example.com/myns' AS x,
                              'http://example.com/b' AS "B"),
             '/x:example/x:item'
                PASSING (SELECT data FROM xmldata)
                COLUMNS foo int PATH '@foo',
                  bar int PATH '@B:bar');
 foo | bar
-----+-----
   1 |   2
   3 |   4
   4 |   5
(3 rows)
]]></screen>
    </para>
   </sect3>
  </sect2>

  <sect2 id="functions-xml-mapping">
   <title>Mapping Tables to XML</title>

   <indexterm zone="functions-xml-mapping">
    <primary>XML export</primary>
   </indexterm>

   <para>
    The following functions map the contents of relational tables to
    XML values.  They can be thought of as XML export functionality:
<synopsis>
<function>table_to_xml</function> ( <parameter>table</parameter> <type>regclass</type>, <parameter>nulls</parameter> <type>boolean</type>,
               <parameter>tableforest</parameter> <type>boolean</type>, <parameter>targetns</parameter> <type>text</type> ) <returnvalue>xml</returnvalue>
<function>query_to_xml</function> ( <parameter>query</parameter> <type>text</type>, <parameter>nulls</parameter> <type>boolean</type>,
               <parameter>tableforest</parameter> <type>boolean</type>, <parameter>targetns</parameter> <type>text</type> ) <returnvalue>xml</returnvalue>
<function>cursor_to_xml</function> ( <parameter>cursor</parameter> <type>refcursor</type>, <parameter>count</parameter> <type>integer</type>, <parameter>nulls</parameter> <type>boolean</type>,
                <parameter>tableforest</parameter> <type>boolean</type>, <parameter>targetns</parameter> <type>text</type> ) <returnvalue>xml</returnvalue>
</synopsis>
   </para>

   <para>
    <function>table_to_xml</function> maps the content of the named
    table, passed as parameter <parameter>table</parameter>.  The
    <type>regclass</type> type accepts strings identifying tables using the
    usual notation, including optional schema qualifications and
    double quotes.  <function>query_to_xml</function> executes the
    query whose text is passed as parameter
    <parameter>query</parameter> and maps the result set.
    <function>cursor_to_xml</function> fetches the indicated number of
    rows from the cursor specified by the parameter
    <parameter>cursor</parameter>.  This variant is recommended if
    large tables have to be mapped, because the result value is built
    up in memory by each function.
   </para>

   <para>
    If <parameter>tableforest</parameter> is false, then the resulting
    XML document looks like this:
<screen><![CDATA[
<tablename>
  <row>
    <columnname1>data</columnname1>
    <columnname2>data</columnname2>
  </row>

  <row>
    ...
  </row>

  ...
</tablename>
]]></screen>

    If <parameter>tableforest</parameter> is true, the result is an
    XML content fragment that looks like this:
<screen><![CDATA[
<tablename>
  <columnname1>data</columnname1>
  <columnname2>data</columnname2>
</tablename>

<tablename>
  ...
</tablename>

...
]]></screen>

    If no table name is available, that is, when mapping a query or a
    cursor, the string <literal>table</literal> is used in the first
    format, <literal>row</literal> in the second format.
   </para>

   <para>
    The choice between these formats is up to the user.  The first
    format is a proper XML document, which will be important in many
    applications.  The second format tends to be more useful in the
    <function>cursor_to_xml</function> function if the result values are to be
    reassembled into one document later on.  The functions for
    producing XML content discussed above, in particular
    <function>xmlelement</function>, can be used to alter the results
    to taste.
   </para>

   <para>
    The data values are mapped in the same way as described for the
    function <function>xmlelement</function> above.
   </para>

   <para>
    The parameter <parameter>nulls</parameter> determines whether null
    values should be included in the output.  If true, null values in
    columns are represented as:
<screen><![CDATA[
<columnname xsi:nil="true"/>
]]></screen>
    where <literal>xsi</literal> is the XML namespace prefix for XML
    Schema Instance.  An appropriate namespace declaration will be
    added to the result value.  If false, columns containing null
    values are simply omitted from the output.
   </para>

   <para>
    The parameter <parameter>targetns</parameter> specifies the
    desired XML namespace of the result.  If no particular namespace
    is wanted, an empty string should be passed.
   </para>

   <para>
    The following functions return XML Schema documents describing the
    mappings performed by the corresponding functions above:
<synopsis>
<function>table_to_xmlschema</function> ( <parameter>table</parameter> <type>regclass</type>, <parameter>nulls</parameter> <type>boolean</type>,
                     <parameter>tableforest</parameter> <type>boolean</type>, <parameter>targetns</parameter> <type>text</type> ) <returnvalue>xml</returnvalue>
<function>query_to_xmlschema</function> ( <parameter>query</parameter> <type>text</type>, <parameter>nulls</parameter> <type>boolean</type>,
                     <parameter>tableforest</parameter> <type>boolean</type>, <parameter>targetns</parameter> <type>text</type> ) <returnvalue>xml</returnvalue>
<function>cursor_to_xmlschema</function> ( <parameter>cursor</parameter> <type>refcursor</type>, <parameter>nulls</parameter> <type>boolean</type>,
                      <parameter>tableforest</parameter> <type>boolean</type>, <parameter>targetns</parameter> <type>text</type> ) <returnvalue>xml</returnvalue>
</synopsis>
    It is essential that the same parameters are passed in order to
    obtain matching XML data mappings and XML Schema documents.
   </para>

   <para>
    The following functions produce XML data mappings and the
    corresponding XML Schema in one document (or forest), linked
    together.  They can be useful where self-contained and
    self-describing results are wanted:
<synopsis>
<function>table_to_xml_and_xmlschema</function> ( <parameter>table</parameter> <type>regclass</type>, <parameter>nulls</parameter> <type>boolean</type>,
                             <parameter>tableforest</parameter> <type>boolean</type>, <parameter>targetns</parameter> <type>text</type> ) <returnvalue>xml</returnvalue>
<function>query_to_xml_and_xmlschema</function> ( <parameter>query</parameter> <type>text</type>, <parameter>nulls</parameter> <type>boolean</type>,
                             <parameter>tableforest</parameter> <type>boolean</type>, <parameter>targetns</parameter> <type>text</type> ) <returnvalue>xml</returnvalue>
</synopsis>
   </para>

   <para>
    In addition, the following functions are available to produce
    analogous mappings of entire schemas or the entire current
    database:
<synopsis>
<function>schema_to_xml</function> ( <parameter>schema</parameter> <type>name</type>, <parameter>nulls</parameter> <type>boolean</type>,
                <parameter>tableforest</parameter> <type>boolean</type>, <parameter>targetns</parameter> <type>text</type> ) <returnvalue>xml</returnvalue>
<function>schema_to_xmlschema</function> ( <parameter>schema</parameter> <type>name</type>, <parameter>nulls</parameter> <type>boolean</type>,
                      <parameter>tableforest</parameter> <type>boolean</type>, <parameter>targetns</parameter> <type>text</type> ) <returnvalue>xml</returnvalue>
<function>schema_to_xml_and_xmlschema</function> ( <parameter>schema</parameter> <type>name</type>, <parameter>nulls</parameter> <type>boolean</type>,
                              <parameter>tableforest</parameter> <type>boolean</type>, <parameter>targetns</parameter> <type>text</type> ) <returnvalue>xml</returnvalue>

<function>database_to_xml</function> ( <parameter>nulls</parameter> <type>boolean</type>,
                  <parameter>tableforest</parameter> <type>boolean</type>, <parameter>targetns</parameter> <type>text</type> ) <returnvalue>xml</returnvalue>
<function>database_to_xmlschema</function> ( <parameter>nulls</parameter> <type>boolean</type>,
                        <parameter>tableforest</parameter> <type>boolean</type>, <parameter>targetns</parameter> <type>text</type> ) <returnvalue>xml</returnvalue>
<function>database_to_xml_and_xmlschema</function> ( <parameter>nulls</parameter> <type>boolean</type>,
                                <parameter>tableforest</parameter> <type>boolean</type>, <parameter>targetns</parameter> <type>text</type> ) <returnvalue>xml</returnvalue>
</synopsis>

    These functions ignore tables that are not readable by the current user.
    The database-wide functions additionally ignore schemas that the current
    user does not have <literal>USAGE</literal> (lookup) privilege for.
   </para>

   <para>
    Note that these potentially produce a lot of data, which needs to
    be built up in memory.  When requesting content mappings of large
    schemas or databases, it might be worthwhile to consider mapping the
    tables separately instead, possibly even through a cursor.
   </para>

   <para>
    The result of a schema content mapping looks like this:

<screen><![CDATA[
<schemaname>

table1-mapping

table2-mapping

...

</schemaname>]]></screen>

    where the format of a table mapping depends on the
    <parameter>tableforest</parameter> parameter as explained above.
   </para>

   <para>
    The result of a database content mapping looks like this:

<screen><![CDATA[
<dbname>

<schema1name>
  ...
</schema1name>

<schema2name>
  ...
</schema2name>

...

</dbname>]]></screen>

    where the schema mapping is as above.
   </para>

   <para>
    As an example of using the output produced by these functions,
    <xref linkend="xslt-xml-html"/> shows an XSLT stylesheet that
    converts the output of
    <function>table_to_xml_and_xmlschema</function> to an HTML
    document containing a tabular rendition of the table data.  In a
    similar manner, the results from these functions can be
    converted into other XML-based formats.
   </para>

   <example id="xslt-xml-html">
    <title>XSLT Stylesheet for Converting SQL/XML Output to HTML</title>
<programlisting><![CDATA[
<?xml version="1.0"?>
<xsl:stylesheet version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:xsd="http://www.w3.org/2001/XMLSchema"
    xmlns="http://www.w3.org/1999/xhtml"
>

  <xsl:output method="xml"
      doctype-system="http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"
      doctype-public="-//W3C/DTD XHTML 1.0 Strict//EN"
      indent="yes"/>

  <xsl:template match="/*">
    <xsl:variable name="schema" select="//xsd:schema"/>
    <xsl:variable name="tabletypename"
                  select="$schema/xsd:element[@name=name(current())]/@type"/>
    <xsl:variable name="rowtypename"
                  select="$schema/xsd:complexType[@name=$tabletypename]/xsd:sequence/xsd:element[@name='row']/@type"/>

    <html>
      <head>
        <title><xsl:value-of select="name(current())"/></title>
      </head>
      <body>
        <table>
          <tr>
            <xsl:for-each select="$schema/xsd:complexType[@name=$rowtypename]/xsd:sequence/xsd:element/@name">
              <th><xsl:value-of select="."/></th>
            </xsl:for-each>
          </tr>

          <xsl:for-each select="row">
            <tr>
              <xsl:for-each select="*">
                <td><xsl:value-of select="."/></td>
              </xsl:for-each>
            </tr>
          </xsl:for-each>
        </table>
      </body>
    </html>
  </xsl:template>

</xsl:stylesheet>
]]></programlisting>
   </example>
  </sect2>
 </sect1>

 <sect1 id="functions-json">
  <title>JSON Functions and Operators</title>

  <indexterm zone="functions-json">
   <primary>JSON</primary>
   <secondary>functions and operators</secondary>
  </indexterm>

  <para>
   This section describes:

   <itemizedlist>
    <listitem>
     <para>
      functions and operators for processing and creating JSON data
     </para>
    </listitem>
    <listitem>
     <para>
      the SQL/JSON path language
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   To learn more about the SQL/JSON standard, see
   <xref linkend="sqltr-19075-6"/>. For details on JSON types
   supported in <productname>PostgreSQL</productname>,
   see <xref linkend="datatype-json"/>.
  </para>

  <sect2 id="functions-json-processing">
   <title>Processing and Creating JSON Data</title>

  <para>
   <xref linkend="functions-json-op-table"/> shows the operators that
   are available for use with JSON data types (see <xref
   linkend="datatype-json"/>).
   In addition, the usual comparison operators shown in <xref
   linkend="functions-comparison-op-table"/> are available for
   <type>jsonb</type>, though not for <type>json</type>.  The comparison
   operators follow the ordering rules for B-tree operations outlined in
   <xref linkend="json-indexing"/>.
  </para>

  <table id="functions-json-op-table">
    <title><type>json</type> and <type>jsonb</type> Operators</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Operator
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>json</type> <literal>-&gt;</literal> <type>integer</type>
        <returnvalue>json</returnvalue>
       </para>
       <para role="func_signature">
        <type>jsonb</type> <literal>-&gt;</literal> <type>integer</type>
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Extracts <parameter>n</parameter>'th element of JSON array
        (array elements are indexed from zero, but negative integers count
        from the end).
       </para>
       <para>
        <literal>'[{"a":"foo"},{"b":"bar"},{"c":"baz"}]'::json -&gt; 2</literal>
        <returnvalue>{"c":"baz"}</returnvalue>
       </para>
       <para>
        <literal>'[{"a":"foo"},{"b":"bar"},{"c":"baz"}]'::json -&gt; -3</literal>
        <returnvalue>{"a":"foo"}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>json</type> <literal>-&gt;</literal> <type>text</type>
        <returnvalue>json</returnvalue>
       </para>
       <para role="func_signature">
        <type>jsonb</type> <literal>-&gt;</literal> <type>text</type>
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Extracts JSON object field with the given key.
       </para>
       <para>
        <literal>'{"a": {"b":"foo"}}'::json -&gt; 'a'</literal>
        <returnvalue>{"b":"foo"}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>json</type> <literal>-&gt;&gt;</literal> <type>integer</type>
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <type>jsonb</type> <literal>-&gt;&gt;</literal> <type>integer</type>
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Extracts <parameter>n</parameter>'th element of JSON array,
        as <type>text</type>.
       </para>
       <para>
        <literal>'[1,2,3]'::json -&gt;&gt; 2</literal>
        <returnvalue>3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>json</type> <literal>-&gt;&gt;</literal> <type>text</type>
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <type>jsonb</type> <literal>-&gt;&gt;</literal> <type>text</type>
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Extracts JSON object field with the given key, as <type>text</type>.
       </para>
       <para>
        <literal>'{"a":1,"b":2}'::json -&gt;&gt; 'b'</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>json</type> <literal>#&gt;</literal> <type>text[]</type>
        <returnvalue>json</returnvalue>
       </para>
       <para role="func_signature">
        <type>jsonb</type> <literal>#&gt;</literal> <type>text[]</type>
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Extracts JSON sub-object at the specified path, where path elements
        can be either field keys or array indexes.
       </para>
       <para>
        <literal>'{"a": {"b": ["foo","bar"]}}'::json #&gt; '{a,b,1}'</literal>
        <returnvalue>"bar"</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>json</type> <literal>#&gt;&gt;</literal> <type>text[]</type>
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <type>jsonb</type> <literal>#&gt;&gt;</literal> <type>text[]</type>
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Extracts JSON sub-object at the specified path as <type>text</type>.
       </para>
       <para>
        <literal>'{"a": {"b": ["foo","bar"]}}'::json #&gt;&gt; '{a,b,1}'</literal>
        <returnvalue>bar</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <note>
   <para>
    The field/element/path extraction operators return NULL, rather than
    failing, if the JSON input does not have the right structure to match
    the request; for example if no such key or array element exists.
   </para>
  </note>

  <para>
   Some further operators exist only for <type>jsonb</type>, as shown
   in <xref linkend="functions-jsonb-op-table"/>.
   <xref linkend="json-indexing"/>
   describes how these operators can be used to effectively search indexed
   <type>jsonb</type> data.
  </para>

  <table id="functions-jsonb-op-table">
    <title>Additional <type>jsonb</type> Operators</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Operator
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>jsonb</type> <literal>@&gt;</literal> <type>jsonb</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does the first JSON value contain the second?
        (See <xref linkend="json-containment"/> for details about containment.)
       </para>
       <para>
        <literal>'{"a":1, "b":2}'::jsonb &#64;&gt; '{"b":2}'::jsonb</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>jsonb</type> <literal>&lt;@</literal> <type>jsonb</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is the first JSON value contained in the second?
       </para>
       <para>
        <literal>'{"b":2}'::jsonb &lt;@ '{"a":1, "b":2}'::jsonb</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>jsonb</type> <literal>?</literal> <type>text</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does the text string exist as a top-level key or array element within
        the JSON value?
       </para>
       <para>
        <literal>'{"a":1, "b":2}'::jsonb ? 'b'</literal>
        <returnvalue>t</returnvalue>
       </para>
       <para>
        <literal>'["a", "b", "c"]'::jsonb ? 'b'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>jsonb</type> <literal>?|</literal> <type>text[]</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Do any of the strings in the text array exist as top-level keys or
        array elements?
       </para>
       <para>
        <literal>'{"a":1, "b":2, "c":3}'::jsonb ?| array['b', 'd']</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>jsonb</type> <literal>?&amp;</literal> <type>text[]</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Do all of the strings in the text array exist as top-level keys or
        array elements?
       </para>
       <para>
        <literal>'["a", "b", "c"]'::jsonb ?&amp; array['a', 'b']</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>jsonb</type> <literal>||</literal> <type>jsonb</type>
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Concatenates two <type>jsonb</type> values.
        Concatenating two objects generates an object with the union of their
        keys, taking the second object's value when there are duplicate keys.
        Does not operate recursively: only the top-level array or object
        structure is merged.
       </para>
       <para>
        <literal>'["a", "b"]'::jsonb || '["a", "d"]'::jsonb</literal>
        <returnvalue>["a", "b", "a", "d"]</returnvalue>
       </para>
       <para>
        <literal>'{"a": "b"}'::jsonb || '{"c": "d"}'::jsonb</literal>
        <returnvalue>{"a": "b", "c": "d"}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>jsonb</type> <literal>-</literal> <type>text</type>
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Deletes a key (and its value) from a JSON object, or matching string
        value(s) from a JSON array.
       </para>
       <para>
        <literal>'{"a": "b", "c": "d"}'::jsonb - 'a'</literal>
        <returnvalue>{"c": "d"}</returnvalue>
       </para>
       <para>
        <literal>'["a", "b", "c", "b"]'::jsonb - 'b'</literal>
        <returnvalue>["a", "c"]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>jsonb</type> <literal>-</literal> <type>text[]</type>
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Deletes all matching keys or array elements from the left operand.
       </para>
       <para>
        <literal>'{"a": "b", "c": "d"}'::jsonb - '{a,c}'::text[]</literal>
        <returnvalue>{}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>jsonb</type> <literal>-</literal> <type>integer</type>
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Deletes the array element with specified index (negative
        integers count from the end).  Throws an error if JSON value
        is not an array.
       </para>
       <para>
        <literal>'["a", "b"]'::jsonb - 1 </literal>
        <returnvalue>["a"]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>jsonb</type> <literal>#-</literal> <type>text[]</type>
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Deletes the field or array element at the specified path, where path
        elements can be either field keys or array indexes.
       </para>
       <para>
        <literal>'["a", {"b":1}]'::jsonb #- '{1,b}'</literal>
        <returnvalue>["a", {}]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>jsonb</type> <literal>@?</literal> <type>jsonpath</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does JSON path return any item for the specified JSON value?
       </para>
       <para>
        <literal>'{"a":[1,2,3,4,5]}'::jsonb @? '$.a[*] ? (@ > 2)'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>jsonb</type> <literal>@@</literal> <type>jsonpath</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Returns the result of a JSON path predicate check for the
        specified JSON value.  Only the first item of the result is taken into
        account.  If the result is not Boolean, then <literal>NULL</literal>
        is returned.
       </para>
       <para>
        <literal>'{"a":[1,2,3,4,5]}'::jsonb @@ '$.a[*] > 2'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <note>
   <para>
    The <type>jsonpath</type> operators <literal>@?</literal>
    and <literal>@@</literal> suppress the following errors: missing object
    field or array element, unexpected JSON item type, datetime and numeric
    errors.  The <type>jsonpath</type>-related functions described below can
    also be told to suppress these types of errors.  This behavior might be
    helpful when searching JSON document collections of varying structure.
   </para>
  </note>

  <para>
   <xref linkend="functions-json-creation-table"/> shows the functions that are
   available for constructing <type>json</type> and <type>jsonb</type> values.
  </para>

  <table id="functions-json-creation-table">
    <title>JSON Creation Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_json</primary>
        </indexterm>
        <function>to_json</function> ( <type>anyelement</type> )
        <returnvalue>json</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>to_jsonb</primary>
        </indexterm>
        <function>to_jsonb</function> ( <type>anyelement</type> )
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Converts any SQL value to <type>json</type> or <type>jsonb</type>.
        Arrays and composites are converted recursively to arrays and
        objects (multidimensional arrays become arrays of arrays in JSON).
        Otherwise, if there is a cast from the SQL data type
        to <type>json</type>, the cast function will be used to perform the
        conversion;<footnote>
         <para>
          For example, the <xref linkend="hstore"/> extension has a cast
          from <type>hstore</type> to <type>json</type>, so that
          <type>hstore</type> values converted via the JSON creation functions
          will be represented as JSON objects, not as primitive string values.
         </para>
        </footnote>
        otherwise, a scalar JSON value is produced.  For any scalar other than
        a number, a Boolean, or a null value, the text representation will be
        used, with escaping as necessary to make it a valid JSON string value.
       </para>
       <para>
        <literal>to_json('Fred said "Hi."'::text)</literal>
        <returnvalue>"Fred said \"Hi.\""</returnvalue>
       </para>
       <para>
        <literal>to_jsonb(row(42, 'Fred said "Hi."'::text))</literal>
        <returnvalue>{"f1": 42, "f2": "Fred said \"Hi.\""}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>array_to_json</primary>
        </indexterm>
        <function>array_to_json</function> ( <type>anyarray</type> <optional>, <type>boolean</type> </optional> )
        <returnvalue>json</returnvalue>
       </para>
       <para>
        Converts a SQL array to a JSON array.  The behavior is the same
        as <function>to_json</function> except that line feeds will be added
        between top-level array elements if the optional boolean parameter is
        true.
       </para>
       <para>
        <literal>array_to_json('{{1,5},{99,100}}'::int[])</literal>
        <returnvalue>[[1,5],[99,100]]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>row_to_json</primary>
        </indexterm>
        <function>row_to_json</function> ( <type>record</type> <optional>, <type>boolean</type> </optional> )
        <returnvalue>json</returnvalue>
       </para>
       <para>
        Converts a SQL composite value to a JSON object.  The behavior is the
        same as <function>to_json</function> except that line feeds will be
        added between top-level elements if the optional boolean parameter is
        true.
       </para>
       <para>
        <literal>row_to_json(row(1,'foo'))</literal>
        <returnvalue>{"f1":1,"f2":"foo"}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_build_array</primary>
        </indexterm>
        <function>json_build_array</function> ( <literal>VARIADIC</literal> <type>"any"</type> )
        <returnvalue>json</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_build_array</primary>
        </indexterm>
        <function>jsonb_build_array</function> ( <literal>VARIADIC</literal> <type>"any"</type> )
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Builds a possibly-heterogeneously-typed JSON array out of a variadic
        argument list.  Each argument is converted as
        per <function>to_json</function> or <function>to_jsonb</function>.
       </para>
       <para>
        <literal>json_build_array(1,2,'foo',4,5)</literal>
        <returnvalue>[1, 2, "foo", 4, 5]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_build_object</primary>
        </indexterm>
        <function>json_build_object</function> ( <literal>VARIADIC</literal> <type>"any"</type> )
        <returnvalue>json</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_build_object</primary>
        </indexterm>
        <function>jsonb_build_object</function> ( <literal>VARIADIC</literal> <type>"any"</type> )
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Builds a JSON object out of a variadic argument list.  By convention,
        the argument list consists of alternating keys and values.  Key
        arguments are coerced to text; value arguments are converted as
        per <function>to_json</function> or <function>to_jsonb</function>.
       </para>
       <para>
        <literal>json_build_object('foo',1,2,row(3,'bar'))</literal>
        <returnvalue>{"foo" : 1, "2" : {"f1":3,"f2":"bar"}}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_object</primary>
        </indexterm>
        <function>json_object</function> ( <type>text[]</type> )
        <returnvalue>json</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_object</primary>
        </indexterm>
        <function>jsonb_object</function> ( <type>text[]</type> )
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Builds a JSON object out of a text array.  The array must have either
        exactly one dimension with an even number of members, in which case
        they are taken as alternating key/value pairs, or two dimensions
        such that each inner array has exactly two elements, which
        are taken as a key/value pair.  All values are converted to JSON
        strings.
       </para>
       <para>
        <literal>json_object('{a, 1, b, "def", c, 3.5}')</literal>
        <returnvalue>{"a" : "1", "b" : "def", "c" : "3.5"}</returnvalue>
       </para>
        <para><literal>json_object('{{a, 1},{b, "def"},{c, 3.5}}')</literal>
        <returnvalue>{"a" : "1", "b" : "def", "c" : "3.5"}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>json_object</function> ( <parameter>keys</parameter> <type>text[]</type>, <parameter>values</parameter> <type>text[]</type> )
        <returnvalue>json</returnvalue>
       </para>
       <para role="func_signature">
        <function>jsonb_object</function> ( <parameter>keys</parameter> <type>text[]</type>, <parameter>values</parameter> <type>text[]</type> )
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        This form of <function>json_object</function> takes keys and values
        pairwise from separate text arrays.  Otherwise it is identical to
        the one-argument form.
       </para>
       <para>
        <literal>json_object('{a, b}', '{1,2}')</literal>
        <returnvalue>{"a": "1", "b": "2"}</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   <xref linkend="functions-json-processing-table"/> shows the functions that
   are available for processing <type>json</type> and <type>jsonb</type> values.
  </para>

  <table id="functions-json-processing-table">
    <title>JSON Processing Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_array_elements</primary>
        </indexterm>
        <function>json_array_elements</function> ( <type>json</type> )
        <returnvalue>setof json</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_array_elements</primary>
        </indexterm>
        <function>jsonb_array_elements</function> ( <type>jsonb</type> )
        <returnvalue>setof jsonb</returnvalue>
       </para>
       <para>
        Expands the top-level JSON array into a set of JSON values.
       </para>
       <para>
        <literal>select * from json_array_elements('[1,true, [2,false]]')</literal>
        <returnvalue></returnvalue>
<programlisting>
   value
-----------
 1
 true
 [2,false]
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_array_elements_text</primary>
        </indexterm>
        <function>json_array_elements_text</function> ( <type>json</type> )
        <returnvalue>setof text</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_array_elements_text</primary>
        </indexterm>
        <function>jsonb_array_elements_text</function> ( <type>jsonb</type> )
        <returnvalue>setof text</returnvalue>
       </para>
       <para>
        Expands the top-level JSON array into a set of <type>text</type> values.
       </para>
       <para>
        <literal>select * from json_array_elements_text('["foo", "bar"]')</literal>
        <returnvalue></returnvalue>
<programlisting>
   value
-----------
 foo
 bar
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_array_length</primary>
        </indexterm>
        <function>json_array_length</function> ( <type>json</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_array_length</primary>
        </indexterm>
        <function>jsonb_array_length</function> ( <type>jsonb</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the number of elements in the top-level JSON array.
       </para>
       <para>
        <literal>json_array_length('[1,2,3,{"f1":1,"f2":[5,6]},4]')</literal>
        <returnvalue>5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_each</primary>
        </indexterm>
        <function>json_each</function> ( <type>json</type> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>key</parameter> <type>text</type>,
        <parameter>value</parameter> <type>json</type> )
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_each</primary>
        </indexterm>
        <function>jsonb_each</function> ( <type>jsonb</type> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>key</parameter> <type>text</type>,
        <parameter>value</parameter> <type>jsonb</type> )
       </para>
       <para>
        Expands the top-level JSON object into a set of key/value pairs.
       </para>
       <para>
        <literal>select * from json_each('{"a":"foo", "b":"bar"}')</literal>
        <returnvalue></returnvalue>
<programlisting>
 key | value
-----+-------
 a   | "foo"
 b   | "bar"
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_each_text</primary>
        </indexterm>
        <function>json_each_text</function> ( <type>json</type> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>key</parameter> <type>text</type>,
        <parameter>value</parameter> <type>text</type> )
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_each_text</primary>
        </indexterm>
        <function>jsonb_each_text</function> ( <type>jsonb</type> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>key</parameter> <type>text</type>,
        <parameter>value</parameter> <type>text</type> )
       </para>
       <para>
        Expands the top-level JSON object into a set of key/value pairs.
        The returned <parameter>value</parameter>s will be of
        type <type>text</type>.
       </para>
       <para>
        <literal>select * from json_each_text('{"a":"foo", "b":"bar"}')</literal>
        <returnvalue></returnvalue>
<programlisting>
 key | value
-----+-------
 a   | foo
 b   | bar
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_extract_path</primary>
        </indexterm>
        <function>json_extract_path</function> ( <parameter>from_json</parameter> <type>json</type>, <literal>VARIADIC</literal> <parameter>path_elems</parameter> <type>text[]</type> )
        <returnvalue>json</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_extract_path</primary>
        </indexterm>
        <function>jsonb_extract_path</function> ( <parameter>from_json</parameter> <type>jsonb</type>, <literal>VARIADIC</literal> <parameter>path_elems</parameter> <type>text[]</type> )
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Extracts JSON sub-object at the specified path.
        (This is functionally equivalent to the <literal>#&gt;</literal>
        operator, but writing the path out as a variadic list can be more
        convenient in some cases.)
       </para>
       <para>
        <literal>json_extract_path('{"f2":{"f3":1},"f4":{"f5":99,"f6":"foo"}}', 'f4', 'f6')</literal>
        <returnvalue>"foo"</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_extract_path_text</primary>
        </indexterm>
        <function>json_extract_path_text</function> ( <parameter>from_json</parameter> <type>json</type>, <literal>VARIADIC</literal> <parameter>path_elems</parameter> <type>text[]</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_extract_path_text</primary>
        </indexterm>
        <function>jsonb_extract_path_text</function> ( <parameter>from_json</parameter> <type>jsonb</type>, <literal>VARIADIC</literal> <parameter>path_elems</parameter> <type>text[]</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Extracts JSON sub-object at the specified path as <type>text</type>.
        (This is functionally equivalent to the <literal>#&gt;&gt;</literal>
        operator.)
       </para>
       <para>
        <literal>json_extract_path_text('{"f2":{"f3":1},"f4":{"f5":99,"f6":"foo"}}', 'f4', 'f6')</literal>
        <returnvalue>foo</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_object_keys</primary>
        </indexterm>
        <function>json_object_keys</function> ( <type>json</type> )
        <returnvalue>setof text</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_object_keys</primary>
        </indexterm>
        <function>jsonb_object_keys</function> ( <type>jsonb</type> )
        <returnvalue>setof text</returnvalue>
       </para>
       <para>
        Returns the set of keys in the top-level JSON object.
       </para>
       <para>
        <literal>select * from json_object_keys('{"f1":"abc","f2":{"f3":"a", "f4":"b"}}')</literal>
        <returnvalue></returnvalue>
<programlisting>
 json_object_keys
------------------
 f1
 f2
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_populate_record</primary>
        </indexterm>
        <function>json_populate_record</function> ( <parameter>base</parameter> <type>anyelement</type>, <parameter>from_json</parameter> <type>json</type> )
        <returnvalue>anyelement</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_populate_record</primary>
        </indexterm>
        <function>jsonb_populate_record</function> ( <parameter>base</parameter> <type>anyelement</type>, <parameter>from_json</parameter> <type>jsonb</type> )
        <returnvalue>anyelement</returnvalue>
       </para>
       <para>
        Expands the top-level JSON object to a row having the composite type
        of the <parameter>base</parameter> argument.  The JSON object
        is scanned for fields whose names match column names of the output row
        type, and their values are inserted into those columns of the output.
        (Fields that do not correspond to any output column name are ignored.)
        In typical use, the value of <parameter>base</parameter> is just
        <literal>NULL</literal>, which means that any output columns that do
        not match any object field will be filled with nulls.  However,
        if <parameter>base</parameter> isn't <literal>NULL</literal> then
        the values it contains will be used for unmatched columns.
       </para>
       <para>
        To convert a JSON value to the SQL type of an output column, the
        following rules are applied in sequence:
        <itemizedlist spacing="compact">
         <listitem>
          <para>
           A JSON null value is converted to a SQL null in all cases.
          </para>
         </listitem>
         <listitem>
          <para>
           If the output column is of type <type>json</type>
           or <type>jsonb</type>, the JSON value is just reproduced exactly.
          </para>
         </listitem>
         <listitem>
          <para>
           If the output column is a composite (row) type, and the JSON value
           is a JSON object, the fields of the object are converted to columns
           of the output row type by recursive application of these rules.
          </para>
         </listitem>
         <listitem>
          <para>
           Likewise, if the output column is an array type and the JSON value
           is a JSON array, the elements of the JSON array are converted to
           elements of the output array by recursive application of these
           rules.
          </para>
         </listitem>
         <listitem>
          <para>
           Otherwise, if the JSON value is a string, the contents of the
           string are fed to the input conversion function for the column's
           data type.
          </para>
         </listitem>
         <listitem>
          <para>
           Otherwise, the ordinary text representation of the JSON value is
           fed to the input conversion function for the column's data type.
          </para>
         </listitem>
        </itemizedlist>
       </para>
       <para>
        While the example below uses a constant JSON value, typical use would
        be to reference a <type>json</type> or <type>jsonb</type> column
        laterally from another table in the query's <literal>FROM</literal>
        clause.  Writing <function>json_populate_record</function> in
        the <literal>FROM</literal> clause is good practice, since all of the
        extracted columns are available for use without duplicate function
        calls.
       </para>
       <para>
        <literal>create type subrowtype as (d int, e text);</literal>
        <literal>create type myrowtype as (a int, b text[], c subrowtype);</literal>
       </para>
       <para>
        <literal>select * from json_populate_record(null::myrowtype,
         '{"a": 1, "b": ["2", "a b"], "c": {"d": 4, "e": "a  b c"}, "x": "foo"}')</literal>
        <returnvalue></returnvalue>
<programlisting>
 a |   b       |      c
---+-----------+-------------
 1 | {2,"a b"} | (4,"a b c")
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_populate_recordset</primary>
        </indexterm>
        <function>json_populate_recordset</function> ( <parameter>base</parameter> <type>anyelement</type>, <parameter>from_json</parameter> <type>json</type> )
        <returnvalue>setof anyelement</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_populate_recordset</primary>
        </indexterm>
        <function>jsonb_populate_recordset</function> ( <parameter>base</parameter> <type>anyelement</type>, <parameter>from_json</parameter> <type>jsonb</type> )
        <returnvalue>setof anyelement</returnvalue>
       </para>
       <para>
        Expands the top-level JSON array of objects to a set of rows having
        the composite type of the <parameter>base</parameter> argument.
        Each element of the JSON array is processed as described above
        for <function>json[b]_populate_record</function>.
       </para>
       <para>
        <literal>create type twoints as (a int, b int);</literal>
       </para>
       <para>
        <literal>select * from json_populate_recordset(null::twoints, '[{"a":1,"b":2},{"a":3,"b":4}]')</literal>
        <returnvalue></returnvalue>
<programlisting>
 a | b
---+---
 1 | 2
 3 | 4
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_to_record</primary>
        </indexterm>
        <function>json_to_record</function> ( <type>json</type> )
        <returnvalue>record</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_to_record</primary>
        </indexterm>
        <function>jsonb_to_record</function> ( <type>jsonb</type> )
        <returnvalue>record</returnvalue>
       </para>
       <para>
        Expands the top-level JSON object to a row having the composite type
        defined by an <literal>AS</literal> clause.  (As with all functions
        returning <type>record</type>, the calling query must explicitly
        define the structure of the record with an <literal>AS</literal>
        clause.)  The output record is filled from fields of the JSON object,
        in the same way as described above
        for <function>json[b]_populate_record</function>.  Since there is no
        input record value, unmatched columns are always filled with nulls.
       </para>
       <para>
        <literal>create type myrowtype as (a int, b text);</literal>
       </para>
       <para>
        <literal>select * from json_to_record('{"a":1,"b":[1,2,3],"c":[1,2,3],"e":"bar","r": {"a": 123, "b": "a b c"}}') as x(a int, b text, c int[], d text, r myrowtype)</literal>
        <returnvalue></returnvalue>
<programlisting>
 a |    b    |    c    | d |       r
---+---------+---------+---+---------------
 1 | [1,2,3] | {1,2,3} |   | (123,"a b c")
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_to_recordset</primary>
        </indexterm>
        <function>json_to_recordset</function> ( <type>json</type> )
        <returnvalue>setof record</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_to_recordset</primary>
        </indexterm>
        <function>jsonb_to_recordset</function> ( <type>jsonb</type> )
        <returnvalue>setof record</returnvalue>
       </para>
       <para>
        Expands the top-level JSON array of objects to a set of rows having
        the composite type defined by an <literal>AS</literal> clause.  (As
        with all functions returning <type>record</type>, the calling query
        must explicitly define the structure of the record with
        an <literal>AS</literal> clause.)  Each element of the JSON array is
        processed as described above
        for <function>json[b]_populate_record</function>.
       </para>
       <para>
        <literal>select * from json_to_recordset('[{"a":1,"b":"foo"},{"a":"2","c":"bar"}]') as x(a int, b text)</literal>
        <returnvalue></returnvalue>
<programlisting>
 a |  b
---+-----
 1 | foo
 2 |
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>jsonb_set</primary>
        </indexterm>
        <function>jsonb_set</function> ( <parameter>target</parameter> <type>jsonb</type>, <parameter>path</parameter> <type>text[]</type>, <parameter>new_value</parameter> <type>jsonb</type> <optional>, <parameter>create_if_missing</parameter> <type>boolean</type> </optional> )
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Returns <parameter>target</parameter>
        with the item designated by <parameter>path</parameter>
        replaced by <parameter>new_value</parameter>, or with
        <parameter>new_value</parameter> added if
        <parameter>create_if_missing</parameter> is true (which is the
        default) and the item designated by <parameter>path</parameter>
        does not exist.
        All earlier steps in the path must exist, or
        the <parameter>target</parameter> is returned unchanged.
        As with the path oriented operators, negative integers that
        appear in the <parameter>path</parameter> count from the end
        of JSON arrays.
        If the last path step is an array index that is out of range,
        and <parameter>create_if_missing</parameter> is true, the new
        value is added at the beginning of the array if the index is negative,
        or at the end of the array if it is positive.
       </para>
       <para>
        <literal>jsonb_set('[{"f1":1,"f2":null},2,null,3]', '{0,f1}','[2,3,4]', false)</literal>
        <returnvalue>[{"f1": [2, 3, 4], "f2": null}, 2, null, 3]</returnvalue>
       </para>
       <para>
        <literal>jsonb_set('[{"f1":1,"f2":null},2]', '{0,f3}','[2,3,4]')</literal>
        <returnvalue>[{"f1": 1, "f2": null, "f3": [2, 3, 4]}, 2]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>jsonb_set_lax</primary>
        </indexterm>
        <function>jsonb_set_lax</function> ( <parameter>target</parameter> <type>jsonb</type>, <parameter>path</parameter> <type>text[]</type>, <parameter>new_value</parameter> <type>jsonb</type> <optional>, <parameter>create_if_missing</parameter> <type>boolean</type> <optional>, <parameter>null_value_treatment</parameter> <type>text</type> </optional></optional> )
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        If <parameter>new_value</parameter> is not <literal>NULL</literal>,
        behaves identically to <literal>jsonb_set</literal>. Otherwise behaves
        according to the value
        of <parameter>null_value_treatment</parameter> which must be one
        of <literal>'raise_exception'</literal>,
        <literal>'use_json_null'</literal>, <literal>'delete_key'</literal>, or
        <literal>'return_target'</literal>. The default is
        <literal>'use_json_null'</literal>.
       </para>
       <para>
        <literal>jsonb_set_lax('[{"f1":1,"f2":null},2,null,3]', '{0,f1}', null)</literal>
        <returnvalue>[{"f1":null,"f2":null},2,null,3]</returnvalue>
       </para>
       <para>
        <literal>jsonb_set_lax('[{"f1":99,"f2":null},2]', '{0,f3}', null, true, 'return_target')</literal>
        <returnvalue>[{"f1": 99, "f2": null}, 2]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>jsonb_insert</primary>
        </indexterm>
        <function>jsonb_insert</function> ( <parameter>target</parameter> <type>jsonb</type>, <parameter>path</parameter> <type>text[]</type>, <parameter>new_value</parameter> <type>jsonb</type> <optional>, <parameter>insert_after</parameter> <type>boolean</type> </optional> )
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Returns <parameter>target</parameter>
        with <parameter>new_value</parameter> inserted.  If the item
        designated by the <parameter>path</parameter> is an array
        element, <parameter>new_value</parameter> will be inserted before
        that item if <parameter>insert_after</parameter> is false (which
        is the default), or after it
        if <parameter>insert_after</parameter> is true.  If the item
        designated by the <parameter>path</parameter> is an object
        field, <parameter>new_value</parameter> will be inserted only if
        the object does not already contain that key.
        All earlier steps in the path must exist, or
        the <parameter>target</parameter> is returned unchanged.
        As with the path oriented operators, negative integers that
        appear in the <parameter>path</parameter> count from the end
        of JSON arrays.
        If the last path step is an array index that is out of range, the new
        value is added at the beginning of the array if the index is negative,
        or at the end of the array if it is positive.
       </para>
       <para>
        <literal>jsonb_insert('{"a": [0,1,2]}', '{a, 1}', '"new_value"')</literal>
        <returnvalue>{"a": [0, "new_value", 1, 2]}</returnvalue>
       </para>
       <para>
        <literal>jsonb_insert('{"a": [0,1,2]}', '{a, 1}', '"new_value"', true)</literal>
        <returnvalue>{"a": [0, 1, "new_value", 2]}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_strip_nulls</primary>
        </indexterm>
        <function>json_strip_nulls</function> ( <type>json</type> )
        <returnvalue>json</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_strip_nulls</primary>
        </indexterm>
        <function>jsonb_strip_nulls</function> ( <type>jsonb</type> )
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Deletes all object fields that have null values from the given JSON
        value, recursively.  Null values that are not object fields are
        untouched.
       </para>
       <para>
        <literal>json_strip_nulls('[{"f1":1,"f2":null},2,null,3]')</literal>
        <returnvalue>[{"f1":1},2,null,3]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>jsonb_path_exists</primary>
        </indexterm>
        <function>jsonb_path_exists</function> ( <parameter>target</parameter> <type>jsonb</type>, <parameter>path</parameter> <type>jsonpath</type> <optional>, <parameter>vars</parameter> <type>jsonb</type> <optional>, <parameter>silent</parameter> <type>boolean</type> </optional></optional> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Checks whether the JSON path returns any item for the specified JSON
        value.
        If the <parameter>vars</parameter> argument is specified, it must
        be a JSON object, and its fields provide named values to be
        substituted into the <type>jsonpath</type> expression.
        If the <parameter>silent</parameter> argument is specified and
        is <literal>true</literal>, the function suppresses the same errors
        as the <literal>@?</literal> and <literal>@@</literal> operators do.
       </para>
       <para>
        <literal>jsonb_path_exists('{"a":[1,2,3,4,5]}', '$.a[*] ? (@ >= $min &amp;&amp; @ &lt;= $max)', '{"min":2,"max":4}')</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>jsonb_path_match</primary>
        </indexterm>
        <function>jsonb_path_match</function> ( <parameter>target</parameter> <type>jsonb</type>, <parameter>path</parameter> <type>jsonpath</type> <optional>, <parameter>vars</parameter> <type>jsonb</type> <optional>, <parameter>silent</parameter> <type>boolean</type> </optional></optional> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Returns the result of a JSON path predicate check for the specified
        JSON value.  Only the first item of the result is taken into account.
        If the result is not Boolean, then <literal>NULL</literal> is returned.
        The optional <parameter>vars</parameter>
        and <parameter>silent</parameter> arguments act the same as
        for <function>jsonb_path_exists</function>.
       </para>
       <para>
        <literal>jsonb_path_match('{"a":[1,2,3,4,5]}', 'exists($.a[*] ? (@ >= $min &amp;&amp; @ &lt;= $max))', '{"min":2,"max":4}')</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>jsonb_path_query</primary>
        </indexterm>
        <function>jsonb_path_query</function> ( <parameter>target</parameter> <type>jsonb</type>, <parameter>path</parameter> <type>jsonpath</type> <optional>, <parameter>vars</parameter> <type>jsonb</type> <optional>, <parameter>silent</parameter> <type>boolean</type> </optional></optional> )
        <returnvalue>setof jsonb</returnvalue>
       </para>
       <para>
        Returns all JSON items returned by the JSON path for the specified
        JSON value.
        The optional <parameter>vars</parameter>
        and <parameter>silent</parameter> arguments act the same as
        for <function>jsonb_path_exists</function>.
       </para>
       <para>
        <literal>select * from jsonb_path_query('{"a":[1,2,3,4,5]}', '$.a[*] ? (@ >= $min &amp;&amp; @ &lt;= $max)', '{"min":2,"max":4}')</literal>
        <returnvalue></returnvalue>
<programlisting>
 jsonb_path_query
------------------
 2
 3
 4
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>jsonb_path_query_array</primary>
        </indexterm>
        <function>jsonb_path_query_array</function> ( <parameter>target</parameter> <type>jsonb</type>, <parameter>path</parameter> <type>jsonpath</type> <optional>, <parameter>vars</parameter> <type>jsonb</type> <optional>, <parameter>silent</parameter> <type>boolean</type> </optional></optional> )
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Returns all JSON items returned by the JSON path for the specified
        JSON value, as a JSON array.
        The optional <parameter>vars</parameter>
        and <parameter>silent</parameter> arguments act the same as
        for <function>jsonb_path_exists</function>.
       </para>
       <para>
        <literal>jsonb_path_query_array('{"a":[1,2,3,4,5]}', '$.a[*] ? (@ >= $min &amp;&amp; @ &lt;= $max)', '{"min":2,"max":4}')</literal>
        <returnvalue>[2, 3, 4]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>jsonb_path_query_first</primary>
        </indexterm>
        <function>jsonb_path_query_first</function> ( <parameter>target</parameter> <type>jsonb</type>, <parameter>path</parameter> <type>jsonpath</type> <optional>, <parameter>vars</parameter> <type>jsonb</type> <optional>, <parameter>silent</parameter> <type>boolean</type> </optional></optional> )
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Returns the first JSON item returned by the JSON path for the
        specified JSON value.  Returns <literal>NULL</literal> if there are no
        results.
        The optional <parameter>vars</parameter>
        and <parameter>silent</parameter> arguments act the same as
        for <function>jsonb_path_exists</function>.
       </para>
       <para>
        <literal>jsonb_path_query_first('{"a":[1,2,3,4,5]}', '$.a[*] ? (@ >= $min &amp;&amp; @ &lt;= $max)', '{"min":2,"max":4}')</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>jsonb_path_exists_tz</primary>
        </indexterm>
        <function>jsonb_path_exists_tz</function> ( <parameter>target</parameter> <type>jsonb</type>, <parameter>path</parameter> <type>jsonpath</type> <optional>, <parameter>vars</parameter> <type>jsonb</type> <optional>, <parameter>silent</parameter> <type>boolean</type> </optional></optional> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_path_match_tz</primary>
        </indexterm>
        <function>jsonb_path_match_tz</function> ( <parameter>target</parameter> <type>jsonb</type>, <parameter>path</parameter> <type>jsonpath</type> <optional>, <parameter>vars</parameter> <type>jsonb</type> <optional>, <parameter>silent</parameter> <type>boolean</type> </optional></optional> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_path_query_tz</primary>
        </indexterm>
        <function>jsonb_path_query_tz</function> ( <parameter>target</parameter> <type>jsonb</type>, <parameter>path</parameter> <type>jsonpath</type> <optional>, <parameter>vars</parameter> <type>jsonb</type> <optional>, <parameter>silent</parameter> <type>boolean</type> </optional></optional> )
        <returnvalue>setof jsonb</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_path_query_array_tz</primary>
        </indexterm>
        <function>jsonb_path_query_array_tz</function> ( <parameter>target</parameter> <type>jsonb</type>, <parameter>path</parameter> <type>jsonpath</type> <optional>, <parameter>vars</parameter> <type>jsonb</type> <optional>, <parameter>silent</parameter> <type>boolean</type> </optional></optional> )
        <returnvalue>jsonb</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_path_query_first_tz</primary>
        </indexterm>
        <function>jsonb_path_query_first_tz</function> ( <parameter>target</parameter> <type>jsonb</type>, <parameter>path</parameter> <type>jsonpath</type> <optional>, <parameter>vars</parameter> <type>jsonb</type> <optional>, <parameter>silent</parameter> <type>boolean</type> </optional></optional> )
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        These functions act like their counterparts described above without
        the <literal>_tz</literal> suffix, except that these functions support
        comparisons of date/time values that require timezone-aware
        conversions.  The example below requires interpretation of the
        date-only value <literal>2015-08-02</literal> as a timestamp with time
        zone, so the result depends on the current
        <xref linkend="guc-timezone"/> setting.  Due to this dependency, these
        functions are marked as stable, which means these functions cannot be
        used in indexes.  Their counterparts are immutable, and so can be used
        in indexes; but they will throw errors if asked to make such
        comparisons.
       </para>
       <para>
        <literal>jsonb_path_exists_tz('["2015-08-01 12:00:00 -05"]', '$[*] ? (@.datetime() &lt; "2015-08-02".datetime())')</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>jsonb_pretty</primary>
        </indexterm>
        <function>jsonb_pretty</function> ( <type>jsonb</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the given JSON value to pretty-printed, indented text.
       </para>
       <para>
        <literal>jsonb_pretty('[{"f1":1,"f2":null},2]')</literal>
        <returnvalue></returnvalue>
<programlisting>
[
    {
        "f1": 1,
        "f2": null
    },
    2
]
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_typeof</primary>
        </indexterm>
        <function>json_typeof</function> ( <type>json</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_typeof</primary>
        </indexterm>
        <function>jsonb_typeof</function> ( <type>jsonb</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the type of the top-level JSON value as a text string.
        Possible types are
        <literal>object</literal>, <literal>array</literal>,
        <literal>string</literal>, <literal>number</literal>,
        <literal>boolean</literal>, and <literal>null</literal>.
        (The <literal>null</literal> result should not be confused
        with a SQL NULL; see the examples.)
       </para>
       <para>
        <literal>json_typeof('-123.4')</literal>
        <returnvalue>number</returnvalue>
       </para>
       <para>
        <literal>json_typeof('null'::json)</literal>
        <returnvalue>null</returnvalue>
       </para>
       <para>
        <literal>json_typeof(NULL::json) IS NULL</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
    See also <xref linkend="functions-aggregate"/> for the aggregate
    function <function>json_agg</function> which aggregates record
    values as JSON, the aggregate function
    <function>json_object_agg</function> which aggregates pairs of values
    into a JSON object, and their <type>jsonb</type> equivalents,
    <function>jsonb_agg</function> and <function>jsonb_object_agg</function>.
  </para>
 </sect2>

 <sect2 id="functions-sqljson-path">
  <title>The SQL/JSON Path Language</title>

  <indexterm zone="functions-sqljson-path">
   <primary>SQL/JSON path language</primary>
  </indexterm>

  <para>
   SQL/JSON path expressions specify the items to be retrieved
   from the JSON data, similar to XPath expressions used
   for SQL access to XML. In <productname>PostgreSQL</productname>,
   path expressions are implemented as the <type>jsonpath</type>
   data type and can use any elements described in
   <xref linkend="datatype-jsonpath"/>.
  </para>

  <para>
   JSON query functions and operators
   pass the provided path expression to the <firstterm>path engine</firstterm>
   for evaluation. If the expression matches the queried JSON data,
   the corresponding JSON item, or set of items, is returned.
   Path expressions are written in the SQL/JSON path language
   and can include arithmetic expressions and functions.
  </para>

  <para>
   A path expression consists of a sequence of elements allowed
   by the <type>jsonpath</type> data type.
   The path expression is normally evaluated from left to right, but
   you can use parentheses to change the order of operations.
   If the evaluation is successful, a sequence of JSON items is produced,
   and the evaluation result is returned to the JSON query function
   that completes the specified computation.
  </para>

  <para>
   To refer to the JSON value being queried (the
   <firstterm>context item</firstterm>), use the <literal>$</literal> variable
   in the path expression. It can be followed by one or more
   <link linkend="type-jsonpath-accessors">accessor operators</link>,
   which go down the JSON structure level by level to retrieve sub-items
   of the context item. Each operator that follows deals with the
   result of the previous evaluation step.
  </para>

  <para>
   For example, suppose you have some JSON data from a GPS tracker that you
   would like to parse, such as:
<programlisting>
{
  "track": {
    "segments": [
      {
        "location":   [ 47.763, 13.4034 ],
        "start time": "2018-10-14 10:05:14",
        "HR": 73
      },
      {
        "location":   [ 47.706, 13.2635 ],
        "start time": "2018-10-14 10:39:21",
        "HR": 135
      }
    ]
  }
}
</programlisting>
  </para>

  <para>
   To retrieve the available track segments, you need to use the
   <literal>.<replaceable>key</replaceable></literal> accessor
   operator to descend through surrounding JSON objects:
<programlisting>
$.track.segments
</programlisting>
  </para>

  <para>
   To retrieve the contents of an array, you typically use the
   <literal>[*]</literal> operator. For example,
   the following path will return the location coordinates for all
   the available track segments:
<programlisting>
$.track.segments[*].location
</programlisting>
  </para>

  <para>
   To return the coordinates of the first segment only, you can
   specify the corresponding subscript in the <literal>[]</literal>
   accessor operator. Recall that JSON array indexes are 0-relative:
<programlisting>
$.track.segments[0].location
</programlisting>
  </para>

  <para>
   The result of each path evaluation step can be processed
   by one or more <type>jsonpath</type> operators and methods
   listed in <xref linkend="functions-sqljson-path-operators"/>.
   Each method name must be preceded by a dot. For example,
   you can get the size of an array:
<programlisting>
$.track.segments.size()
</programlisting>
   More examples of using <type>jsonpath</type> operators
   and methods within path expressions appear below in
   <xref linkend="functions-sqljson-path-operators"/>.
  </para>

  <para>
   When defining a path, you can also use one or more
   <firstterm>filter expressions</firstterm> that work similarly to the
   <literal>WHERE</literal> clause in SQL. A filter expression begins with
   a question mark and provides a condition in parentheses:

<programlisting>
? (<replaceable>condition</replaceable>)
</programlisting>
  </para>

  <para>
   Filter expressions must be written just after the path evaluation step
   to which they should apply. The result of that step is filtered to include
   only those items that satisfy the provided condition. SQL/JSON defines
   three-valued logic, so the condition can be <literal>true</literal>, <literal>false</literal>,
   or <literal>unknown</literal>. The <literal>unknown</literal> value
   plays the same role as SQL <literal>NULL</literal> and can be tested
   for with the <literal>is unknown</literal> predicate. Further path
   evaluation steps use only those items for which the filter expression
   returned <literal>true</literal>.
  </para>

  <para>
   The functions and operators that can be used in filter expressions are
   listed in <xref linkend="functions-sqljson-filter-ex-table"/>.  Within a
   filter expression, the <literal>@</literal> variable denotes the value
   being filtered (i.e., one result of the preceding path step).  You can
   write accessor operators after <literal>@</literal> to retrieve component
   items.
  </para>

  <para>
   For example, suppose you would like to retrieve all heart rate values higher
   than 130. You can achieve this using the following expression:
<programlisting>
$.track.segments[*].HR ? (@ &gt; 130)
</programlisting>
  </para>

  <para>
   To get the start times of segments with such values, you have to
   filter out irrelevant segments before returning the start times, so the
   filter expression is applied to the previous step, and the path used
   in the condition is different:
<programlisting>
$.track.segments[*] ? (@.HR &gt; 130)."start time"
</programlisting>
  </para>

  <para>
   You can use several filter expressions in sequence, if required. For
   example, the following expression selects start times of all segments that
   contain locations with relevant coordinates and high heart rate values:
<programlisting>
$.track.segments[*] ? (@.location[1] &lt; 13.4) ? (@.HR &gt; 130)."start time"
</programlisting>
  </para>

  <para>
   Using filter expressions at different nesting levels is also allowed.
   The following example first filters all segments by location, and then
   returns high heart rate values for these segments, if available:
<programlisting>
$.track.segments[*] ? (@.location[1] &lt; 13.4).HR ? (@ &gt; 130)
</programlisting>
  </para>

  <para>
   You can also nest filter expressions within each other:
<programlisting>
$.track ? (exists(@.segments[*] ? (@.HR &gt; 130))).segments.size()
</programlisting>
   This expression returns the size of the track if it contains any
   segments with high heart rate values, or an empty sequence otherwise.
  </para>

  <para>
   <productname>PostgreSQL</productname>'s implementation of the SQL/JSON path
   language has the following deviations from the SQL/JSON standard:
  </para>

  <itemizedlist>
   <listitem>
    <para>
     A path expression can be a Boolean predicate, although the SQL/JSON
     standard allows predicates only in filters.  This is necessary for
     implementation of the <literal>@@</literal> operator. For example,
     the following <type>jsonpath</type> expression is valid in
     <productname>PostgreSQL</productname>:
<programlisting>
$.track.segments[*].HR &lt; 70
</programlisting>
    </para>
   </listitem>

   <listitem>
    <para>
     There are minor differences in the interpretation of regular
     expression patterns used in <literal>like_regex</literal> filters, as
     described in <xref linkend="jsonpath-regular-expressions"/>.
    </para>
   </listitem>
  </itemizedlist>

   <sect3 id="strict-and-lax-modes">
   <title>Strict and Lax Modes</title>
    <para>
     When you query JSON data, the path expression may not match the
     actual JSON data structure. An attempt to access a non-existent
     member of an object or element of an array results in a
     structural error. SQL/JSON path expressions have two modes
     of handling structural errors:
    </para>

   <itemizedlist>
    <listitem>
     <para>
      lax (default) &mdash; the path engine implicitly adapts
      the queried data to the specified path.
      Any remaining structural errors are suppressed and converted
      to empty SQL/JSON sequences.
     </para>
    </listitem>
    <listitem>
     <para>
      strict &mdash; if a structural error occurs, an error is raised.
     </para>
    </listitem>
   </itemizedlist>

   <para>
    The lax mode facilitates matching of a JSON document structure and path
    expression if the JSON data does not conform to the expected schema.
    If an operand does not match the requirements of a particular operation,
    it can be automatically wrapped as an SQL/JSON array or unwrapped by
    converting its elements into an SQL/JSON sequence before performing
    this operation. Besides, comparison operators automatically unwrap their
    operands in the lax mode, so you can compare SQL/JSON arrays
    out-of-the-box. An array of size 1 is considered equal to its sole element.
    Automatic unwrapping is not performed only when:
    <itemizedlist>
     <listitem>
      <para>
       The path expression contains <literal>type()</literal> or
       <literal>size()</literal> methods that return the type
       and the number of elements in the array, respectively.
      </para>
     </listitem>
     <listitem>
      <para>
       The queried JSON data contain nested arrays. In this case, only
       the outermost array is unwrapped, while all the inner arrays
       remain unchanged. Thus, implicit unwrapping can only go one
       level down within each path evaluation step.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    For example, when querying the GPS data listed above, you can
    abstract from the fact that it stores an array of segments
    when using the lax mode:
<programlisting>
lax $.track.segments.location
</programlisting>
   </para>

   <para>
    In the strict mode, the specified path must exactly match the structure of
    the queried JSON document to return an SQL/JSON item, so using this
    path expression will cause an error. To get the same result as in
    the lax mode, you have to explicitly unwrap the
    <literal>segments</literal> array:
<programlisting>
strict $.track.segments[*].location
</programlisting>
   </para>

   </sect3>

   <sect3 id="functions-sqljson-path-operators">
   <title>SQL/JSON Path Operators and Methods</title>

   <para>
    <xref linkend="functions-sqljson-op-table"/> shows the operators and
    methods available in <type>jsonpath</type>.  Note that while the unary
    operators and methods can be applied to multiple values resulting from a
    preceding path step, the binary operators (addition etc.) can only be
    applied to single values.
   </para>

   <table id="functions-sqljson-op-table">
    <title><type>jsonpath</type> Operators and Methods</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Operator/Method
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>number</replaceable> <literal>+</literal> <replaceable>number</replaceable>
        <returnvalue><replaceable>number</replaceable></returnvalue>
       </para>
       <para>
        Addition
       </para>
       <para>
        <literal>jsonb_path_query('[2]', '$[0] + 3')</literal>
        <returnvalue>5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>+</literal> <replaceable>number</replaceable>
        <returnvalue><replaceable>number</replaceable></returnvalue>
       </para>
       <para>
        Unary plus (no operation); unlike addition, this can iterate over
        multiple values
       </para>
       <para>
        <literal>jsonb_path_query_array('{"x": [2,3,4]}', '+ $.x')</literal>
        <returnvalue>[2, 3, 4]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>number</replaceable> <literal>-</literal> <replaceable>number</replaceable>
        <returnvalue><replaceable>number</replaceable></returnvalue>
       </para>
       <para>
        Subtraction
       </para>
       <para>
        <literal>jsonb_path_query('[2]', '7 - $[0]')</literal>
        <returnvalue>5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>-</literal> <replaceable>number</replaceable>
        <returnvalue><replaceable>number</replaceable></returnvalue>
       </para>
       <para>
        Negation; unlike subtraction, this can iterate over
        multiple values
       </para>
       <para>
        <literal>jsonb_path_query_array('{"x": [2,3,4]}', '- $.x')</literal>
        <returnvalue>[-2, -3, -4]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>number</replaceable> <literal>*</literal> <replaceable>number</replaceable>
        <returnvalue><replaceable>number</replaceable></returnvalue>
       </para>
       <para>
        Multiplication
       </para>
       <para>
        <literal>jsonb_path_query('[4]', '2 * $[0]')</literal>
        <returnvalue>8</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>number</replaceable> <literal>/</literal> <replaceable>number</replaceable>
        <returnvalue><replaceable>number</replaceable></returnvalue>
       </para>
       <para>
        Division
       </para>
       <para>
        <literal>jsonb_path_query('[8.5]', '$[0] / 2')</literal>
        <returnvalue>4.2500000000000000</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>number</replaceable> <literal>%</literal> <replaceable>number</replaceable>
        <returnvalue><replaceable>number</replaceable></returnvalue>
       </para>
       <para>
        Modulo (remainder)
       </para>
       <para>
        <literal>jsonb_path_query('[32]', '$[0] % 10')</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>value</replaceable> <literal>.</literal> <literal>type()</literal>
        <returnvalue><replaceable>string</replaceable></returnvalue>
       </para>
       <para>
        Type of the JSON item (see <function>json_typeof</function>)
       </para>
       <para>
        <literal>jsonb_path_query_array('[1, "2", {}]', '$[*].type()')</literal>
        <returnvalue>["number", "string", "object"]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>value</replaceable> <literal>.</literal> <literal>size()</literal>
        <returnvalue><replaceable>number</replaceable></returnvalue>
       </para>
       <para>
        Size of the JSON item (number of array elements, or 1 if not an
        array)
       </para>
       <para>
        <literal>jsonb_path_query('{"m": [11, 15]}', '$.m.size()')</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>value</replaceable> <literal>.</literal> <literal>double()</literal>
        <returnvalue><replaceable>number</replaceable></returnvalue>
       </para>
       <para>
        Approximate floating-point number converted from a JSON number or
        string
       </para>
       <para>
        <literal>jsonb_path_query('{"len": "1.9"}', '$.len.double() * 2')</literal>
        <returnvalue>3.8</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>number</replaceable> <literal>.</literal> <literal>ceiling()</literal>
        <returnvalue><replaceable>number</replaceable></returnvalue>
       </para>
       <para>
        Nearest integer greater than or equal to the given number
       </para>
       <para>
        <literal>jsonb_path_query('{"h": 1.3}', '$.h.ceiling()')</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>number</replaceable> <literal>.</literal> <literal>floor()</literal>
        <returnvalue><replaceable>number</replaceable></returnvalue>
       </para>
       <para>
        Nearest integer less than or equal to the given number
       </para>
       <para>
        <literal>jsonb_path_query('{"h": 1.7}', '$.h.floor()')</literal>
        <returnvalue>1</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>number</replaceable> <literal>.</literal> <literal>abs()</literal>
        <returnvalue><replaceable>number</replaceable></returnvalue>
       </para>
       <para>
        Absolute value of the given number
       </para>
       <para>
        <literal>jsonb_path_query('{"z": -0.3}', '$.z.abs()')</literal>
        <returnvalue>0.3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>string</replaceable> <literal>.</literal> <literal>datetime()</literal>
        <returnvalue><replaceable>datetime_type</replaceable></returnvalue>
        (see note)
       </para>
       <para>
        Date/time value converted from a string
       </para>
       <para>
        <literal>jsonb_path_query('["2015-8-1", "2015-08-12"]', '$[*] ? (@.datetime() &lt; "2015-08-2".datetime())')</literal>
        <returnvalue>"2015-8-1"</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>string</replaceable> <literal>.</literal> <literal>datetime(<replaceable>template</replaceable>)</literal>
        <returnvalue><replaceable>datetime_type</replaceable></returnvalue>
        (see note)
       </para>
       <para>
        Date/time value converted from a string using the
        specified <function>to_timestamp</function> template
       </para>
       <para>
        <literal>jsonb_path_query_array('["12:30", "18:40"]', '$[*].datetime("HH24:MI")')</literal>
        <returnvalue>["12:30:00", "18:40:00"]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>object</replaceable> <literal>.</literal> <literal>keyvalue()</literal>
        <returnvalue><replaceable>array</replaceable></returnvalue>
       </para>
       <para>
        The object's key-value pairs, represented as an array of objects
        containing three fields: <literal>"key"</literal>,
        <literal>"value"</literal>, and <literal>"id"</literal>;
        <literal>"id"</literal> is a unique identifier of the object the
        key-value pair belongs to
       </para>
       <para>
        <literal>jsonb_path_query_array('{"x": "20", "y": 32}', '$.keyvalue()')</literal>
        <returnvalue>[{"id": 0, "key": "x", "value": "20"}, {"id": 0, "key": "y", "value": 32}]</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

    <note>
     <para>
      The result type of the <literal>datetime()</literal> and
      <literal>datetime(<replaceable>template</replaceable>)</literal>
      methods can be <type>date</type>, <type>timetz</type>, <type>time</type>,
      <type>timestamptz</type>, or <type>timestamp</type>.
      Both methods determine their result type dynamically.
     </para>
     <para>
      The <literal>datetime()</literal> method sequentially tries to
      match its input string to the ISO formats
      for <type>date</type>, <type>timetz</type>, <type>time</type>,
      <type>timestamptz</type>, and <type>timestamp</type>. It stops on
      the first matching format and emits the corresponding data type.
     </para>
     <para>
      The <literal>datetime(<replaceable>template</replaceable>)</literal>
      method determines the result type according to the fields used in the
      provided template string.
     </para>
     <para>
      The <literal>datetime()</literal> and
      <literal>datetime(<replaceable>template</replaceable>)</literal> methods
      use the same parsing rules as the <literal>to_timestamp</literal> SQL
      function does (see <xref linkend="functions-formatting"/>), with three
      exceptions.  First, these methods don't allow unmatched template
      patterns.  Second, only the following separators are allowed in the
      template string: minus sign, period, solidus (slash), comma, apostrophe,
      semicolon, colon and space.  Third, separators in the template string
      must exactly match the input string.
     </para>
     <para>
      If different date/time types need to be compared, an implicit cast is
      applied. A <type>date</type> value can be cast to <type>timestamp</type>
      or <type>timestamptz</type>, <type>timestamp</type> can be cast to
      <type>timestamptz</type>, and <type>time</type> to <type>timetz</type>.
      However, all but the first of these conversions depend on the current
      <xref linkend="guc-timezone"/> setting, and thus can only be performed
      within timezone-aware <type>jsonpath</type> functions.
     </para>
    </note>

   <para>
    <xref linkend="functions-sqljson-filter-ex-table"/> shows the available
    filter expression elements.
   </para>

   <table id="functions-sqljson-filter-ex-table">
    <title><type>jsonpath</type> Filter Expression Elements</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Predicate/Value
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>value</replaceable> <literal>==</literal> <replaceable>value</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Equality comparison (this, and the other comparison operators, work on
        all JSON scalar values)
       </para>
       <para>
        <literal>jsonb_path_query_array('[1, "a", 1, 3]', '$[*] ? (@ == 1)')</literal>
        <returnvalue>[1, 1]</returnvalue>
       </para>
       <para>
        <literal>jsonb_path_query_array('[1, "a", 1, 3]', '$[*] ? (@ == "a")')</literal>
        <returnvalue>["a"]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>value</replaceable> <literal>!=</literal> <replaceable>value</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para role="func_signature">
        <replaceable>value</replaceable> <literal>&lt;&gt;</literal> <replaceable>value</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Non-equality comparison
       </para>
       <para>
        <literal>jsonb_path_query_array('[1, 2, 1, 3]', '$[*] ? (@ != 1)')</literal>
        <returnvalue>[2, 3]</returnvalue>
       </para>
       <para>
        <literal>jsonb_path_query_array('["a", "b", "c"]', '$[*] ? (@ &lt;&gt; "b")')</literal>
        <returnvalue>["a", "c"]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>value</replaceable> <literal>&lt;</literal> <replaceable>value</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Less-than comparison
       </para>
       <para>
        <literal>jsonb_path_query_array('[1, 2, 3]', '$[*] ? (@ &lt; 2)')</literal>
        <returnvalue>[1]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>value</replaceable> <literal>&lt;=</literal> <replaceable>value</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Less-than-or-equal-to comparison
       </para>
       <para>
        <literal>jsonb_path_query_array('["a", "b", "c"]', '$[*] ? (@ &lt;= "b")')</literal>
        <returnvalue>["a", "b"]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>value</replaceable> <literal>&gt;</literal> <replaceable>value</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Greater-than comparison
       </para>
       <para>
        <literal>jsonb_path_query_array('[1, 2, 3]', '$[*] ? (@ &gt; 2)')</literal>
        <returnvalue>[3]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>value</replaceable> <literal>&gt;=</literal> <replaceable>value</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Greater-than-or-equal-to comparison
       </para>
       <para>
        <literal>jsonb_path_query_array('[1, 2, 3]', '$[*] ? (@ &gt;= 2)')</literal>
        <returnvalue>[2, 3]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>true</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        JSON constant <literal>true</literal>
       </para>
       <para>
        <literal>jsonb_path_query('[{"name": "John", "parent": false}, {"name": "Chris", "parent": true}]', '$[*] ? (@.parent == true)')</literal>
        <returnvalue>{"name": "Chris", "parent": true}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>false</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        JSON constant <literal>false</literal>
       </para>
       <para>
        <literal>jsonb_path_query('[{"name": "John", "parent": false}, {"name": "Chris", "parent": true}]', '$[*] ? (@.parent == false)')</literal>
        <returnvalue>{"name": "John", "parent": false}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>null</literal>
        <returnvalue><replaceable>value</replaceable></returnvalue>
       </para>
       <para>
        JSON constant <literal>null</literal> (note that, unlike in SQL,
        comparison to <literal>null</literal> works normally)
       </para>
       <para>
        <literal>jsonb_path_query('[{"name": "Mary", "job": null}, {"name": "Michael", "job": "driver"}]', '$[*] ? (@.job == null) .name')</literal>
        <returnvalue>"Mary"</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>boolean</replaceable> <literal>&amp;&amp;</literal> <replaceable>boolean</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Boolean AND
       </para>
       <para>
        <literal>jsonb_path_query('[1, 3, 7]', '$[*] ? (@ &gt; 1 &amp;&amp; @ &lt; 5)')</literal>
        <returnvalue>3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>boolean</replaceable> <literal>||</literal> <replaceable>boolean</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Boolean OR
       </para>
       <para>
        <literal>jsonb_path_query('[1, 3, 7]', '$[*] ? (@ &lt; 1 || @ &gt; 5)')</literal>
        <returnvalue>7</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>!</literal> <replaceable>boolean</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Boolean NOT
       </para>
       <para>
        <literal>jsonb_path_query('[1, 3, 7]', '$[*] ? (!(@ &lt; 5))')</literal>
        <returnvalue>7</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>boolean</replaceable> <literal>is unknown</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Tests whether a Boolean condition is <literal>unknown</literal>.
       </para>
       <para>
        <literal>jsonb_path_query('[-1, 2, 7, "foo"]', '$[*] ? ((@ > 0) is unknown)')</literal>
        <returnvalue>"foo"</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>string</replaceable> <literal>like_regex</literal> <replaceable>string</replaceable> <optional> <literal>flag</literal> <replaceable>string</replaceable> </optional>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Tests whether the first operand matches the regular expression
        given by the second operand, optionally with modifications
        described by a string of <literal>flag</literal> characters (see
        <xref linkend="jsonpath-regular-expressions"/>).
       </para>
       <para>
        <literal>jsonb_path_query_array('["abc", "abd", "aBdC", "abdacb", "babc"]', '$[*] ? (@ like_regex "^ab.*c")')</literal>
        <returnvalue>["abc", "abdacb"]</returnvalue>
       </para>
       <para>
        <literal>jsonb_path_query_array('["abc", "abd", "aBdC", "abdacb", "babc"]', '$[*] ? (@ like_regex "^ab.*c" flag "i")')</literal>
        <returnvalue>["abc", "aBdC", "abdacb"]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>string</replaceable> <literal>starts with</literal> <replaceable>string</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Tests whether the second operand is an initial substring of the first
        operand.
       </para>
       <para>
        <literal>jsonb_path_query('["John Smith", "Mary Stone", "Bob Johnson"]', '$[*] ? (@ starts with "John")')</literal>
        <returnvalue>"John Smith"</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <literal>exists</literal> <literal>(</literal> <replaceable>path_expression</replaceable> <literal>)</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Tests whether a path expression matches at least one SQL/JSON item.
        Returns <literal>unknown</literal> if the path expression would result
        in an error; the second example uses this to avoid a no-such-key error
        in strict mode.
       </para>
       <para>
        <literal>jsonb_path_query('{"x": [1, 2], "y": [2, 4]}', 'strict $.* ? (exists (@ ? (@[*] &gt; 2)))')</literal>
        <returnvalue>[2, 4]</returnvalue>
       </para>
       <para>
        <literal>jsonb_path_query_array('{"value": 41}', 'strict $ ? (exists (@.name)) .name')</literal>
        <returnvalue>[]</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   </sect3>

   <sect3 id="jsonpath-regular-expressions">
    <title>SQL/JSON Regular Expressions</title>

    <indexterm zone="jsonpath-regular-expressions">
     <primary><literal>LIKE_REGEX</literal></primary>
     <secondary>in SQL/JSON</secondary>
    </indexterm>

    <para>
     SQL/JSON path expressions allow matching text to a regular expression
     with the <literal>like_regex</literal> filter.  For example, the
     following SQL/JSON path query would case-insensitively match all
     strings in an array that start with an English vowel:
<programlisting>
$[*] ? (@ like_regex "^[aeiou]" flag "i")
</programlisting>
    </para>

    <para>
     The optional <literal>flag</literal> string may include one or more of
     the characters
     <literal>i</literal> for case-insensitive match,
     <literal>m</literal> to allow <literal>^</literal>
     and <literal>$</literal> to match at newlines,
     <literal>s</literal> to allow <literal>.</literal> to match a newline,
     and <literal>q</literal> to quote the whole pattern (reducing the
     behavior to a simple substring match).
    </para>

    <para>
     The SQL/JSON standard borrows its definition for regular expressions
     from the <literal>LIKE_REGEX</literal> operator, which in turn uses the
     XQuery standard.  PostgreSQL does not currently support the
     <literal>LIKE_REGEX</literal> operator.  Therefore,
     the <literal>like_regex</literal> filter is implemented using the
     POSIX regular expression engine described in
     <xref linkend="functions-posix-regexp"/>.  This leads to various minor
     discrepancies from standard SQL/JSON behavior, which are cataloged in
     <xref linkend="posix-vs-xquery"/>.
     Note, however, that the flag-letter incompatibilities described there
     do not apply to SQL/JSON, as it translates the XQuery flag letters to
     match what the POSIX engine expects.
    </para>

    <para>
     Keep in mind that the pattern argument of <literal>like_regex</literal>
     is a JSON path string literal, written according to the rules given in
     <xref linkend="datatype-jsonpath"/>.  This means in particular that any
     backslashes you want to use in the regular expression must be doubled.
     For example, to match strings that contain only digits:
<programlisting>
$ ? (@ like_regex "^\\d+$")
</programlisting>
    </para>
   </sect3>
  </sect2>
 </sect1>

 <sect1 id="functions-sequence">
  <title>Sequence Manipulation Functions</title>

  <indexterm>
   <primary>sequence</primary>
  </indexterm>

  <para>
   This section describes functions for operating on <firstterm>sequence
   objects</firstterm>, also called sequence generators or just sequences.
   Sequence objects are special single-row tables created with <xref
   linkend="sql-createsequence"/>.
   Sequence objects are commonly used to generate unique identifiers
   for rows of a table.  The sequence functions, listed in <xref
   linkend="functions-sequence-table"/>, provide simple, multiuser-safe
   methods for obtaining successive sequence values from sequence
   objects.
  </para>

   <table id="functions-sequence-table">
    <title>Sequence Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>nextval</primary>
        </indexterm>
        <function>nextval</function> ( <type>regclass</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Advances the sequence object to its next value and returns that value.
        This is done atomically: even if multiple sessions
        execute <function>nextval</function> concurrently, each will safely
        receive a distinct sequence value.
        If the sequence object has been created with default parameters,
        successive <function>nextval</function> calls will return successive
        values beginning with 1.  Other behaviors can be obtained by using
        appropriate parameters in the <xref linkend="sql-createsequence"/>
        command.
      </para>
       <para>
        This function requires <literal>USAGE</literal>
        or <literal>UPDATE</literal> privilege on the sequence.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>setval</primary>
        </indexterm>
        <function>setval</function> ( <type>regclass</type>, <type>bigint</type> <optional>, <type>boolean</type> </optional> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Sets the sequence object's current value, and optionally
        its <literal>is_called</literal> flag.  The two-parameter
        form sets the sequence's <literal>last_value</literal> field to the
        specified value and sets its <literal>is_called</literal> field to
        <literal>true</literal>, meaning that the next
        <function>nextval</function> will advance the sequence before
        returning a value.  The value that will be reported
        by <function>currval</function> is also set to the specified value.
        In the three-parameter form, <literal>is_called</literal> can be set
        to either <literal>true</literal>
        or <literal>false</literal>.  <literal>true</literal> has the same
        effect as the two-parameter form. If it is set
        to <literal>false</literal>, the next <function>nextval</function>
        will return exactly the specified value, and sequence advancement
        commences with the following <function>nextval</function>.
        Furthermore, the value reported by <function>currval</function> is not
        changed in this case.  For example,
<programlisting>
SELECT setval('myseq', 42);           <lineannotation>Next <function>nextval</function> will return 43</lineannotation>
SELECT setval('myseq', 42, true);     <lineannotation>Same as above</lineannotation>
SELECT setval('myseq', 42, false);    <lineannotation>Next <function>nextval</function> will return 42</lineannotation>
</programlisting>
        The result returned by <function>setval</function> is just the value of its
        second argument.
       </para>
       <para>
        This function requires <literal>UPDATE</literal> privilege on the
        sequence.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>currval</primary>
        </indexterm>
        <function>currval</function> ( <type>regclass</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Returns the value most recently obtained
        by <function>nextval</function> for this sequence in the current
        session.  (An error is reported if <function>nextval</function> has
        never been called for this sequence in this session.)  Because this is
        returning a session-local value, it gives a predictable answer whether
        or not other sessions have executed <function>nextval</function> since
        the current session did.
       </para>
       <para>
        This function requires <literal>USAGE</literal>
        or <literal>SELECT</literal> privilege on the sequence.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>lastval</primary>
        </indexterm>
        <function>lastval</function> ()
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Returns the value most recently returned by
        <function>nextval</function> in the current session. This function is
        identical to <function>currval</function>, except that instead
        of taking the sequence name as an argument it refers to whichever
        sequence <function>nextval</function> was most recently applied to
        in the current session. It is an error to call
        <function>lastval</function> if <function>nextval</function>
        has not yet been called in the current session.
       </para>
       <para>
        This function requires <literal>USAGE</literal>
        or <literal>SELECT</literal> privilege on the last used sequence.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <caution>
   <para>
    To avoid blocking concurrent transactions that obtain numbers from
    the same sequence, a <function>nextval</function> operation is never
    rolled back; that is, once a value has been fetched it is considered
    used and will not be returned again.  This is true even if the
    surrounding transaction later aborts, or if the calling query ends
    up not using the value.  For example an <command>INSERT</command> with
    an <literal>ON CONFLICT</literal> clause will compute the to-be-inserted
    tuple, including doing any required <function>nextval</function>
    calls, before detecting any conflict that would cause it to follow
    the <literal>ON CONFLICT</literal> rule instead.  Such cases will leave
    unused <quote>holes</quote> in the sequence of assigned values.
    Thus, <productname>PostgreSQL</productname> sequence
    objects <emphasis>cannot be used to obtain <quote>gapless</quote>
    sequences</emphasis>.
   </para>

   <para>
    Likewise, any sequence state changes made by <function>setval</function>
    are not undone if the transaction rolls back.
   </para>
  </caution>

  <para>
   The sequence to be operated on by a sequence function is specified by
   a <type>regclass</type> argument, which is simply the OID of the sequence in the
   <structname>pg_class</structname> system catalog.  You do not have to look up the
   OID by hand, however, since the <type>regclass</type> data type's input
   converter will do the work for you.  Just write the sequence name enclosed
   in single quotes so that it looks like a literal constant.  For
   compatibility with the handling of ordinary
   <acronym>SQL</acronym> names, the string will be converted to lower case
   unless it contains double quotes around the sequence name.  Thus:
<programlisting>
nextval('foo')      <lineannotation>operates on sequence <literal>foo</literal></lineannotation>
nextval('FOO')      <lineannotation>operates on sequence <literal>foo</literal></lineannotation>
nextval('"Foo"')    <lineannotation>operates on sequence <literal>Foo</literal></lineannotation>
</programlisting>
   The sequence name can be schema-qualified if necessary:
<programlisting>
nextval('myschema.foo')     <lineannotation>operates on <literal>myschema.foo</literal></lineannotation>
nextval('"myschema".foo')   <lineannotation>same as above</lineannotation>
nextval('foo')              <lineannotation>searches search path for <literal>foo</literal></lineannotation>
</programlisting>
   See <xref linkend="datatype-oid"/> for more information about
   <type>regclass</type>.
  </para>

  <note>
   <para>
    Before <productname>PostgreSQL</productname> 8.1, the arguments of the
    sequence functions were of type <type>text</type>, not <type>regclass</type>, and
    the above-described conversion from a text string to an OID value would
    happen at run time during each call.  For backward compatibility, this
    facility still exists, but internally it is now handled as an implicit
    coercion from <type>text</type> to <type>regclass</type> before the function is
    invoked.
   </para>

   <para>
    When you write the argument of a sequence function as an unadorned
    literal string, it becomes a constant of type <type>regclass</type>.
    Since this is really just an OID, it will track the originally
    identified sequence despite later renaming, schema reassignment,
    etc.  This <quote>early binding</quote> behavior is usually desirable for
    sequence references in column defaults and views.  But sometimes you might
    want <quote>late binding</quote> where the sequence reference is resolved
    at run time.  To get late-binding behavior, force the constant to be
    stored as a <type>text</type> constant instead of <type>regclass</type>:
<programlisting>
nextval('foo'::text)      <lineannotation><literal>foo</literal> is looked up at runtime</lineannotation>
</programlisting>
    Note that late binding was the only behavior supported in
    <productname>PostgreSQL</productname> releases before 8.1, so you
    might need to do this to preserve the semantics of old applications.
   </para>

   <para>
    Of course, the argument of a sequence function can be an expression
    as well as a constant.  If it is a text expression then the implicit
    coercion will result in a run-time lookup.
   </para>
  </note>

 </sect1>


 <sect1 id="functions-conditional">
  <title>Conditional Expressions</title>

  <indexterm>
   <primary>CASE</primary>
  </indexterm>

  <indexterm>
   <primary>conditional expression</primary>
  </indexterm>

  <para>
   This section describes the <acronym>SQL</acronym>-compliant conditional expressions
   available in <productname>PostgreSQL</productname>.
  </para>

  <tip>
   <para>
    If your needs go beyond the capabilities of these conditional
    expressions, you might want to consider writing a server-side function
    in a more expressive programming language.
   </para>
  </tip>

   <note>
    <para>
     Although <token>COALESCE</token>, <token>GREATEST</token>, and
     <token>LEAST</token> are syntactically similar to functions, they are
     not ordinary functions, and thus cannot be used with explicit
     <token>VARIADIC</token> array arguments.
    </para>
   </note>

  <sect2 id="functions-case">
   <title><literal>CASE</literal></title>

  <para>
   The <acronym>SQL</acronym> <token>CASE</token> expression is a
   generic conditional expression, similar to if/else statements in
   other programming languages:

<synopsis>
CASE WHEN <replaceable>condition</replaceable> THEN <replaceable>result</replaceable>
     <optional>WHEN ...</optional>
     <optional>ELSE <replaceable>result</replaceable></optional>
END
</synopsis>

   <token>CASE</token> clauses can be used wherever
   an expression is valid.  Each <replaceable>condition</replaceable> is an
   expression that returns a <type>boolean</type> result.  If the condition's
   result is true, the value of the <token>CASE</token> expression is the
   <replaceable>result</replaceable> that follows the condition, and the
   remainder of the <token>CASE</token> expression is not processed.  If the
   condition's result is not true, any subsequent <token>WHEN</token> clauses
   are examined in the same manner.  If no <token>WHEN</token>
   <replaceable>condition</replaceable> yields true, the value of the
   <token>CASE</token> expression is the <replaceable>result</replaceable> of the
   <token>ELSE</token> clause.  If the <token>ELSE</token> clause is
   omitted and no condition is true, the result is null.
  </para>

   <para>
    An example:
<screen>
SELECT * FROM test;

 a
---
 1
 2
 3


SELECT a,
       CASE WHEN a=1 THEN 'one'
            WHEN a=2 THEN 'two'
            ELSE 'other'
       END
    FROM test;

 a | case
---+-------
 1 | one
 2 | two
 3 | other
</screen>
   </para>

  <para>
   The data types of all the <replaceable>result</replaceable>
   expressions must be convertible to a single output type.
   See <xref linkend="typeconv-union-case"/> for more details.
  </para>

  <para>
   There is a <quote>simple</quote> form of <token>CASE</token> expression
   that is a variant of the general form above:

<synopsis>
CASE <replaceable>expression</replaceable>
    WHEN <replaceable>value</replaceable> THEN <replaceable>result</replaceable>
    <optional>WHEN ...</optional>
    <optional>ELSE <replaceable>result</replaceable></optional>
END
</synopsis>

   The first
   <replaceable>expression</replaceable> is computed, then compared to
   each of the <replaceable>value</replaceable> expressions in the
   <token>WHEN</token> clauses until one is found that is equal to it.  If
   no match is found, the <replaceable>result</replaceable> of the
   <token>ELSE</token> clause (or a null value) is returned.  This is similar
   to the <function>switch</function> statement in C.
  </para>

   <para>
    The example above can be written using the simple
    <token>CASE</token> syntax:
<screen>
SELECT a,
       CASE a WHEN 1 THEN 'one'
              WHEN 2 THEN 'two'
              ELSE 'other'
       END
    FROM test;

 a | case
---+-------
 1 | one
 2 | two
 3 | other
</screen>
   </para>

   <para>
    A <token>CASE</token> expression does not evaluate any subexpressions
    that are not needed to determine the result.  For example, this is a
    possible way of avoiding a division-by-zero failure:
<programlisting>
SELECT ... WHERE CASE WHEN x &lt;&gt; 0 THEN y/x &gt; 1.5 ELSE false END;
</programlisting>
   </para>

   <note>
    <para>
     As described in <xref linkend="syntax-express-eval"/>, there are various
     situations in which subexpressions of an expression are evaluated at
     different times, so that the principle that <quote><token>CASE</token>
     evaluates only necessary subexpressions</quote> is not ironclad.  For
     example a constant <literal>1/0</literal> subexpression will usually result in
     a division-by-zero failure at planning time, even if it's within
     a <token>CASE</token> arm that would never be entered at run time.
    </para>
   </note>
  </sect2>

  <sect2 id="functions-coalesce-nvl-ifnull">
   <title><literal>COALESCE</literal></title>

  <indexterm>
   <primary>COALESCE</primary>
  </indexterm>

  <indexterm>
   <primary>NVL</primary>
  </indexterm>

  <indexterm>
   <primary>IFNULL</primary>
  </indexterm>

<synopsis>
<function>COALESCE</function>(<replaceable>value</replaceable> <optional>, ...</optional>)
</synopsis>

  <para>
   The <function>COALESCE</function> function returns the first of its
   arguments that is not null.  Null is returned only if all arguments
   are null.  It is often used to substitute a default value for
   null values when data is retrieved for display, for example:
<programlisting>
SELECT COALESCE(description, short_description, '(none)') ...
</programlisting>
   This returns <varname>description</varname> if it is not null, otherwise
   <varname>short_description</varname> if it is not null, otherwise <literal>(none)</literal>.
  </para>

   <para>
    Like a <token>CASE</token> expression, <function>COALESCE</function> only
    evaluates the arguments that are needed to determine the result;
    that is, arguments to the right of the first non-null argument are
    not evaluated.  This SQL-standard function provides capabilities similar
    to <function>NVL</function> and <function>IFNULL</function>, which are used in some other
    database systems.
   </para>
  </sect2>

  <sect2 id="functions-nullif">
   <title><literal>NULLIF</literal></title>

  <indexterm>
   <primary>NULLIF</primary>
  </indexterm>

<synopsis>
<function>NULLIF</function>(<replaceable>value1</replaceable>, <replaceable>value2</replaceable>)
</synopsis>

  <para>
   The <function>NULLIF</function> function returns a null value if
   <replaceable>value1</replaceable> equals <replaceable>value2</replaceable>;
   otherwise it returns <replaceable>value1</replaceable>.
   This can be used to perform the inverse operation of the
   <function>COALESCE</function> example given above:
<programlisting>
SELECT NULLIF(value, '(none)') ...
</programlisting>
  </para>
  <para>
   In this example, if <literal>value</literal> is <literal>(none)</literal>,
   null is returned, otherwise the value of <literal>value</literal>
   is returned.
  </para>

  </sect2>

  <sect2 id="functions-greatest-least">
   <title><literal>GREATEST</literal> and <literal>LEAST</literal></title>

  <indexterm>
   <primary>GREATEST</primary>
  </indexterm>
  <indexterm>
   <primary>LEAST</primary>
  </indexterm>

<synopsis>
<function>GREATEST</function>(<replaceable>value</replaceable> <optional>, ...</optional>)
</synopsis>
<synopsis>
<function>LEAST</function>(<replaceable>value</replaceable> <optional>, ...</optional>)
</synopsis>

   <para>
    The <function>GREATEST</function> and <function>LEAST</function> functions select the
    largest or smallest value from a list of any number of expressions.
    The expressions must all be convertible to a common data type, which
    will be the type of the result
    (see <xref linkend="typeconv-union-case"/> for details).  NULL values
    in the list are ignored.  The result will be NULL only if all the
    expressions evaluate to NULL.
   </para>

   <para>
    Note that <function>GREATEST</function> and <function>LEAST</function> are not in
    the SQL standard, but are a common extension.  Some other databases
    make them return NULL if any argument is NULL, rather than only when
    all are NULL.
   </para>
  </sect2>
 </sect1>

 <sect1 id="functions-array">
  <title>Array Functions and Operators</title>

  <para>
   <xref linkend="array-operators-table"/> shows the specialized operators
   available for array types.
   In addition to those, the usual comparison operators shown in <xref
   linkend="functions-comparison-op-table"/> are available for
   arrays.  The comparison operators compare the array contents
   element-by-element, using the default B-tree comparison function for
   the element data type, and sort based on the first difference.
   In multidimensional arrays the elements are visited in row-major order
   (last subscript varies most rapidly).
   If the contents of two arrays are equal but the dimensionality is
   different, the first difference in the dimensionality information
   determines the sort order.  (This is a change from versions of
   <productname>PostgreSQL</productname> prior to 8.2: older versions would claim
   that two arrays with the same contents were equal, even if the
   number of dimensions or subscript ranges were different.)
  </para>

   <table id="array-operators-table">
    <title>Array Operators</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Operator
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyarray</type> <literal>@&gt;</literal> <type>anyarray</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does the first array contain the second, that is, does each element
        appearing in the second array equal some element of the first array?
        (Duplicates are not treated specially,
        thus <literal>ARRAY[1]</literal> and <literal>ARRAY[1,1]</literal> are
        each considered to contain the other.)
       </para>
       <para>
        <literal>ARRAY[1,4,3] @&gt; ARRAY[3,1,3]</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyarray</type> <literal>&lt;@</literal> <type>anyarray</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is the first array contained by the second?
       </para>
       <para>
        <literal>ARRAY[2,2,7] &lt;@ ARRAY[1,7,4,2,6]</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyarray</type> <literal>&amp;&amp;</literal> <type>anyarray</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Do the arrays overlap, that is, have any elements in common?
       </para>
       <para>
        <literal>ARRAY[1,4,3] &amp;&amp; ARRAY[2,1]</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyarray</type> <literal>||</literal> <type>anyarray</type>
        <returnvalue>anyarray</returnvalue>
       </para>
       <para>
        Concatenates the two arrays.  Concatenating a null or empty array is a
        no-op; otherwise the arrays must have the same number of dimensions
        (as illustrated by the first example) or differ in number of
        dimensions by one (as illustrated by the second).
       </para>
       <para>
        <literal>ARRAY[1,2,3] || ARRAY[4,5,6,7]</literal>
        <returnvalue>{1,2,3,4,5,6,7}</returnvalue>
       </para>
       <para>
        <literal>ARRAY[1,2,3] || ARRAY[[4,5,6],[7,8,9]]</literal>
        <returnvalue>{{1,2,3},{4,5,6},{7,8,9}}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyelement</type> <literal>||</literal> <type>anyarray</type>
        <returnvalue>anyarray</returnvalue>
       </para>
       <para>
        Concatenates an element onto the front of an array (which must be
        empty or one-dimensional).
       </para>
       <para>
        <literal>3 || ARRAY[4,5,6]</literal>
        <returnvalue>{3,4,5,6}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyarray</type> <literal>||</literal> <type>anyelement</type>
        <returnvalue>anyarray</returnvalue>
       </para>
       <para>
        Concatenates an element onto the end of an array (which must be
        empty or one-dimensional).
       </para>
       <para>
        <literal>ARRAY[4,5,6] || 7</literal>
        <returnvalue>{4,5,6,7}</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   See <xref linkend="arrays"/> for more details about array operator
   behavior.  See <xref linkend="indexes-types"/> for more details about
   which operators support indexed operations.
  </para>

  <para>
   <xref linkend="array-functions-table"/> shows the functions
   available for use with array types. See <xref linkend="arrays"/>
   for more information  and examples of the use of these functions.
  </para>

   <table id="array-functions-table">
    <title>Array Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>array_append</primary>
        </indexterm>
        <function>array_append</function> ( <type>anyarray</type>, <type>anyelement</type> )
        <returnvalue>anyarray</returnvalue>
       </para>
       <para>
        Appends an element to the end of an array (same as
        the <type>anyarray</type> <literal>||</literal> <type>anyelement</type>
        operator).
       </para>
       <para>
        <literal>array_append(ARRAY[1,2], 3)</literal>
        <returnvalue>{1,2,3}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>array_cat</primary>
        </indexterm>
        <function>array_cat</function> ( <type>anyarray</type>, <type>anyarray</type> )
        <returnvalue>anyarray</returnvalue>
       </para>
       <para>
        Concatenates two arrays (same as
        the <type>anyarray</type> <literal>||</literal> <type>anyarray</type>
        operator).
       </para>
       <para>
        <literal>array_cat(ARRAY[1,2,3], ARRAY[4,5])</literal>
        <returnvalue>{1,2,3,4,5}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>array_dims</primary>
        </indexterm>
        <function>array_dims</function> ( <type>anyarray</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns a text representation of the array's dimensions.
       </para>
       <para>
        <literal>array_dims(ARRAY[[1,2,3], [4,5,6]])</literal>
        <returnvalue>[1:2][1:3]</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>array_fill</primary>
        </indexterm>
        <function>array_fill</function> ( <type>anyelement</type>, <type>integer[]</type>
          <optional>, <type>integer[]</type> </optional> )
        <returnvalue>anyarray</returnvalue>
       </para>
       <para>
        Returns an array filled with copies of the given value, having
        dimensions of the lengths specified by the second argument.
        The optional third argument supplies lower-bound values for each
        dimension (which default to all <literal>1</literal>).
       </para>
       <para>
        <literal>array_fill(11, ARRAY[2,3])</literal>
        <returnvalue>{{11,11,11},{11,11,11}}</returnvalue>
       </para>
       <para>
        <literal>array_fill(7, ARRAY[3], ARRAY[2])</literal>
        <returnvalue>[2:4]={7,7,7}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>array_length</primary>
        </indexterm>
        <function>array_length</function> ( <type>anyarray</type>, <type>integer</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the length of the requested array dimension.
       </para>
       <para>
        <literal>array_length(array[1,2,3], 1)</literal>
        <returnvalue>3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>array_lower</primary>
        </indexterm>
        <function>array_lower</function> ( <type>anyarray</type>, <type>integer</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the lower bound of the requested array dimension.
       </para>
       <para>
        <literal>array_lower('[0:2]={1,2,3}'::integer[], 1)</literal>
        <returnvalue>0</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>array_ndims</primary>
        </indexterm>
        <function>array_ndims</function> ( <type>anyarray</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the number of dimensions of the array.
       </para>
       <para>
        <literal>array_ndims(ARRAY[[1,2,3], [4,5,6]])</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>array_position</primary>
        </indexterm>
        <function>array_position</function> ( <type>anyarray</type>, <type>anyelement</type> <optional>, <type>integer</type> </optional> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the subscript of the first occurrence of the second argument
        in the array, or <literal>NULL</literal> if it's not present.
        If the third argument is given, the search begins at that subscript.
        The array must be one-dimensional.
        Comparisons are done using <literal>IS NOT DISTINCT FROM</literal>
        semantics, so it is possible to search for <literal>NULL</literal>.
       </para>
       <para>
        <literal>array_position(ARRAY['sun', 'mon', 'tue', 'wed', 'thu', 'fri', 'sat'], 'mon')</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>array_positions</primary>
        </indexterm>
        <function>array_positions</function> ( <type>anyarray</type>, <type>anyelement</type> )
        <returnvalue>integer[]</returnvalue>
       </para>
       <para>
        Returns an array of the subscripts of all occurrences of the second
        argument in the array given as first argument.
        The array must be one-dimensional.
        Comparisons are done using <literal>IS NOT DISTINCT FROM</literal>
        semantics, so it is possible to search for <literal>NULL</literal>.
        <literal>NULL</literal> is returned only if the array
        is <literal>NULL</literal>; if the value is not found in the array, an
        empty array is returned.
       </para>
       <para>
        <literal>array_positions(ARRAY['A','A','B','A'], 'A')</literal>
        <returnvalue>{1,2,4}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>array_prepend</primary>
        </indexterm>
        <function>array_prepend</function> ( <type>anyelement</type>, <type>anyarray</type> )
        <returnvalue>anyarray</returnvalue>
       </para>
       <para>
        Prepends an element to the beginning of an array (same as
        the <type>anyelement</type> <literal>||</literal> <type>anyarray</type>
        operator).
       </para>
       <para>
        <literal>array_prepend(1, ARRAY[2,3])</literal>
        <returnvalue>{1,2,3}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>array_remove</primary>
        </indexterm>
        <function>array_remove</function> ( <type>anyarray</type>, <type>anyelement</type> )
        <returnvalue>anyarray</returnvalue>
       </para>
       <para>
        Removes all elements equal to the given value from the array.
        The array must be one-dimensional.
        Comparisons are done using <literal>IS NOT DISTINCT FROM</literal>
        semantics, so it is possible to remove <literal>NULL</literal>s.
       </para>
       <para>
        <literal>array_remove(ARRAY[1,2,3,2], 2)</literal>
        <returnvalue>{1,3}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>array_replace</primary>
        </indexterm>
        <function>array_replace</function> ( <type>anyarray</type>, <type>anyelement</type>, <type>anyelement</type> )
        <returnvalue>anyarray</returnvalue>
       </para>
       <para>
        Replaces each array element equal to the second argument with the
        third argument.
       </para>
       <para>
        <literal>array_replace(ARRAY[1,2,5,4], 5, 3)</literal>
        <returnvalue>{1,2,3,4}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>array_to_string</primary>
        </indexterm>
        <function>array_to_string</function> ( <parameter>array</parameter> <type>anyarray</type>, <parameter>delimiter</parameter> <type>text</type> <optional>, <parameter>null_string</parameter> <type>text</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts each array element to its text representation, and
        concatenates those separated by
        the <parameter>delimiter</parameter> string.
        If <parameter>null_string</parameter> is given and is
        not <literal>NULL</literal>, then <literal>NULL</literal> array
        entries are represented by that string; otherwise, they are omitted.
       </para>
       <para>
        <literal>array_to_string(ARRAY[1, 2, 3, NULL, 5], ',', '*')</literal>
        <returnvalue>1,2,3,*,5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>array_upper</primary>
        </indexterm>
        <function>array_upper</function> ( <type>anyarray</type>, <type>integer</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the upper bound of the requested array dimension.
       </para>
       <para>
        <literal>array_upper(ARRAY[1,8,3,7], 1)</literal>
        <returnvalue>4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>cardinality</primary>
        </indexterm>
        <function>cardinality</function> ( <type>anyarray</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the total number of elements in the array, or 0 if the array
        is empty.
       </para>
       <para>
        <literal>cardinality(ARRAY[[1,2],[3,4]])</literal>
        <returnvalue>4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>unnest</primary>
        </indexterm>
        <function>unnest</function> ( <type>anyarray</type> )
        <returnvalue>setof anyelement</returnvalue>
       </para>
       <para>
        Expands an array to a set of rows.
       </para>
       <para>
        <literal>unnest(ARRAY[1,2])</literal>
        <returnvalue></returnvalue>
<programlisting>
 1
 2
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>unnest</function> ( <type>anyarray</type>, <type>anyarray</type> <optional>, ... </optional> )
        <returnvalue>setof anyelement, anyelement [, ... ]</returnvalue>
       </para>
       <para>
        Expands multiple arrays (possibly of different data types) to a set of
        rows.  If the arrays are not all the same length then the shorter ones
        are padded with <literal>NULL</literal>s.  This is only allowed in a
        query's FROM clause; see <xref linkend="queries-tablefunctions"/>.
       </para>
       <para>
        <literal>select * from unnest(ARRAY[1,2], ARRAY['foo','bar','baz']) as x(a,b)</literal>
        <returnvalue></returnvalue>
<programlisting>
 a |  b
---+-----
 1 | foo
 2 | bar
   | baz
</programlisting>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <note>
    <para>
     There are two differences in the behavior of <function>string_to_array</function>
     from pre-9.1 versions of <productname>PostgreSQL</productname>.
     First, it will return an empty (zero-element) array rather
     than <literal>NULL</literal> when the input string is of zero length.
     Second, if the delimiter string is <literal>NULL</literal>, the function
     splits the input into individual characters, rather than
     returning <literal>NULL</literal> as before.
    </para>
   </note>

   <para>
    See also <xref linkend="functions-aggregate"/> about the aggregate
    function <function>array_agg</function> for use with arrays.
   </para>
  </sect1>

 <sect1 id="functions-range">
  <title>Range Functions and Operators</title>

  <para>
   See <xref linkend="rangetypes"/> for an overview of range types.
  </para>

  <para>
   <xref linkend="range-operators-table"/> shows the specialized operators
   available for range types.
   In addition to those, the usual comparison operators shown in
   <xref linkend="functions-comparison-op-table"/> are available for range
   types.  The comparison operators order first by the range lower bounds, and
   only if those are equal do they compare the upper bounds.  This does not
   usually result in a useful overall ordering, but the operators are provided
   to allow unique indexes to be constructed on ranges.
  </para>

   <table id="range-operators-table">
    <title>Range Operators</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Operator
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyrange</type> <literal>@&gt;</literal> <type>anyrange</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does the first range contain the second?
       </para>
       <para>
        <literal>int4range(2,4) @&gt; int4range(2,3)</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyrange</type> <literal>@&gt;</literal> <type>anyelement</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does the range contain the element?
       </para>
       <para>
        <literal>'[2011-01-01,2011-03-01)'::tsrange @&gt; '2011-01-10'::timestamp</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyrange</type> <literal>&lt;@</literal> <type>anyrange</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is the first range contained by the second?
       </para>
       <para>
        <literal>int4range(2,4) &lt;@ int4range(1,7)</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyelement</type> <literal>&lt;@</literal> <type>anyrange</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is the element contained in the range?
       </para>
       <para>
        <literal>42 &lt;@ int4range(1,7)</literal>
        <returnvalue>f</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyrange</type> <literal>&amp;&amp;</literal> <type>anyrange</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Do the ranges overlap, that is, have any elements in common?
       </para>
       <para>
        <literal>int8range(3,7) &amp;&amp; int8range(4,12)</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyrange</type> <literal>&lt;&lt;</literal> <type>anyrange</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is the first range strictly left of the second?
       </para>
       <para>
        <literal>int8range(1,10) &lt;&lt; int8range(100,110)</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyrange</type> <literal>&gt;&gt;</literal> <type>anyrange</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is the first range strictly right of the second?
       </para>
       <para>
        <literal>int8range(50,60) &gt;&gt; int8range(20,30)</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyrange</type> <literal>&amp;&lt;</literal> <type>anyrange</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does the first range not extend to the right of the second?
       </para>
       <para>
        <literal>int8range(1,20) &amp;&lt; int8range(18,20)</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyrange</type> <literal>&amp;&gt;</literal> <type>anyrange</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does the first range not extend to the left of the second?
       </para>
       <para>
        <literal>int8range(7,20) &amp;&gt; int8range(5,10)</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyrange</type> <literal>-|-</literal> <type>anyrange</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Are the ranges adjacent?
       </para>
       <para>
        <literal>numrange(1.1,2.2) -|- numrange(2.2,3.3)</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyrange</type> <literal>+</literal> <type>anyrange</type>
        <returnvalue>anyrange</returnvalue>
       </para>
       <para>
        Computes the union of the ranges.  The ranges must overlap or be
        adjacent, so that the union is a single range (but
        see <function>range_merge()</function>).
       </para>
       <para>
        <literal>numrange(5,15) + numrange(10,20)</literal>
        <returnvalue>[5,20)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyrange</type> <literal>*</literal> <type>anyrange</type>
        <returnvalue>anyrange</returnvalue>
       </para>
       <para>
        Computes the intersection of the ranges.
       </para>
       <para>
        <literal>int8range(5,15) * int8range(10,20)</literal>
        <returnvalue>[10,15)</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>anyrange</type> <literal>-</literal> <type>anyrange</type>
        <returnvalue>anyrange</returnvalue>
       </para>
       <para>
        Computes the difference of the ranges.  The second range must not be
        contained in the first in such a way that the difference would not be
        a single range.
       </para>
       <para>
        <literal>int8range(5,15) - int8range(10,20)</literal>
        <returnvalue>[5,10)</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   The left-of/right-of/adjacent operators always return false when an empty
   range is involved; that is, an empty range is not considered to be either
   before or after any other range.
  </para>

  <para>
   <xref linkend="range-functions-table"/> shows the functions
   available for use with range types.
  </para>

   <table id="range-functions-table">
    <title>Range Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>lower</primary>
        </indexterm>
        <function>lower</function> ( <type>anyrange</type> )
        <returnvalue>anyelement</returnvalue>
       </para>
       <para>
        Extracts the lower bound of the range (<literal>NULL</literal> if the
        range is empty or the lower bound is infinite).
       </para>
       <para>
        <literal>lower(numrange(1.1,2.2))</literal>
        <returnvalue>1.1</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>upper</primary>
        </indexterm>
        <function>upper</function> ( <type>anyrange</type> )
        <returnvalue>anyelement</returnvalue>
       </para>
       <para>
        Extracts the upper bound of the range (<literal>NULL</literal> if the
        range is empty or the upper bound is infinite).
       </para>
       <para>
        <literal>upper(numrange(1.1,2.2))</literal>
        <returnvalue>2.2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>isempty</primary>
        </indexterm>
        <function>isempty</function> ( <type>anyrange</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is the range empty?
       </para>
       <para>
        <literal>isempty(numrange(1.1,2.2))</literal>
        <returnvalue>f</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>lower_inc</primary>
        </indexterm>
        <function>lower_inc</function> ( <type>anyrange</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is the range's lower bound inclusive?
       </para>
       <para>
        <literal>lower_inc(numrange(1.1,2.2))</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>upper_inc</primary>
        </indexterm>
        <function>upper_inc</function> ( <type>anyrange</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is the range's upper bound inclusive?
       </para>
       <para>
        <literal>upper_inc(numrange(1.1,2.2))</literal>
        <returnvalue>f</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>lower_inf</primary>
        </indexterm>
        <function>lower_inf</function> ( <type>anyrange</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is the range's lower bound infinite?
       </para>
       <para>
        <literal>lower_inf('(,)'::daterange)</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>upper_inf</primary>
        </indexterm>
        <function>upper_inf</function> ( <type>anyrange</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is the range's upper bound infinite?
       </para>
       <para>
        <literal>upper_inf('(,)'::daterange)</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>range_merge</primary>
        </indexterm>
        <function>range_merge</function> ( <type>anyrange</type>, <type>anyrange</type> )
        <returnvalue>anyrange</returnvalue>
       </para>
       <para>
        Computes the smallest range that includes both of the given ranges.
       </para>
       <para>
        <literal>range_merge('[1,2)'::int4range, '[3,4)'::int4range)</literal>
        <returnvalue>[1,4)</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   The <function>lower_inc</function>, <function>upper_inc</function>,
   <function>lower_inf</function>, and <function>upper_inf</function>
   functions all return false for an empty range.
  </para>
  </sect1>

 <sect1 id="functions-aggregate">
  <title>Aggregate Functions</title>

  <indexterm zone="functions-aggregate">
   <primary>aggregate function</primary>
   <secondary>built-in</secondary>
  </indexterm>

  <para>
   <firstterm>Aggregate functions</firstterm> compute a single result
   from a set of input values.  The built-in general-purpose aggregate
   functions are listed in <xref linkend="functions-aggregate-table"/>
   while statistical aggregates are in <xref
   linkend="functions-aggregate-statistics-table"/>.
   The built-in within-group ordered-set aggregate functions
   are listed in <xref linkend="functions-orderedset-table"/>
   while the built-in within-group hypothetical-set ones are in <xref
   linkend="functions-hypothetical-table"/>.  Grouping operations,
   which are closely related to aggregate functions, are listed in
   <xref linkend="functions-grouping-table"/>.
   The special syntax considerations for aggregate
   functions are explained in <xref linkend="syntax-aggregates"/>.
   Consult <xref linkend="tutorial-agg"/> for additional introductory
   information.
  </para>

  <para>
   Aggregate functions that support <firstterm>Partial Mode</firstterm>
   are eligible to participate in various optimizations, such as parallel
   aggregation.
  </para>

   <table id="functions-aggregate-table">
    <title>General-Purpose Aggregate Functions</title>
    <tgroup cols="2">
     <colspec colname="col1" colwidth="10*"/>
     <colspec colname="col2" colwidth="1*"/>
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
       <entry>Partial Mode</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>array_agg</primary>
        </indexterm>
        <function>array_agg</function> ( <type>anynonarray</type> )
        <returnvalue>anyarray</returnvalue>
       </para>
       <para>
        Collects all the input values, including nulls, into an array.
       </para></entry>
       <entry>No</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>array_agg</function> ( <type>anyarray</type> )
        <returnvalue>anyarray</returnvalue>
       </para>
       <para>
        Concatenates all the input arrays into an array of one higher
        dimension.  (The inputs must all have the same dimensionality, and
        cannot be empty or null.)
       </para></entry>
       <entry>No</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>average</primary>
        </indexterm>
        <indexterm>
         <primary>avg</primary>
        </indexterm>
        <function>avg</function> ( <type>smallint</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <function>avg</function> ( <type>integer</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <function>avg</function> ( <type>bigint</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <function>avg</function> ( <type>numeric</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <function>avg</function> ( <type>real</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para role="func_signature">
        <function>avg</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para role="func_signature">
        <function>avg</function> ( <type>interval</type> )
        <returnvalue>interval</returnvalue>
       </para>
       <para>
        Computes the average (arithmetic mean) of all the non-null input
        values.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>bit_and</primary>
        </indexterm>
        <function>bit_and</function> ( <type>smallint</type> )
        <returnvalue>smallint</returnvalue>
       </para>
       <para role="func_signature">
        <function>bit_and</function> ( <type>integer</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para role="func_signature">
        <function>bit_and</function> ( <type>bigint</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para role="func_signature">
        <function>bit_and</function> ( <type>bit</type> )
        <returnvalue>bit</returnvalue>
       </para>
       <para>
        Computes the bitwise AND of all non-null input values.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>bit_or</primary>
        </indexterm>
        <function>bit_or</function> ( <type>smallint</type> )
        <returnvalue>smallint</returnvalue>
       </para>
       <para role="func_signature">
        <function>bit_or</function> ( <type>integer</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para role="func_signature">
        <function>bit_or</function> ( <type>bigint</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para role="func_signature">
        <function>bit_or</function> ( <type>bit</type> )
        <returnvalue>bit</returnvalue>
       </para>
       <para>
        Computes the bitwise OR of all non-null input values.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>bool_and</primary>
        </indexterm>
        <function>bool_and</function> ( <type>boolean</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Returns true if all non-null input values are true, otherwise false.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>bool_or</primary>
        </indexterm>
        <function>bool_or</function> ( <type>boolean</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Returns true if any non-null input value is true, otherwise false.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>count</primary>
        </indexterm>
        <function>count</function> ( <literal>*</literal> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Computes the number of input rows.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>count</function> ( <type>"any"</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Computes the number of input rows in which the input value is not
        null.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>every</primary>
        </indexterm>
        <function>every</function> ( <type>boolean</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        This is the SQL standard's equivalent to <function>bool_and</function>.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_agg</primary>
        </indexterm>
        <function>json_agg</function> ( <type>anyelement</type> )
        <returnvalue>json</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_agg</primary>
        </indexterm>
        <function>jsonb_agg</function> ( <type>anyelement</type> )
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Collects all the input values, including nulls, into a JSON array.
        Values are converted to JSON as per <function>to_json</function>
        or <function>to_jsonb</function>.
       </para></entry>
       <entry>No</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>json_object_agg</primary>
        </indexterm>
        <function>json_object_agg</function> ( <parameter>key</parameter>
         <type>"any"</type>, <parameter>value</parameter>
         <type>"any"</type> )
        <returnvalue>json</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>jsonb_object_agg</primary>
        </indexterm>
        <function>jsonb_object_agg</function> ( <parameter>key</parameter>
         <type>"any"</type>, <parameter>value</parameter>
         <type>"any"</type> )
        <returnvalue>jsonb</returnvalue>
       </para>
       <para>
        Collects all the key/value pairs into a JSON object.  Key arguments
        are coerced to text; value arguments are converted as
        per <function>to_json</function> or <function>to_jsonb</function>.
        Values can be null, but not keys.
       </para></entry>
       <entry>No</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>max</primary>
        </indexterm>
        <function>max</function> ( <replaceable>see text</replaceable> )
        <returnvalue><replaceable>same as input type</replaceable></returnvalue>
       </para>
       <para>
        Computes the maximum of the non-null input
        values.  Available for any numeric, string, date/time, or enum type,
        as well as <type>inet</type>, <type>interval</type>,
        <type>money</type>, <type>oid</type>, <type>pg_lsn</type>,
        <type>tid</type>,
        and arrays of any of these types.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>min</primary>
        </indexterm>
        <function>min</function> ( <replaceable>see text</replaceable> )
        <returnvalue><replaceable>same as input type</replaceable></returnvalue>
       </para>
       <para>
        Computes the minimum of the non-null input
        values.  Available for any numeric, string, date/time, or enum type,
        as well as <type>inet</type>, <type>interval</type>,
        <type>money</type>, <type>oid</type>, <type>pg_lsn</type>,
        <type>tid</type>,
        and arrays of any of these types.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>string_agg</primary>
        </indexterm>
        <function>string_agg</function> ( <parameter>value</parameter>
         <type>text</type>, <parameter>delimiter</parameter> <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <function>string_agg</function> ( <parameter>value</parameter>
         <type>bytea</type>, <parameter>delimiter</parameter> <type>bytea</type> )
        <returnvalue>bytea</returnvalue>
       </para>
       <para>
        Concatenates the non-null input values into a string.  Each value
        after the first is preceded by the
        corresponding <parameter>delimiter</parameter> (if it's not null).
       </para></entry>
       <entry>No</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>sum</primary>
        </indexterm>
        <function>sum</function> ( <type>smallint</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para role="func_signature">
        <function>sum</function> ( <type>integer</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para role="func_signature">
        <function>sum</function> ( <type>bigint</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <function>sum</function> ( <type>numeric</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para role="func_signature">
        <function>sum</function> ( <type>real</type> )
        <returnvalue>real</returnvalue>
       </para>
       <para role="func_signature">
        <function>sum</function> ( <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para role="func_signature">
        <function>sum</function> ( <type>interval</type> )
        <returnvalue>interval</returnvalue>
       </para>
       <para role="func_signature">
        <function>sum</function> ( <type>money</type> )
        <returnvalue>money</returnvalue>
       </para>
       <para>
        Computes the sum of the non-null input values.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>xmlagg</primary>
        </indexterm>
        <function>xmlagg</function> ( <type>xml</type> )
        <returnvalue>xml</returnvalue>
       </para>
       <para>
        Concatenates the non-null XML input values (see
        <xref linkend="functions-xml-xmlagg"/>).
       </para></entry>
       <entry>No</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   It should be noted that except for <function>count</function>,
   these functions return a null value when no rows are selected.  In
   particular, <function>sum</function> of no rows returns null, not
   zero as one might expect, and <function>array_agg</function>
   returns null rather than an empty array when there are no input
   rows.  The <function>coalesce</function> function can be used to
   substitute zero or an empty array for null when necessary.
  </para>

  <para>
   The aggregate functions <function>array_agg</function>,
   <function>json_agg</function>, <function>jsonb_agg</function>,
   <function>json_object_agg</function>, <function>jsonb_object_agg</function>,
   <function>string_agg</function>,
   and <function>xmlagg</function>, as well as similar user-defined
   aggregate functions, produce meaningfully different result values
   depending on the order of the input values.  This ordering is
   unspecified by default, but can be controlled by writing an
   <literal>ORDER BY</literal> clause within the aggregate call, as shown in
   <xref linkend="syntax-aggregates"/>.
   Alternatively, supplying the input values from a sorted subquery
   will usually work.  For example:

<screen><![CDATA[
SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab;
]]></screen>

   Beware that this approach can fail if the outer query level contains
   additional processing, such as a join, because that might cause the
   subquery's output to be reordered before the aggregate is computed.
  </para>

  <note>
    <indexterm>
      <primary>ANY</primary>
    </indexterm>
    <indexterm>
      <primary>SOME</primary>
    </indexterm>
    <para>
      The boolean aggregates <function>bool_and</function> and
      <function>bool_or</function> correspond to the standard SQL aggregates
      <function>every</function> and <function>any</function> or
      <function>some</function>.
      <productname>PostgreSQL</productname>
      supports <function>every</function>, but not <function>any</function>
      or <function>some</function>, because there is an ambiguity built into
      the standard syntax:
<programlisting>
SELECT b1 = ANY((SELECT b2 FROM t2 ...)) FROM t1 ...;
</programlisting>
      Here <function>ANY</function> can be considered either as introducing
      a subquery, or as being an aggregate function, if the subquery
      returns one row with a Boolean value.
      Thus the standard name cannot be given to these aggregates.
    </para>
  </note>

  <note>
   <para>
    Users accustomed to working with other SQL database management
    systems might be disappointed by the performance of the
    <function>count</function> aggregate when it is applied to the
    entire table. A query like:
<programlisting>
SELECT count(*) FROM sometable;
</programlisting>
    will require effort proportional to the size of the table:
    <productname>PostgreSQL</productname> will need to scan either the
    entire table or the entirety of an index that includes all rows in
    the table.
   </para>
  </note>

  <para>
   <xref linkend="functions-aggregate-statistics-table"/> shows
   aggregate functions typically used in statistical analysis.
   (These are separated out merely to avoid cluttering the listing
   of more-commonly-used aggregates.)  Functions shown as
   accepting <replaceable>numeric_type</replaceable> are available for all
   the types <type>smallint</type>, <type>integer</type>,
   <type>bigint</type>, <type>numeric</type>, <type>real</type>,
   and <type>double precision</type>.
   Where the description mentions
   <parameter>N</parameter>, it means the
   number of input rows for which all the input expressions are non-null.
   In all cases, null is returned if the computation is meaningless,
   for example when <parameter>N</parameter> is zero.
  </para>

  <indexterm>
   <primary>statistics</primary>
  </indexterm>
  <indexterm>
   <primary>linear regression</primary>
  </indexterm>

   <table id="functions-aggregate-statistics-table">
    <title>Aggregate Functions for Statistics</title>
    <tgroup cols="2">
     <colspec colname="col1" colwidth="10*"/>
     <colspec colname="col2" colwidth="1*"/>
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
       <entry>Partial Mode</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>correlation</primary>
        </indexterm>
        <indexterm>
         <primary>corr</primary>
        </indexterm>
        <function>corr</function> ( <parameter>Y</parameter> <type>double precision</type>, <parameter>X</parameter> <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes the correlation coefficient.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>covariance</primary>
         <secondary>population</secondary>
        </indexterm>
        <indexterm>
         <primary>covar_pop</primary>
        </indexterm>
        <function>covar_pop</function> ( <parameter>Y</parameter> <type>double precision</type>, <parameter>X</parameter> <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes the population covariance.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>covariance</primary>
         <secondary>sample</secondary>
        </indexterm>
        <indexterm>
         <primary>covar_samp</primary>
        </indexterm>
        <function>covar_samp</function> ( <parameter>Y</parameter> <type>double precision</type>, <parameter>X</parameter> <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes the sample covariance.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regr_avgx</primary>
        </indexterm>
        <function>regr_avgx</function> ( <parameter>Y</parameter> <type>double precision</type>, <parameter>X</parameter> <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes the average of the independent variable,
        <literal>sum(<parameter>X</parameter>)/<parameter>N</parameter></literal>.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regr_avgy</primary>
        </indexterm>
        <function>regr_avgy</function> ( <parameter>Y</parameter> <type>double precision</type>, <parameter>X</parameter> <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes the average of the dependent variable,
        <literal>sum(<parameter>Y</parameter>)/<parameter>N</parameter></literal>.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regr_count</primary>
        </indexterm>
        <function>regr_count</function> ( <parameter>Y</parameter> <type>double precision</type>, <parameter>X</parameter> <type>double precision</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Computes the number of rows in which both inputs are non-null.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regression intercept</primary>
        </indexterm>
        <indexterm>
         <primary>regr_intercept</primary>
        </indexterm>
        <function>regr_intercept</function> ( <parameter>Y</parameter> <type>double precision</type>, <parameter>X</parameter> <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes the y-intercept of the least-squares-fit linear equation
        determined by the
        (<parameter>X</parameter>, <parameter>Y</parameter>) pairs.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regr_r2</primary>
        </indexterm>
        <function>regr_r2</function> ( <parameter>Y</parameter> <type>double precision</type>, <parameter>X</parameter> <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes the square of the correlation coefficient.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regression slope</primary>
        </indexterm>
        <indexterm>
         <primary>regr_slope</primary>
        </indexterm>
        <function>regr_slope</function> ( <parameter>Y</parameter> <type>double precision</type>, <parameter>X</parameter> <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes the slope of the least-squares-fit linear equation determined
        by the (<parameter>X</parameter>, <parameter>Y</parameter>)
        pairs.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regr_sxx</primary>
        </indexterm>
        <function>regr_sxx</function> ( <parameter>Y</parameter> <type>double precision</type>, <parameter>X</parameter> <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes the <quote>sum of squares</quote> of the independent
        variable,
        <literal>sum(<parameter>X</parameter>^2) - sum(<parameter>X</parameter>)^2/<parameter>N</parameter></literal>.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regr_sxy</primary>
        </indexterm>
        <function>regr_sxy</function> ( <parameter>Y</parameter> <type>double precision</type>, <parameter>X</parameter> <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes the <quote>sum of products</quote> of independent times
        dependent variables,
        <literal>sum(<parameter>X</parameter>*<parameter>Y</parameter>) - sum(<parameter>X</parameter>) * sum(<parameter>Y</parameter>)/<parameter>N</parameter></literal>.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regr_syy</primary>
        </indexterm>
        <function>regr_syy</function> ( <parameter>Y</parameter> <type>double precision</type>, <parameter>X</parameter> <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes the <quote>sum of squares</quote> of the dependent
        variable,
        <literal>sum(<parameter>Y</parameter>^2) - sum(<parameter>Y</parameter>)^2/<parameter>N</parameter></literal>.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>standard deviation</primary>
        </indexterm>
        <indexterm>
         <primary>stddev</primary>
        </indexterm>
        <function>stddev</function> ( <replaceable>numeric_type</replaceable> )
        <returnvalue></returnvalue> <type>double precision</type>
        for <type>real</type> or <type>double precision</type>,
        otherwise <type>numeric</type>
       </para>
       <para>
        This is a historical alias for <function>stddev_samp</function>.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>standard deviation</primary>
         <secondary>population</secondary>
        </indexterm>
        <indexterm>
         <primary>stddev_pop</primary>
        </indexterm>
        <function>stddev_pop</function> ( <replaceable>numeric_type</replaceable> )
        <returnvalue></returnvalue> <type>double precision</type>
        for <type>real</type> or <type>double precision</type>,
        otherwise <type>numeric</type>
       </para>
       <para>
        Computes the population standard deviation of the input values.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>standard deviation</primary>
         <secondary>sample</secondary>
        </indexterm>
        <indexterm>
         <primary>stddev_samp</primary>
        </indexterm>
        <function>stddev_samp</function> ( <replaceable>numeric_type</replaceable> )
        <returnvalue></returnvalue> <type>double precision</type>
        for <type>real</type> or <type>double precision</type>,
        otherwise <type>numeric</type>
       </para>
       <para>
        Computes the sample standard deviation of the input values.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>variance</primary>
        </indexterm>
        <function>variance</function> ( <replaceable>numeric_type</replaceable> )
        <returnvalue></returnvalue> <type>double precision</type>
        for <type>real</type> or <type>double precision</type>,
        otherwise <type>numeric</type>
       </para>
       <para>
        This is a historical alias for <function>var_samp</function>.
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>variance</primary>
         <secondary>population</secondary>
        </indexterm>
        <indexterm>
         <primary>var_pop</primary>
        </indexterm>
        <function>var_pop</function> ( <replaceable>numeric_type</replaceable> )
        <returnvalue></returnvalue> <type>double precision</type>
        for <type>real</type> or <type>double precision</type>,
        otherwise <type>numeric</type>
       </para>
       <para>
        Computes the population variance of the input values (square of the
        population standard deviation).
       </para></entry>
       <entry>Yes</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>variance</primary>
         <secondary>sample</secondary>
        </indexterm>
        <indexterm>
         <primary>var_samp</primary>
        </indexterm>
        <function>var_samp</function> ( <replaceable>numeric_type</replaceable> )
        <returnvalue></returnvalue> <type>double precision</type>
        for <type>real</type> or <type>double precision</type>,
        otherwise <type>numeric</type>
       </para>
       <para>
        Computes the sample variance of the input values (square of the sample
        standard deviation).
       </para></entry>
       <entry>Yes</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   <xref linkend="functions-orderedset-table"/> shows some
   aggregate functions that use the <firstterm>ordered-set aggregate</firstterm>
   syntax.  These functions are sometimes referred to as <quote>inverse
   distribution</quote> functions.  Their aggregated input is introduced by
   <literal>ORDER BY</literal>, and they may also take a <firstterm>direct
   argument</firstterm> that is not aggregated, but is computed only once.
   All these functions ignore null values in their aggregated input.
   For those that take a <parameter>fraction</parameter> parameter, the
   fraction value must be between 0 and 1; an error is thrown if not.
   However, a null <parameter>fraction</parameter> value simply produces a
   null result.
  </para>

  <indexterm>
   <primary>ordered-set aggregate</primary>
   <secondary>built-in</secondary>
  </indexterm>
  <indexterm>
   <primary>inverse distribution</primary>
  </indexterm>

   <table id="functions-orderedset-table">
    <title>Ordered-Set Aggregate Functions</title>
    <tgroup cols="2">
     <colspec colname="col1" colwidth="10*"/>
     <colspec colname="col2" colwidth="1*"/>
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
       <entry>Partial Mode</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>mode</primary>
         <secondary>statistical</secondary>
        </indexterm>
        <function>mode</function> () <literal>WITHIN GROUP</literal> ( <literal>ORDER BY</literal> <type>anyelement</type> )
        <returnvalue>anyelement</returnvalue>
       </para>
       <para>
        Computes the <firstterm>mode</firstterm>, the most frequent
        value of the aggregated argument (arbitrarily choosing the first one
        if there are multiple equally-frequent values).  The aggregated
        argument must be of a sortable type.
       </para></entry>
       <entry>No</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>percentile</primary>
         <secondary>continuous</secondary>
        </indexterm>
        <function>percentile_cont</function> ( <parameter>fraction</parameter> <type>double precision</type> ) <literal>WITHIN GROUP</literal> ( <literal>ORDER BY</literal> <type>double precision</type> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para role="func_signature">
        <function>percentile_cont</function> ( <parameter>fraction</parameter> <type>double precision</type> ) <literal>WITHIN GROUP</literal> ( <literal>ORDER BY</literal> <type>interval</type> )
        <returnvalue>interval</returnvalue>
       </para>
       <para>
        Computes the <firstterm>continuous percentile</firstterm>, a value
        corresponding to the specified <parameter>fraction</parameter>
        within the ordered set of aggregated argument values.  This will
        interpolate between adjacent input items if needed.
       </para></entry>
       <entry>No</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>percentile_cont</function> ( <parameter>fractions</parameter> <type>double precision[]</type> ) <literal>WITHIN GROUP</literal> ( <literal>ORDER BY</literal> <type>double precision</type> )
        <returnvalue>double precision[]</returnvalue>
       </para>
       <para role="func_signature">
        <function>percentile_cont</function> ( <parameter>fractions</parameter> <type>double precision[]</type> ) <literal>WITHIN GROUP</literal> ( <literal>ORDER BY</literal> <type>interval</type> )
        <returnvalue>interval[]</returnvalue>
       </para>
       <para>
        Computes multiple continuous percentiles.  The result is an array of
        the same dimensions as the <parameter>fractions</parameter>
        parameter, with each non-null element replaced by the (possibly
        interpolated) value corresponding to that percentile.
       </para></entry>
       <entry>No</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>percentile</primary>
         <secondary>discrete</secondary>
        </indexterm>
        <function>percentile_disc</function> ( <parameter>fraction</parameter> <type>double precision</type> ) <literal>WITHIN GROUP</literal> ( <literal>ORDER BY</literal> <type>anyelement</type> )
        <returnvalue>anyelement</returnvalue>
       </para>
       <para>
        Computes the <firstterm>discrete percentile</firstterm>, the first
        value within the ordered set of aggregated argument values whose
        position in the ordering equals or exceeds the
        specified <parameter>fraction</parameter>.  The aggregated
        argument must be of a sortable type.
       </para></entry>
       <entry>No</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>percentile_disc</function> ( <parameter>fractions</parameter> <type>double precision[]</type> ) <literal>WITHIN GROUP</literal> ( <literal>ORDER BY</literal> <type>anyelement</type> )
        <returnvalue>anyarray</returnvalue>
       </para>
       <para>
        Computes multiple discrete percentiles.  The result is an array of the
        same dimensions as the <parameter>fractions</parameter> parameter,
        with each non-null element replaced by the input value corresponding
        to that percentile.
        The aggregated argument must be of a sortable type.
       </para></entry>
       <entry>No</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <indexterm>
   <primary>hypothetical-set aggregate</primary>
   <secondary>built-in</secondary>
  </indexterm>

  <para>
   Each of the <quote>hypothetical-set</quote> aggregates listed in
   <xref linkend="functions-hypothetical-table"/> is associated with a
   window function of the same name defined in
   <xref linkend="functions-window"/>.  In each case, the aggregate's result
   is the value that the associated window function would have
   returned for the <quote>hypothetical</quote> row constructed from
   <replaceable>args</replaceable>, if such a row had been added to the sorted
   group of rows represented by the <replaceable>sorted_args</replaceable>.
   For each of these functions, the list of direct arguments
   given in <replaceable>args</replaceable> must match the number and types of
   the aggregated arguments given in <replaceable>sorted_args</replaceable>.
   Unlike most built-in aggregates, these aggregates are not strict, that is
   they do not drop input rows containing nulls.  Null values sort according
   to the rule specified in the <literal>ORDER BY</literal> clause.
  </para>

   <table id="functions-hypothetical-table">
    <title>Hypothetical-Set Aggregate Functions</title>
    <tgroup cols="2">
     <colspec colname="col1" colwidth="10*"/>
     <colspec colname="col2" colwidth="1*"/>
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
       <entry>Partial Mode</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>rank</primary>
         <secondary>hypothetical</secondary>
        </indexterm>
        <function>rank</function> ( <replaceable>args</replaceable> ) <literal>WITHIN GROUP</literal> ( <literal>ORDER BY</literal> <replaceable>sorted_args</replaceable> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Computes the rank of the hypothetical row, with gaps; that is, the row
        number of the first row in its peer group.
       </para></entry>
       <entry>No</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>dense_rank</primary>
         <secondary>hypothetical</secondary>
        </indexterm>
        <function>dense_rank</function> ( <replaceable>args</replaceable> ) <literal>WITHIN GROUP</literal> ( <literal>ORDER BY</literal> <replaceable>sorted_args</replaceable> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Computes the rank of the hypothetical row, without gaps; this function
        effectively counts peer groups.
       </para></entry>
       <entry>No</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>percent_rank</primary>
         <secondary>hypothetical</secondary>
        </indexterm>
        <function>percent_rank</function> ( <replaceable>args</replaceable> ) <literal>WITHIN GROUP</literal> ( <literal>ORDER BY</literal> <replaceable>sorted_args</replaceable> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes the relative rank of the hypothetical row, that is
        (<function>rank</function> - 1) / (total rows - 1).
        The value thus ranges from 0 to 1 inclusive.
       </para></entry>
       <entry>No</entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>cume_dist</primary>
         <secondary>hypothetical</secondary>
        </indexterm>
        <function>cume_dist</function> ( <replaceable>args</replaceable> ) <literal>WITHIN GROUP</literal> ( <literal>ORDER BY</literal> <replaceable>sorted_args</replaceable> )
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Computes the cumulative distribution, that is (number of rows
        preceding or peers with hypothetical row) / (total rows).  The value
        thus ranges from 1/<parameter>N</parameter> to 1.
       </para></entry>
       <entry>No</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <table id="functions-grouping-table">
   <title>Grouping Operations</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>GROUPING</primary>
        </indexterm>
        <function>GROUPING</function> ( <replaceable>group_by_expression(s)</replaceable> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns a bit mask indicating which <literal>GROUP BY</literal>
        expressions are not included in the current grouping set.
        Bits are assigned with the rightmost argument corresponding to the
        least-significant bit; each bit is 0 if the corresponding expression
        is included in the grouping criteria of the grouping set generating
        the current result row, and 1 if it is not included.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    The grouping operations shown in
    <xref linkend="functions-grouping-table"/> are used in conjunction with
    grouping sets (see <xref linkend="queries-grouping-sets"/>) to distinguish
    result rows.  The arguments to the <literal>GROUPING</literal> function
    are not actually evaluated, but they must exactly match expressions given
    in the <literal>GROUP BY</literal> clause of the associated query level.
    For example:
<screen>
<prompt>=&gt;</prompt> <userinput>SELECT * FROM items_sold;</userinput>
 make  | model | sales
-------+-------+-------
 Foo   | GT    |  10
 Foo   | Tour  |  20
 Bar   | City  |  15
 Bar   | Sport |  5
(4 rows)

<prompt>=&gt;</prompt> <userinput>SELECT make, model, GROUPING(make,model), sum(sales) FROM items_sold GROUP BY ROLLUP(make,model);</userinput>
 make  | model | grouping | sum
-------+-------+----------+-----
 Foo   | GT    |        0 | 10
 Foo   | Tour  |        0 | 20
 Bar   | City  |        0 | 15
 Bar   | Sport |        0 | 5
 Foo   |       |        1 | 30
 Bar   |       |        1 | 20
       |       |        3 | 50
(7 rows)
</screen>
    Here, the <literal>grouping</literal> value <literal>0</literal> in the
    first four rows shows that those have been grouped normally, over both the
    grouping columns.  The value <literal>1</literal> indicates
    that <literal>model</literal> was not grouped by in the next-to-last two
    rows, and the value <literal>3</literal> indicates that
    neither <literal>make</literal> nor <literal>model</literal> was grouped
    by in the last row (which therefore is an aggregate over all the input
    rows).
   </para>

 </sect1>

 <sect1 id="functions-window">
  <title>Window Functions</title>

  <indexterm zone="functions-window">
   <primary>window function</primary>
   <secondary>built-in</secondary>
  </indexterm>

  <para>
   <firstterm>Window functions</firstterm> provide the ability to perform
   calculations across sets of rows that are related to the current query
   row.  See <xref linkend="tutorial-window"/> for an introduction to this
   feature, and <xref linkend="syntax-window-functions"/> for syntax
   details.
  </para>

  <para>
   The built-in window functions are listed in
   <xref linkend="functions-window-table"/>.  Note that these functions
   <emphasis>must</emphasis> be invoked using window function syntax, i.e., an
   <literal>OVER</literal> clause is required.
  </para>

  <para>
   In addition to these functions, any built-in or user-defined
   ordinary aggregate (i.e., not ordered-set or hypothetical-set aggregates)
   can be used as a window function; see
   <xref linkend="functions-aggregate"/> for a list of the built-in aggregates.
   Aggregate functions act as window functions only when an <literal>OVER</literal>
   clause follows the call; otherwise they act as plain aggregates
   and return a single row for the entire set.
  </para>

   <table id="functions-window-table">
    <title>General-Purpose Window Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>row_number</primary>
        </indexterm>
        <function>row_number</function> ()
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Returns the number of the current row within its partition, counting
        from 1.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>rank</primary>
        </indexterm>
        <function>rank</function> ()
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Returns the rank of the current row, with gaps; that is,
        the <function>row_number</function> of the first row in its peer
        group.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>dense_rank</primary>
        </indexterm>
        <function>dense_rank</function> ()
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Returns the rank of the current row, without gaps; this function
        effectively counts peer groups.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>percent_rank</primary>
        </indexterm>
        <function>percent_rank</function> ()
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Returns the relative rank of the current row, that is
        (<function>rank</function> - 1) / (total partition rows - 1).
        The value thus ranges from 0 to 1 inclusive.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>cume_dist</primary>
        </indexterm>
        <function>cume_dist</function> ()
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Returns the cumulative distribution, that is (number of partition rows
        preceding or peers with current row) / (total partition rows).
        The value thus ranges from 1/<parameter>N</parameter> to 1.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ntile</primary>
        </indexterm>
        <function>ntile</function> ( <parameter>num_buckets</parameter> <type>integer</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns an integer ranging from 1 to the argument value, dividing the
        partition as equally as possible.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>lag</primary>
        </indexterm>
        <function>lag</function> ( <parameter>value</parameter> <type>anyelement</type>
          <optional>, <parameter>offset</parameter> <type>integer</type>
          <optional>, <parameter>default</parameter> <type>anyelement</type> </optional></optional> )
        <returnvalue>anyelement</returnvalue>
       </para>
       <para>
        Returns <parameter>value</parameter> evaluated at
        the row that is <parameter>offset</parameter>
        rows before the current row within the partition; if there is no such
        row, instead returns <parameter>default</parameter>
        (which must be of the same type as
        <parameter>value</parameter>).
        Both <parameter>offset</parameter> and
        <parameter>default</parameter> are evaluated
        with respect to the current row.  If omitted,
        <parameter>offset</parameter> defaults to 1 and
        <parameter>default</parameter> to <literal>NULL</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>lead</primary>
        </indexterm>
        <function>lead</function> ( <parameter>value</parameter> <type>anyelement</type>
          <optional>, <parameter>offset</parameter> <type>integer</type>
          <optional>, <parameter>default</parameter> <type>anyelement</type> </optional></optional> )
        <returnvalue>anyelement</returnvalue>
       </para>
       <para>
        Returns <parameter>value</parameter> evaluated at
        the row that is <parameter>offset</parameter>
        rows after the current row within the partition; if there is no such
        row, instead returns <parameter>default</parameter>
        (which must be of the same type as
        <parameter>value</parameter>).
        Both <parameter>offset</parameter> and
        <parameter>default</parameter> are evaluated
        with respect to the current row.  If omitted,
        <parameter>offset</parameter> defaults to 1 and
        <parameter>default</parameter> to <literal>NULL</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>first_value</primary>
        </indexterm>
        <function>first_value</function> ( <parameter>value</parameter> <type>anyelement</type> )
        <returnvalue>anyelement</returnvalue>
       </para>
       <para>
        Returns <parameter>value</parameter> evaluated
        at the row that is the first row of the window frame.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>last_value</primary>
        </indexterm>
        <function>last_value</function> ( <parameter>value</parameter> <type>anyelement</type> )
        <returnvalue>anyelement</returnvalue>
       </para>
       <para>
        Returns <parameter>value</parameter> evaluated
        at the row that is the last row of the window frame.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>nth_value</primary>
        </indexterm>
        <function>nth_value</function> ( <parameter>value</parameter> <type>anyelement</type>, <parameter>n</parameter> <type>integer</type> )
        <returnvalue>anyelement</returnvalue>
       </para>
       <para>
        Returns <parameter>value</parameter> evaluated
        at the row that is the <parameter>n</parameter>'th
        row of the window frame (counting from 1);
        returns <literal>NULL</literal> if there is no such row.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   All of the functions listed in
   <xref linkend="functions-window-table"/> depend on the sort ordering
   specified by the <literal>ORDER BY</literal> clause of the associated window
   definition.  Rows that are not distinct when considering only the
   <literal>ORDER BY</literal> columns are said to be <firstterm>peers</firstterm>.
   The four ranking functions (including <function>cume_dist</function>) are
   defined so that they give the same answer for all rows of a peer group.
  </para>

  <para>
   Note that <function>first_value</function>, <function>last_value</function>, and
   <function>nth_value</function> consider only the rows within the <quote>window
   frame</quote>, which by default contains the rows from the start of the
   partition through the last peer of the current row.  This is
   likely to give unhelpful results for <function>last_value</function> and
   sometimes also <function>nth_value</function>.  You can redefine the frame by
   adding a suitable frame specification (<literal>RANGE</literal>,
   <literal>ROWS</literal> or <literal>GROUPS</literal>) to
   the <literal>OVER</literal> clause.
   See <xref linkend="syntax-window-functions"/> for more information
   about frame specifications.
  </para>

  <para>
   When an aggregate function is used as a window function, it aggregates
   over the rows within the current row's window frame.
   An aggregate used with <literal>ORDER BY</literal> and the default window frame
   definition produces a <quote>running sum</quote> type of behavior, which may or
   may not be what's wanted.  To obtain
   aggregation over the whole partition, omit <literal>ORDER BY</literal> or use
   <literal>ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING</literal>.
   Other frame specifications can be used to obtain other effects.
  </para>

  <note>
   <para>
    The SQL standard defines a <literal>RESPECT NULLS</literal> or
    <literal>IGNORE NULLS</literal> option for <function>lead</function>, <function>lag</function>,
    <function>first_value</function>, <function>last_value</function>, and
    <function>nth_value</function>.  This is not implemented in
    <productname>PostgreSQL</productname>: the behavior is always the
    same as the standard's default, namely <literal>RESPECT NULLS</literal>.
    Likewise, the standard's <literal>FROM FIRST</literal> or <literal>FROM LAST</literal>
    option for <function>nth_value</function> is not implemented: only the
    default <literal>FROM FIRST</literal> behavior is supported.  (You can achieve
    the result of <literal>FROM LAST</literal> by reversing the <literal>ORDER BY</literal>
    ordering.)
   </para>
  </note>

 </sect1>

 <sect1 id="functions-subquery">
  <title>Subquery Expressions</title>

  <indexterm>
   <primary>EXISTS</primary>
  </indexterm>

  <indexterm>
   <primary>IN</primary>
  </indexterm>

  <indexterm>
   <primary>NOT IN</primary>
  </indexterm>

  <indexterm>
   <primary>ANY</primary>
  </indexterm>

  <indexterm>
   <primary>ALL</primary>
  </indexterm>

  <indexterm>
   <primary>SOME</primary>
  </indexterm>

  <indexterm>
   <primary>subquery</primary>
  </indexterm>

  <para>
   This section describes the <acronym>SQL</acronym>-compliant subquery
   expressions available in <productname>PostgreSQL</productname>.
   All of the expression forms documented in this section return
   Boolean (true/false) results.
  </para>

  <sect2 id="functions-subquery-exists">
   <title><literal>EXISTS</literal></title>

<synopsis>
EXISTS (<replaceable>subquery</replaceable>)
</synopsis>

  <para>
   The argument of <token>EXISTS</token> is an arbitrary <command>SELECT</command> statement,
   or <firstterm>subquery</firstterm>.  The
   subquery is evaluated to determine whether it returns any rows.
   If it returns at least one row, the result of <token>EXISTS</token> is
   <quote>true</quote>; if the subquery returns no rows, the result of <token>EXISTS</token>
   is <quote>false</quote>.
  </para>

  <para>
   The subquery can refer to variables from the surrounding query,
   which will act as constants during any one evaluation of the subquery.
  </para>

  <para>
   The subquery will generally only be executed long enough to determine
   whether at least one row is returned, not all the way to completion.
   It is unwise to write a subquery that has side effects (such as
   calling sequence functions); whether the side effects occur
   might be unpredictable.
  </para>

  <para>
   Since the result depends only on whether any rows are returned,
   and not on the contents of those rows, the output list of the
   subquery is normally unimportant.  A common coding convention is
   to write all <literal>EXISTS</literal> tests in the form
   <literal>EXISTS(SELECT 1 WHERE ...)</literal>.  There are exceptions to
   this rule however, such as subqueries that use <token>INTERSECT</token>.
  </para>

  <para>
   This simple example is like an inner join on <literal>col2</literal>, but
   it produces at most one output row for each <literal>tab1</literal> row,
   even if there are several matching <literal>tab2</literal> rows:
<screen>
SELECT col1
FROM tab1
WHERE EXISTS (SELECT 1 FROM tab2 WHERE col2 = tab1.col2);
</screen>
  </para>
  </sect2>

  <sect2 id="functions-subquery-in">
   <title><literal>IN</literal></title>

<synopsis>
<replaceable>expression</replaceable> IN (<replaceable>subquery</replaceable>)
</synopsis>

  <para>
   The right-hand side is a parenthesized
   subquery, which must return exactly one column.  The left-hand expression
   is evaluated and compared to each row of the subquery result.
   The result of <token>IN</token> is <quote>true</quote> if any equal subquery row is found.
   The result is <quote>false</quote> if no equal row is found (including the
   case where the subquery returns no rows).
  </para>

  <para>
   Note that if the left-hand expression yields null, or if there are
   no equal right-hand values and at least one right-hand row yields
   null, the result of the <token>IN</token> construct will be null, not false.
   This is in accordance with SQL's normal rules for Boolean combinations
   of null values.
  </para>

  <para>
   As with <token>EXISTS</token>, it's unwise to assume that the subquery will
   be evaluated completely.
  </para>

<synopsis>
<replaceable>row_constructor</replaceable> IN (<replaceable>subquery</replaceable>)
</synopsis>

  <para>
   The left-hand side of this form of <token>IN</token> is a row constructor,
   as described in <xref linkend="sql-syntax-row-constructors"/>.
   The right-hand side is a parenthesized
   subquery, which must return exactly as many columns as there are
   expressions in the left-hand row.  The left-hand expressions are
   evaluated and compared row-wise to each row of the subquery result.
   The result of <token>IN</token> is <quote>true</quote> if any equal subquery row is found.
   The result is <quote>false</quote> if no equal row is found (including the
   case where the subquery returns no rows).
  </para>

  <para>
   As usual, null values in the rows are combined per
   the normal rules of SQL Boolean expressions.  Two rows are considered
   equal if all their corresponding members are non-null and equal; the rows
   are unequal if any corresponding members are non-null and unequal;
   otherwise the result of that row comparison is unknown (null).
   If all the per-row results are either unequal or null, with at least one
   null, then the result of <token>IN</token> is null.
  </para>
  </sect2>

  <sect2 id="functions-subquery-notin">
   <title><literal>NOT IN</literal></title>

<synopsis>
<replaceable>expression</replaceable> NOT IN (<replaceable>subquery</replaceable>)
</synopsis>

  <para>
   The right-hand side is a parenthesized
   subquery, which must return exactly one column.  The left-hand expression
   is evaluated and compared to each row of the subquery result.
   The result of <token>NOT IN</token> is <quote>true</quote> if only unequal subquery rows
   are found (including the case where the subquery returns no rows).
   The result is <quote>false</quote> if any equal row is found.
  </para>

  <para>
   Note that if the left-hand expression yields null, or if there are
   no equal right-hand values and at least one right-hand row yields
   null, the result of the <token>NOT IN</token> construct will be null, not true.
   This is in accordance with SQL's normal rules for Boolean combinations
   of null values.
  </para>

  <para>
   As with <token>EXISTS</token>, it's unwise to assume that the subquery will
   be evaluated completely.
  </para>

<synopsis>
<replaceable>row_constructor</replaceable> NOT IN (<replaceable>subquery</replaceable>)
</synopsis>

  <para>
   The left-hand side of this form of <token>NOT IN</token> is a row constructor,
   as described in <xref linkend="sql-syntax-row-constructors"/>.
   The right-hand side is a parenthesized
   subquery, which must return exactly as many columns as there are
   expressions in the left-hand row.  The left-hand expressions are
   evaluated and compared row-wise to each row of the subquery result.
   The result of <token>NOT IN</token> is <quote>true</quote> if only unequal subquery rows
   are found (including the case where the subquery returns no rows).
   The result is <quote>false</quote> if any equal row is found.
  </para>

  <para>
   As usual, null values in the rows are combined per
   the normal rules of SQL Boolean expressions.  Two rows are considered
   equal if all their corresponding members are non-null and equal; the rows
   are unequal if any corresponding members are non-null and unequal;
   otherwise the result of that row comparison is unknown (null).
   If all the per-row results are either unequal or null, with at least one
   null, then the result of <token>NOT IN</token> is null.
  </para>
  </sect2>

  <sect2 id="functions-subquery-any-some">
   <title><literal>ANY</literal>/<literal>SOME</literal></title>

<synopsis>
<replaceable>expression</replaceable> <replaceable>operator</replaceable> ANY (<replaceable>subquery</replaceable>)
<replaceable>expression</replaceable> <replaceable>operator</replaceable> SOME (<replaceable>subquery</replaceable>)
</synopsis>

  <para>
   The right-hand side is a parenthesized
   subquery, which must return exactly one column.  The left-hand expression
   is evaluated and compared to each row of the subquery result using the
   given <replaceable>operator</replaceable>, which must yield a Boolean
   result.
   The result of <token>ANY</token> is <quote>true</quote> if any true result is obtained.
   The result is <quote>false</quote> if no true result is found (including the
   case where the subquery returns no rows).
  </para>

  <para>
   <token>SOME</token> is a synonym for <token>ANY</token>.
   <token>IN</token> is equivalent to <literal>= ANY</literal>.
  </para>

  <para>
   Note that if there are no successes and at least one right-hand row yields
   null for the operator's result, the result of the <token>ANY</token> construct
   will be null, not false.
   This is in accordance with SQL's normal rules for Boolean combinations
   of null values.
  </para>

  <para>
   As with <token>EXISTS</token>, it's unwise to assume that the subquery will
   be evaluated completely.
  </para>

<synopsis>
<replaceable>row_constructor</replaceable> <replaceable>operator</replaceable> ANY (<replaceable>subquery</replaceable>)
<replaceable>row_constructor</replaceable> <replaceable>operator</replaceable> SOME (<replaceable>subquery</replaceable>)
</synopsis>

  <para>
   The left-hand side of this form of <token>ANY</token> is a row constructor,
   as described in <xref linkend="sql-syntax-row-constructors"/>.
   The right-hand side is a parenthesized
   subquery, which must return exactly as many columns as there are
   expressions in the left-hand row.  The left-hand expressions are
   evaluated and compared row-wise to each row of the subquery result,
   using the given <replaceable>operator</replaceable>.
   The result of <token>ANY</token> is <quote>true</quote> if the comparison
   returns true for any subquery row.
   The result is <quote>false</quote> if the comparison returns false for every
   subquery row (including the case where the subquery returns no
   rows).
   The result is NULL if no comparison with a subquery row returns true,
   and at least one comparison returns NULL.
  </para>

  <para>
   See <xref linkend="row-wise-comparison"/> for details about the meaning
   of a row constructor comparison.
  </para>
  </sect2>

  <sect2 id="functions-subquery-all">
   <title><literal>ALL</literal></title>

<synopsis>
<replaceable>expression</replaceable> <replaceable>operator</replaceable> ALL (<replaceable>subquery</replaceable>)
</synopsis>

  <para>
   The right-hand side is a parenthesized
   subquery, which must return exactly one column.  The left-hand expression
   is evaluated and compared to each row of the subquery result using the
   given <replaceable>operator</replaceable>, which must yield a Boolean
   result.
   The result of <token>ALL</token> is <quote>true</quote> if all rows yield true
   (including the case where the subquery returns no rows).
   The result is <quote>false</quote> if any false result is found.
   The result is NULL if no comparison with a subquery row returns false,
   and at least one comparison returns NULL.
  </para>

  <para>
   <token>NOT IN</token> is equivalent to <literal>&lt;&gt; ALL</literal>.
  </para>

  <para>
   As with <token>EXISTS</token>, it's unwise to assume that the subquery will
   be evaluated completely.
  </para>

<synopsis>
<replaceable>row_constructor</replaceable> <replaceable>operator</replaceable> ALL (<replaceable>subquery</replaceable>)
</synopsis>

  <para>
   The left-hand side of this form of <token>ALL</token> is a row constructor,
   as described in <xref linkend="sql-syntax-row-constructors"/>.
   The right-hand side is a parenthesized
   subquery, which must return exactly as many columns as there are
   expressions in the left-hand row.  The left-hand expressions are
   evaluated and compared row-wise to each row of the subquery result,
   using the given <replaceable>operator</replaceable>.
   The result of <token>ALL</token> is <quote>true</quote> if the comparison
   returns true for all subquery rows (including the
   case where the subquery returns no rows).
   The result is <quote>false</quote> if the comparison returns false for any
   subquery row.
   The result is NULL if no comparison with a subquery row returns false,
   and at least one comparison returns NULL.
  </para>

  <para>
   See <xref linkend="row-wise-comparison"/> for details about the meaning
   of a row constructor comparison.
  </para>
  </sect2>

  <sect2>
   <title>Single-Row Comparison</title>

   <indexterm zone="functions-subquery">
    <primary>comparison</primary>
    <secondary>subquery result row</secondary>
   </indexterm>

<synopsis>
<replaceable>row_constructor</replaceable> <replaceable>operator</replaceable> (<replaceable>subquery</replaceable>)
</synopsis>

  <para>
   The left-hand side is a row constructor,
   as described in <xref linkend="sql-syntax-row-constructors"/>.
   The right-hand side is a parenthesized subquery, which must return exactly
   as many columns as there are expressions in the left-hand row. Furthermore,
   the subquery cannot return more than one row.  (If it returns zero rows,
   the result is taken to be null.)  The left-hand side is evaluated and
   compared row-wise to the single subquery result row.
  </para>

  <para>
   See <xref linkend="row-wise-comparison"/> for details about the meaning
   of a row constructor comparison.
  </para>
  </sect2>
 </sect1>


 <sect1 id="functions-comparisons">
  <title>Row and Array Comparisons</title>

  <indexterm>
   <primary>IN</primary>
  </indexterm>

  <indexterm>
   <primary>NOT IN</primary>
  </indexterm>

  <indexterm>
   <primary>ANY</primary>
  </indexterm>

  <indexterm>
   <primary>ALL</primary>
  </indexterm>

  <indexterm>
   <primary>SOME</primary>
  </indexterm>

  <indexterm>
   <primary>composite type</primary>
   <secondary>comparison</secondary>
  </indexterm>

  <indexterm>
   <primary>row-wise comparison</primary>
  </indexterm>

  <indexterm>
   <primary>comparison</primary>
   <secondary>composite type</secondary>
  </indexterm>

  <indexterm>
   <primary>comparison</primary>
   <secondary>row constructor</secondary>
  </indexterm>

  <indexterm>
   <primary>IS DISTINCT FROM</primary>
  </indexterm>

  <indexterm>
   <primary>IS NOT DISTINCT FROM</primary>
  </indexterm>

  <para>
   This section describes several specialized constructs for making
   multiple comparisons between groups of values.  These forms are
   syntactically related to the subquery forms of the previous section,
   but do not involve subqueries.
   The forms involving array subexpressions are
   <productname>PostgreSQL</productname> extensions; the rest are
   <acronym>SQL</acronym>-compliant.
   All of the expression forms documented in this section return
   Boolean (true/false) results.
  </para>

  <sect2 id="functions-comparisons-in-scalar">
   <title><literal>IN</literal></title>

<synopsis>
<replaceable>expression</replaceable> IN (<replaceable>value</replaceable> <optional>, ...</optional>)
</synopsis>

  <para>
   The right-hand side is a parenthesized list
   of scalar expressions.  The result is <quote>true</quote> if the left-hand expression's
   result is equal to any of the right-hand expressions.  This is a shorthand
   notation for

<synopsis>
<replaceable>expression</replaceable> = <replaceable>value1</replaceable>
OR
<replaceable>expression</replaceable> = <replaceable>value2</replaceable>
OR
...
</synopsis>
  </para>

  <para>
   Note that if the left-hand expression yields null, or if there are
   no equal right-hand values and at least one right-hand expression yields
   null, the result of the <token>IN</token> construct will be null, not false.
   This is in accordance with SQL's normal rules for Boolean combinations
   of null values.
  </para>
  </sect2>

  <sect2>
   <title><literal>NOT IN</literal></title>

<synopsis>
<replaceable>expression</replaceable> NOT IN (<replaceable>value</replaceable> <optional>, ...</optional>)
</synopsis>

  <para>
   The right-hand side is a parenthesized list
   of scalar expressions.  The result is <quote>true</quote> if the left-hand expression's
   result is unequal to all of the right-hand expressions.  This is a shorthand
   notation for

<synopsis>
<replaceable>expression</replaceable> &lt;&gt; <replaceable>value1</replaceable>
AND
<replaceable>expression</replaceable> &lt;&gt; <replaceable>value2</replaceable>
AND
...
</synopsis>
  </para>

  <para>
   Note that if the left-hand expression yields null, or if there are
   no equal right-hand values and at least one right-hand expression yields
   null, the result of the <token>NOT IN</token> construct will be null, not true
   as one might naively expect.
   This is in accordance with SQL's normal rules for Boolean combinations
   of null values.
  </para>

  <tip>
  <para>
   <literal>x NOT IN y</literal> is equivalent to <literal>NOT (x IN y)</literal> in all
   cases.  However, null values are much more likely to trip up the novice when
   working with <token>NOT IN</token> than when working with <token>IN</token>.
   It is best to express your condition positively if possible.
  </para>
  </tip>
  </sect2>

  <sect2>
   <title><literal>ANY</literal>/<literal>SOME</literal> (array)</title>

<synopsis>
<replaceable>expression</replaceable> <replaceable>operator</replaceable> ANY (<replaceable>array expression</replaceable>)
<replaceable>expression</replaceable> <replaceable>operator</replaceable> SOME (<replaceable>array expression</replaceable>)
</synopsis>

  <para>
   The right-hand side is a parenthesized expression, which must yield an
   array value.
   The left-hand expression
   is evaluated and compared to each element of the array using the
   given <replaceable>operator</replaceable>, which must yield a Boolean
   result.
   The result of <token>ANY</token> is <quote>true</quote> if any true result is obtained.
   The result is <quote>false</quote> if no true result is found (including the
   case where the array has zero elements).
  </para>

  <para>
   If the array expression yields a null array, the result of
   <token>ANY</token> will be null.  If the left-hand expression yields null,
   the result of <token>ANY</token> is ordinarily null (though a non-strict
   comparison operator could possibly yield a different result).
   Also, if the right-hand array contains any null elements and no true
   comparison result is obtained, the result of <token>ANY</token>
   will be null, not false (again, assuming a strict comparison operator).
   This is in accordance with SQL's normal rules for Boolean combinations
   of null values.
  </para>

  <para>
   <token>SOME</token> is a synonym for <token>ANY</token>.
  </para>
  </sect2>

  <sect2>
   <title><literal>ALL</literal> (array)</title>

<synopsis>
<replaceable>expression</replaceable> <replaceable>operator</replaceable> ALL (<replaceable>array expression</replaceable>)
</synopsis>

  <para>
   The right-hand side is a parenthesized expression, which must yield an
   array value.
   The left-hand expression
   is evaluated and compared to each element of the array using the
   given <replaceable>operator</replaceable>, which must yield a Boolean
   result.
   The result of <token>ALL</token> is <quote>true</quote> if all comparisons yield true
   (including the case where the array has zero elements).
   The result is <quote>false</quote> if any false result is found.
  </para>

  <para>
   If the array expression yields a null array, the result of
   <token>ALL</token> will be null.  If the left-hand expression yields null,
   the result of <token>ALL</token> is ordinarily null (though a non-strict
   comparison operator could possibly yield a different result).
   Also, if the right-hand array contains any null elements and no false
   comparison result is obtained, the result of <token>ALL</token>
   will be null, not true (again, assuming a strict comparison operator).
   This is in accordance with SQL's normal rules for Boolean combinations
   of null values.
  </para>
  </sect2>

  <sect2 id="row-wise-comparison">
   <title>Row Constructor Comparison</title>

<synopsis>
<replaceable>row_constructor</replaceable> <replaceable>operator</replaceable> <replaceable>row_constructor</replaceable>
</synopsis>

  <para>
   Each side is a row constructor,
   as described in <xref linkend="sql-syntax-row-constructors"/>.
   The two row values must have the same number of fields.
   Each side is evaluated and they are compared row-wise.  Row constructor
   comparisons are allowed when the <replaceable>operator</replaceable> is
   <literal>=</literal>,
   <literal>&lt;&gt;</literal>,
   <literal>&lt;</literal>,
   <literal>&lt;=</literal>,
   <literal>&gt;</literal> or
   <literal>&gt;=</literal>.
   Every row element must be of a type which has a default B-tree operator
   class or the attempted comparison may generate an error.
  </para>

  <note>
   <para>
    Errors related to the number or types of elements might not occur if
    the comparison is resolved using earlier columns.
   </para>
  </note>

  <para>
   The <literal>=</literal> and <literal>&lt;&gt;</literal> cases work slightly differently
   from the others.  Two rows are considered
   equal if all their corresponding members are non-null and equal; the rows
   are unequal if any corresponding members are non-null and unequal;
   otherwise the result of the row comparison is unknown (null).
  </para>

  <para>
   For the <literal>&lt;</literal>, <literal>&lt;=</literal>, <literal>&gt;</literal> and
   <literal>&gt;=</literal> cases, the row elements are compared left-to-right,
   stopping as soon as an unequal or null pair of elements is found.
   If either of this pair of elements is null, the result of the
   row comparison is unknown (null); otherwise comparison of this pair
   of elements determines the result.  For example,
   <literal>ROW(1,2,NULL) &lt; ROW(1,3,0)</literal>
   yields true, not null, because the third pair of elements are not
   considered.
  </para>

  <note>
   <para>
    Prior to <productname>PostgreSQL</productname> 8.2, the
    <literal>&lt;</literal>, <literal>&lt;=</literal>, <literal>&gt;</literal> and <literal>&gt;=</literal>
    cases were not handled per SQL specification.  A comparison like
    <literal>ROW(a,b) &lt; ROW(c,d)</literal>
    was implemented as
    <literal>a &lt; c AND b &lt; d</literal>
    whereas the correct behavior is equivalent to
    <literal>a &lt; c OR (a = c AND b &lt; d)</literal>.
   </para>
  </note>

<synopsis>
<replaceable>row_constructor</replaceable> IS DISTINCT FROM <replaceable>row_constructor</replaceable>
</synopsis>

  <para>
   This construct is similar to a <literal>&lt;&gt;</literal> row comparison,
   but it does not yield null for null inputs.  Instead, any null value is
   considered unequal to (distinct from) any non-null value, and any two
   nulls are considered equal (not distinct).  Thus the result will
   either be true or false, never null.
  </para>

<synopsis>
<replaceable>row_constructor</replaceable> IS NOT DISTINCT FROM <replaceable>row_constructor</replaceable>
</synopsis>

  <para>
   This construct is similar to a <literal>=</literal> row comparison,
   but it does not yield null for null inputs.  Instead, any null value is
   considered unequal to (distinct from) any non-null value, and any two
   nulls are considered equal (not distinct).  Thus the result will always
   be either true or false, never null.
  </para>

  </sect2>

  <sect2 id="composite-type-comparison">
   <title>Composite Type Comparison</title>

<synopsis>
<replaceable>record</replaceable> <replaceable>operator</replaceable> <replaceable>record</replaceable>
</synopsis>

  <para>
   The SQL specification requires row-wise comparison to return NULL if the
   result depends on comparing two NULL values or a NULL and a non-NULL.
   <productname>PostgreSQL</productname> does this only when comparing the
   results of two row constructors (as in
   <xref linkend="row-wise-comparison"/>) or comparing a row constructor
   to the output of a subquery (as in <xref linkend="functions-subquery"/>).
   In other contexts where two composite-type values are compared, two
   NULL field values are considered equal, and a NULL is considered larger
   than a non-NULL.  This is necessary in order to have consistent sorting
   and indexing behavior for composite types.
  </para>

  <para>
   Each side is evaluated and they are compared row-wise.  Composite type
   comparisons are allowed when the <replaceable>operator</replaceable> is
   <literal>=</literal>,
   <literal>&lt;&gt;</literal>,
   <literal>&lt;</literal>,
   <literal>&lt;=</literal>,
   <literal>&gt;</literal> or
   <literal>&gt;=</literal>,
   or has semantics similar to one of these.  (To be specific, an operator
   can be a row comparison operator if it is a member of a B-tree operator
   class, or is the negator of the <literal>=</literal> member of a B-tree operator
   class.)  The default behavior of the above operators is the same as for
   <literal>IS [ NOT ] DISTINCT FROM</literal> for row constructors (see
   <xref linkend="row-wise-comparison"/>).
  </para>

  <para>
   To support matching of rows which include elements without a default
   B-tree operator class, the following operators are defined for composite
   type comparison:
   <literal>*=</literal>,
   <literal>*&lt;&gt;</literal>,
   <literal>*&lt;</literal>,
   <literal>*&lt;=</literal>,
   <literal>*&gt;</literal>, and
   <literal>*&gt;=</literal>.
   These operators compare the internal binary representation of the two
   rows.  Two rows might have a different binary representation even
   though comparisons of the two rows with the equality operator is true.
   The ordering of rows under these comparison operators is deterministic
   but not otherwise meaningful.  These operators are used internally
   for materialized views and might be useful for other specialized
   purposes such as replication and B-Tree deduplication (see <xref
   linkend="btree-deduplication"/>).  They are not intended to be
   generally useful for writing queries, though.
  </para>
  </sect2>
 </sect1>

 <sect1 id="functions-srf">
  <title>Set Returning Functions</title>

  <indexterm zone="functions-srf">
   <primary>set returning functions</primary>
   <secondary>functions</secondary>
  </indexterm>

  <para>
   This section describes functions that possibly return more than one row.
   The most widely used functions in this class are series generating
   functions, as detailed in <xref linkend="functions-srf-series"/> and
   <xref linkend="functions-srf-subscripts"/>.  Other, more specialized
   set-returning functions are described elsewhere in this manual.
   See <xref linkend="queries-tablefunctions"/> for ways to combine multiple
   set-returning functions.
  </para>

   <table id="functions-srf-series">
    <title>Series Generating Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>generate_series</primary>
        </indexterm>
        <function>generate_series</function> ( <parameter>start</parameter> <type>integer</type>, <parameter>stop</parameter> <type>integer</type> <optional>, <parameter>step</parameter> <type>integer</type> </optional> )
        <returnvalue>setof integer</returnvalue>
       </para>
       <para role="func_signature">
        <function>generate_series</function> ( <parameter>start</parameter> <type>bigint</type>, <parameter>stop</parameter> <type>bigint</type> <optional>, <parameter>step</parameter> <type>bigint</type> </optional> )
        <returnvalue>setof bigint</returnvalue>
       </para>
       <para role="func_signature">
        <function>generate_series</function> ( <parameter>start</parameter> <type>numeric</type>, <parameter>stop</parameter> <type>numeric</type> <optional>, <parameter>step</parameter> <type>numeric</type> </optional> )
        <returnvalue>setof numeric</returnvalue>
       </para>
       <para>
        Generates a series of values from <parameter>start</parameter>
        to <parameter>stop</parameter>, with a step size
        of <parameter>step</parameter>.  <parameter>step</parameter>
        defaults to 1.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>generate_series</function> ( <parameter>start</parameter> <type>timestamp</type>, <parameter>stop</parameter> <type>timestamp</type>, <parameter>step</parameter> <type>interval</type> )
        <returnvalue>setof timestamp</returnvalue>
       </para>
       <para role="func_signature">
        <function>generate_series</function> ( <parameter>start</parameter> <type>timestamp with time zone</type>, <parameter>stop</parameter> <type>timestamp with time zone</type>, <parameter>step</parameter> <type>interval</type> )
        <returnvalue>setof timestamp with time zone</returnvalue>
       </para>
       <para>
        Generates a series of values from <parameter>start</parameter>
        to <parameter>stop</parameter>, with a step size
        of <parameter>step</parameter>.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   When <parameter>step</parameter> is positive, zero rows are returned if
   <parameter>start</parameter> is greater than <parameter>stop</parameter>.
   Conversely, when <parameter>step</parameter> is negative, zero rows are
   returned if <parameter>start</parameter> is less than <parameter>stop</parameter>.
   Zero rows are also returned if any input is <literal>NULL</literal>.
   It is an error
   for <parameter>step</parameter> to be zero. Some examples follow:
<programlisting>
SELECT * FROM generate_series(2,4);
 generate_series
-----------------
               2
               3
               4
(3 rows)

SELECT * FROM generate_series(5,1,-2);
 generate_series
-----------------
               5
               3
               1
(3 rows)

SELECT * FROM generate_series(4,3);
 generate_series
-----------------
(0 rows)

SELECT generate_series(1.1, 4, 1.3);
 generate_series
-----------------
             1.1
             2.4
             3.7
(3 rows)

-- this example relies on the date-plus-integer operator:
SELECT current_date + s.a AS dates FROM generate_series(0,14,7) AS s(a);
   dates
------------
 2004-02-05
 2004-02-12
 2004-02-19
(3 rows)

SELECT * FROM generate_series('2008-03-01 00:00'::timestamp,
                              '2008-03-04 12:00', '10 hours');
   generate_series
---------------------
 2008-03-01 00:00:00
 2008-03-01 10:00:00
 2008-03-01 20:00:00
 2008-03-02 06:00:00
 2008-03-02 16:00:00
 2008-03-03 02:00:00
 2008-03-03 12:00:00
 2008-03-03 22:00:00
 2008-03-04 08:00:00
(9 rows)
</programlisting>
  </para>

   <table id="functions-srf-subscripts">
    <title>Subscript Generating Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>generate_subscripts</primary>
        </indexterm>
        <function>generate_subscripts</function> ( <parameter>array</parameter> <type>anyarray</type>, <parameter>dim</parameter> <type>integer</type> )
        <returnvalue>setof integer</returnvalue>
       </para>
       <para>
        Generates a series comprising the valid subscripts of
        the <parameter>dim</parameter>'th dimension of the given array.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>generate_subscripts</function> ( <parameter>array</parameter> <type>anyarray</type>, <parameter>dim</parameter> <type>integer</type>,  <parameter>reverse</parameter> <type>boolean</type> )
        <returnvalue>setof integer</returnvalue>
       </para>
       <para>
        Generates a series comprising the valid subscripts of
        the <parameter>dim</parameter>'th dimension of the given array.
        When <parameter>reverse</parameter> is true, returns the series in
        reverse order.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   <function>generate_subscripts</function> is a convenience function that generates
   the set of valid subscripts for the specified dimension of the given
   array.
   Zero rows are returned for arrays that do not have the requested dimension,
   or if any input is <literal>NULL</literal>.
   Some examples follow:
<programlisting>
-- basic usage:
SELECT generate_subscripts('{NULL,1,NULL,2}'::int[], 1) AS s;
 s
---
 1
 2
 3
 4
(4 rows)

-- presenting an array, the subscript and the subscripted
-- value requires a subquery:
SELECT * FROM arrays;
         a
--------------------
 {-1,-2}
 {100,200,300}
(2 rows)

SELECT a AS array, s AS subscript, a[s] AS value
FROM (SELECT generate_subscripts(a, 1) AS s, a FROM arrays) foo;
     array     | subscript | value
---------------+-----------+-------
 {-1,-2}       |         1 |    -1
 {-1,-2}       |         2 |    -2
 {100,200,300} |         1 |   100
 {100,200,300} |         2 |   200
 {100,200,300} |         3 |   300
(5 rows)

-- unnest a 2D array:
CREATE OR REPLACE FUNCTION unnest2(anyarray)
RETURNS SETOF anyelement AS $$
select $1[i][j]
   from generate_subscripts($1,1) g1(i),
        generate_subscripts($1,2) g2(j);
$$ LANGUAGE sql IMMUTABLE;
CREATE FUNCTION
SELECT * FROM unnest2(ARRAY[[1,2],[3,4]]);
 unnest2
---------
       1
       2
       3
       4
(4 rows)
</programlisting>
  </para>

  <indexterm>
   <primary>ordinality</primary>
  </indexterm>

  <para>
   When a function in the <literal>FROM</literal> clause is suffixed
   by <literal>WITH ORDINALITY</literal>, a <type>bigint</type> column is
   appended to the function's output column(s), which starts from 1 and
   increments by 1 for each row of the function's output.
   This is most useful in the case of set returning
   functions such as <function>unnest()</function>.

<programlisting>
-- set returning function WITH ORDINALITY:
SELECT * FROM pg_ls_dir('.') WITH ORDINALITY AS t(ls,n);
       ls        | n
-----------------+----
 pg_serial       |  1
 pg_twophase     |  2
 postmaster.opts |  3
 pg_notify       |  4
 postgresql.conf |  5
 pg_tblspc       |  6
 logfile         |  7
 base            |  8
 postmaster.pid  |  9
 pg_ident.conf   | 10
 global          | 11
 pg_xact         | 12
 pg_snapshots    | 13
 pg_multixact    | 14
 PG_VERSION      | 15
 pg_wal          | 16
 pg_hba.conf     | 17
 pg_stat_tmp     | 18
 pg_subtrans     | 19
(19 rows)
</programlisting>
  </para>

 </sect1>

 <sect1 id="functions-info">
  <title>System Information Functions and Operators</title>

  <para>
   <xref linkend="functions-info-session-table"/> shows several
   functions that extract session and system information.
  </para>

  <para>
   In addition to the functions listed in this section, there are a number of
   functions related to the statistics system that also provide system
   information. See <xref linkend="monitoring-stats-views"/> for more
   information.
  </para>

   <table id="functions-info-session-table">
    <title>Session Information Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>current_catalog</primary>
        </indexterm>
        <function>current_catalog</function>
        <returnvalue>name</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>current_database</primary>
        </indexterm>
        <function>current_database</function> ()
        <returnvalue>name</returnvalue>
       </para>
       <para>
        Returns the name of the current database.  (Databases are
        called <quote>catalogs</quote> in the SQL standard,
        so <function>current_catalog</function> is the standard's
        spelling.)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>current_query</primary>
        </indexterm>
        <function>current_query</function> ()
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the text of the currently executing query, as submitted
        by the client (which might contain more than one statement).
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>current_role</primary>
        </indexterm>
        <function>current_role</function>
        <returnvalue>name</returnvalue>
       </para>
       <para>
        This is equivalent to <function>current_user</function>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>current_schema</primary>
        </indexterm>
        <indexterm>
         <primary>schema</primary>
         <secondary>current</secondary>
        </indexterm>
        <function>current_schema</function>
        <returnvalue>name</returnvalue>
       </para>
       <para role="func_signature">
        <function>current_schema</function> ()
        <returnvalue>name</returnvalue>
       </para>
       <para>
        Returns the name of the schema that is first in the search path (or a
        null value if the search path is empty).  This is the schema that will
        be used for any tables or other named objects that are created without
        specifying a target schema.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>current_schemas</primary>
        </indexterm>
        <indexterm>
         <primary>search path</primary>
         <secondary>current</secondary>
        </indexterm>
        <function>current_schemas</function> ( <parameter>include_implicit</parameter> <type>boolean</type> )
        <returnvalue>name[]</returnvalue>
       </para>
       <para>
        Returns an array of the names of all schemas presently in the
        effective search path, in their priority order.  (Items in the current
        <xref linkend="guc-search-path"/> setting that do not correspond to
        existing, searchable schemas are omitted.)  If the Boolean argument
        is <literal>true</literal>, then implicitly-searched system schemas
        such as <literal>pg_catalog</literal> are included in the result.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>current_user</primary>
        </indexterm>
        <indexterm>
         <primary>user</primary>
         <secondary>current</secondary>
        </indexterm>
        <function>current_user</function>
        <returnvalue>name</returnvalue>
       </para>
       <para>
        Returns the user name of the current execution context.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>inet_client_addr</primary>
        </indexterm>
        <function>inet_client_addr</function> ()
        <returnvalue>inet</returnvalue>
       </para>
       <para>
        Returns the IP address of the current client,
        or <literal>NULL</literal> if the current connection is via a
        Unix-domain socket.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>inet_client_port</primary>
        </indexterm>
        <function>inet_client_port</function> ()
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the IP port number of the current client,
        or <literal>NULL</literal> if the current connection is via a
        Unix-domain socket.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>inet_server_addr</primary>
        </indexterm>
        <function>inet_server_addr</function> ()
        <returnvalue>inet</returnvalue>
       </para>
       <para>
        Returns the IP address on which the server accepted the current
        connection,
        or <literal>NULL</literal> if the current connection is via a
        Unix-domain socket.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>inet_server_port</primary>
        </indexterm>
        <function>inet_server_port</function> ()
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the IP port number on which the server accepted the current
        connection,
        or <literal>NULL</literal> if the current connection is via a
        Unix-domain socket.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_backend_pid</primary>
        </indexterm>
        <function>pg_backend_pid</function> ()
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the process ID of the server process attached to the current
        session.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_blocking_pids</primary>
        </indexterm>
        <function>pg_blocking_pids</function> ( <type>integer</type> )
        <returnvalue>integer[]</returnvalue>
       </para>
       <para>
        Returns an array of the process ID(s) of the sessions that are
        blocking the server process with the specified process ID from
        acquiring a lock, or an empty array if there is no such server process
        or it is not blocked.
       </para>
       <para>
        One server process blocks another if it either holds a lock that
        conflicts with the blocked process's lock request (hard block), or is
        waiting for a lock that would conflict with the blocked process's lock
        request and is ahead of it in the wait queue (soft block).  When using
        parallel queries the result always lists client-visible process IDs
        (that is, <function>pg_backend_pid</function> results) even if the
        actual lock is held or awaited by a child worker process.  As a result
        of that, there may be duplicated PIDs in the result.  Also note that
        when a prepared transaction holds a conflicting lock, it will be
        represented by a zero process ID.
       </para>
       <para>
        Frequent calls to this function could have some impact on database
        performance, because it needs exclusive access to the lock manager's
        shared state for a short time.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_conf_load_time</primary>
        </indexterm>
        <function>pg_conf_load_time</function> ()
        <returnvalue>timestamp with time zone</returnvalue>
       </para>
       <para>
        Returns the time when the server configuration files were last loaded.
        If the current session was alive at the time, this will be the time
        when the session itself re-read the configuration files (so the
        reading will vary a little in different sessions).  Otherwise it is
        the time when the postmaster process re-read the configuration files.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_current_logfile</primary>
        </indexterm>
        <indexterm>
         <primary>Logging</primary>
         <secondary>pg_current_logfile function</secondary>
        </indexterm>
        <indexterm>
          <primary>current_logfiles</primary>
          <secondary>and the pg_current_logfile function</secondary>
        </indexterm>
        <indexterm>
         <primary>Logging</primary>
         <secondary>current_logfiles file and the pg_current_logfile
         function</secondary>
        </indexterm>
        <function>pg_current_logfile</function> ( <optional> <type>text</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the path name of the log file currently in use by the logging
        collector.  The path includes the <xref linkend="guc-log-directory"/>
        directory and the individual log file name.  The result
        is <literal>NULL</literal> if the logging collector is disabled.
        When multiple log files exist, each in a different
        format, <function>pg_current_logfile</function> without an argument
        returns the path of the file having the first format found in the
        ordered list: <literal>stderr</literal>,
        <literal>csvlog</literal>.  <literal>NULL</literal> is returned
        if no log file has any of these formats.
        To request information about a specific log file format, supply
        either <literal>csvlog</literal> or <literal>stderr</literal> as the
        value of the optional parameter. The result is <literal>NULL</literal>
        if the log format requested is not configured in
        <xref linkend="guc-log-destination"/>.
        The result reflects the contents of
        the <filename>current_logfiles</filename> file.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_my_temp_schema</primary>
        </indexterm>
        <function>pg_my_temp_schema</function> ()
        <returnvalue>oid</returnvalue>
       </para>
       <para>
        Returns the OID of the current session's temporary schema, or zero if
        it has none (because it has not created any temporary tables).
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_is_other_temp_schema</primary>
        </indexterm>
        <function>pg_is_other_temp_schema</function> ( <type>oid</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Returns true if the given OID is the OID of another session's
        temporary schema.  (This can be useful, for example, to exclude other
        sessions' temporary tables from a catalog display.)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_jit_available</primary>
        </indexterm>
        <function>pg_jit_available</function> ()
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Returns true if a <acronym>JIT</acronym> compiler extension is
        available (see <xref linkend="jit"/>) and the
        <xref linkend="guc-jit"/> configuration parameter is set to
        <literal>on</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_listening_channels</primary>
        </indexterm>
        <function>pg_listening_channels</function> ()
        <returnvalue>setof text</returnvalue>
       </para>
       <para>
        Returns the set of names of asynchronous notification channels that
        the current session is listening to.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_notification_queue_usage</primary>
        </indexterm>
        <function>pg_notification_queue_usage</function> ()
        <returnvalue>double precision</returnvalue>
       </para>
       <para>
        Returns the fraction (0&ndash;1) of the asynchronous notification
        queue's maximum size that is currently occupied by notifications that
        are waiting to be processed.
        See <xref linkend="sql-listen"/> and <xref linkend="sql-notify"/>
        for more information.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_postmaster_start_time</primary>
        </indexterm>
        <function>pg_postmaster_start_time</function> ()
        <returnvalue>timestamp with time zone</returnvalue>
       </para>
       <para>
        Returns the time when the server started.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_safe_snapshot_blocking_pids</primary>
        </indexterm>
        <function>pg_safe_snapshot_blocking_pids</function> ( <type>integer</type> )
        <returnvalue>integer[]</returnvalue>
       </para>
       <para>
        Returns an array of the process ID(s) of the sessions that are blocking
        the server process with the specified process ID from acquiring a safe
        snapshot, or an empty array if there is no such server process or it
        is not blocked.
       </para>
       <para>
        A session running a <literal>SERIALIZABLE</literal> transaction blocks
        a <literal>SERIALIZABLE READ ONLY DEFERRABLE</literal> transaction
        from acquiring a snapshot until the latter determines that it is safe
        to avoid taking any predicate locks.  See
        <xref linkend="xact-serializable"/> for more information about
        serializable and deferrable transactions.
       </para>
       <para>
        Frequent calls to this function could have some impact on database
        performance, because it needs access to the predicate lock manager's
        shared state for a short time.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_trigger_depth</primary>
        </indexterm>
        <function>pg_trigger_depth</function> ()
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the current nesting level
        of <productname>PostgreSQL</productname> triggers (0 if not called,
        directly or indirectly, from inside a trigger).
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>session_user</primary>
        </indexterm>
        <function>session_user</function>
        <returnvalue>name</returnvalue>
       </para>
       <para>
        Returns the session user's name.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>user</primary>
        </indexterm>
        <function>user</function>
        <returnvalue>name</returnvalue>
       </para>
       <para>
        This is equivalent to <function>current_user</function>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>version</primary>
        </indexterm>
        <function>version</function> ()
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns a string describing the <productname>PostgreSQL</productname>
        server's version.  You can also get this information from
        <xref linkend="guc-server-version"/>, or for a machine-readable
        version use <xref linkend="guc-server-version-num"/>.  Software
        developers should use <varname>server_version_num</varname> (available
        since 8.2) or <xref linkend="libpq-PQserverVersion"/> instead of
        parsing the text version.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <note>
    <para>
     <function>current_catalog</function>,
     <function>current_role</function>,
     <function>current_schema</function>,
     <function>current_user</function>,
     <function>session_user</function>,
     and <function>user</function> have special syntactic status
     in <acronym>SQL</acronym>: they must be called without trailing
     parentheses.  In PostgreSQL, parentheses can optionally be used with
     <function>current_schema</function>, but not with the others.
    </para>
   </note>

   <para>
    The <function>session_user</function> is normally the user who initiated
    the current database connection; but superusers can change this setting
    with <xref linkend="sql-set-session-authorization"/>.
    The <function>current_user</function> is the user identifier
    that is applicable for permission checking. Normally it is equal
    to the session user, but it can be changed with
    <xref linkend="sql-set-role"/>.
    It also changes during the execution of
    functions with the attribute <literal>SECURITY DEFINER</literal>.
    In Unix parlance, the session user is the <quote>real user</quote> and
    the current user is the <quote>effective user</quote>.
    <function>current_role</function> and <function>user</function> are
    synonyms for <function>current_user</function>.  (The SQL standard draws
    a distinction between <function>current_role</function>
    and <function>current_user</function>, but <productname>PostgreSQL</productname>
    does not, since it unifies users and roles into a single kind of entity.)
   </para>

  <indexterm>
   <primary>privilege</primary>
   <secondary>querying</secondary>
  </indexterm>

  <para>
   <xref linkend="functions-info-access-table"/> lists functions that
   allow querying object access privileges programmatically.
   (See <xref linkend="ddl-priv"/> for more information about
   privileges.)
   In these functions, the user whose privileges are being inquired about
   can be specified by name or by OID
   (<structname>pg_authid</structname>.<structfield>oid</structfield>), or if
   the name is given as <literal>public</literal> then the privileges of the
   PUBLIC pseudo-role are checked.  Also, the <parameter>user</parameter>
   argument can be omitted entirely, in which case
   the <function>current_user</function> is assumed.
   The object that is being inquired about can be specified either by name or
   by OID, too.  When specifying by name, a schema name can be included if
   relevant.
   The access privilege of interest is specified by a text string, which must
   evaluate to one of the appropriate privilege keywords for the object's type
   (e.g., <literal>SELECT</literal>).  Optionally, <literal>WITH GRANT
   OPTION</literal> can be added to a privilege type to test whether the
   privilege is held with grant option. Also, multiple privilege types can be
   listed separated by commas, in which case the result will be true if any of
   the listed privileges is held. (Case of the privilege string is not
   significant, and extra whitespace is allowed between but not within
   privilege names.)
   Some examples:
<programlisting>
SELECT has_table_privilege('myschema.mytable', 'select');
SELECT has_table_privilege('joe', 'mytable', 'INSERT, SELECT WITH GRANT OPTION');
</programlisting>
  </para>

   <table id="functions-info-access-table">
    <title>Access Privilege Inquiry Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>has_any_column_privilege</primary>
        </indexterm>
        <function>has_any_column_privilege</function> (
          <optional> <parameter>user</parameter> <type>name</type> or <type>oid</type>, </optional>
          <parameter>table</parameter> <type>text</type> or <type>oid</type>,
          <parameter>privilege</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does user have privilege for any column of table?
        This succeeds either if the privilege is held for the whole table, or
        if there is a column-level grant of the privilege for at least one
        column.
        Allowable privilege types are
        <literal>SELECT</literal>, <literal>INSERT</literal>,
        <literal>UPDATE</literal>, and <literal>REFERENCES</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>has_column_privilege</primary>
        </indexterm>
        <function>has_column_privilege</function> (
          <optional> <parameter>user</parameter> <type>name</type> or <type>oid</type>, </optional>
          <parameter>table</parameter> <type>text</type> or <type>oid</type>,
          <parameter>column</parameter> <type>text</type> or <type>smallint</type>,
          <parameter>privilege</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does user have privilege for the specified table column?
        This succeeds either if the privilege is held for the whole table, or
        if there is a column-level grant of the privilege for the column.
        The column can be specified by name or by attribute number
        (<structname>pg_attribute</structname>.<structfield>attnum</structfield>).
        Allowable privilege types are
        <literal>SELECT</literal>, <literal>INSERT</literal>,
        <literal>UPDATE</literal>, and <literal>REFERENCES</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>has_database_privilege</primary>
        </indexterm>
        <function>has_database_privilege</function> (
          <optional> <parameter>user</parameter> <type>name</type> or <type>oid</type>, </optional>
          <parameter>database</parameter> <type>text</type> or <type>oid</type>,
          <parameter>privilege</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does user have privilege for database?
        Allowable privilege types are
        <literal>CREATE</literal>,
        <literal>CONNECT</literal>,
        <literal>TEMPORARY</literal>, and
        <literal>TEMP</literal> (which is equivalent to
        <literal>TEMPORARY</literal>).
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>has_foreign_data_wrapper_privilege</primary>
        </indexterm>
        <function>has_foreign_data_wrapper_privilege</function> (
          <optional> <parameter>user</parameter> <type>name</type> or <type>oid</type>, </optional>
          <parameter>fdw</parameter> <type>text</type> or <type>oid</type>,
          <parameter>privilege</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does user have privilege for foreign-data wrapper?
        The only allowable privilege type is <literal>USAGE</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>has_function_privilege</primary>
        </indexterm>
        <function>has_function_privilege</function> (
          <optional> <parameter>user</parameter> <type>name</type> or <type>oid</type>, </optional>
          <parameter>function</parameter> <type>text</type> or <type>oid</type>,
          <parameter>privilege</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does user have privilege for function?
        The only allowable privilege type is <literal>EXECUTE</literal>.
       </para>
       <para>
        When specifying a function by name rather than by OID, the allowed
        input is the same as for the <type>regprocedure</type> data type (see
        <xref linkend="datatype-oid"/>).
        An example is:
<programlisting>
SELECT has_function_privilege('joeuser', 'myfunc(int, text)', 'execute');
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>has_language_privilege</primary>
        </indexterm>
        <function>has_language_privilege</function> (
          <optional> <parameter>user</parameter> <type>name</type> or <type>oid</type>, </optional>
          <parameter>language</parameter> <type>text</type> or <type>oid</type>,
          <parameter>privilege</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does user have privilege for language?
        The only allowable privilege type is <literal>USAGE</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>has_schema_privilege</primary>
        </indexterm>
        <function>has_schema_privilege</function> (
          <optional> <parameter>user</parameter> <type>name</type> or <type>oid</type>, </optional>
          <parameter>schema</parameter> <type>text</type> or <type>oid</type>,
          <parameter>privilege</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does user have privilege for schema?
        Allowable privilege types are
        <literal>CREATE</literal> and
        <literal>USAGE</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>has_sequence_privilege</primary>
        </indexterm>
        <function>has_sequence_privilege</function> (
          <optional> <parameter>user</parameter> <type>name</type> or <type>oid</type>, </optional>
          <parameter>sequence</parameter> <type>text</type> or <type>oid</type>,
          <parameter>privilege</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does user have privilege for sequence?
        Allowable privilege types are
        <literal>USAGE</literal>,
        <literal>SELECT</literal>, and
        <literal>UPDATE</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>has_server_privilege</primary>
        </indexterm>
        <function>has_server_privilege</function> (
          <optional> <parameter>user</parameter> <type>name</type> or <type>oid</type>, </optional>
          <parameter>server</parameter> <type>text</type> or <type>oid</type>,
          <parameter>privilege</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does user have privilege for foreign server?
        The only allowable privilege type is <literal>USAGE</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>has_table_privilege</primary>
        </indexterm>
        <function>has_table_privilege</function> (
          <optional> <parameter>user</parameter> <type>name</type> or <type>oid</type>, </optional>
          <parameter>table</parameter> <type>text</type> or <type>oid</type>,
          <parameter>privilege</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does user have privilege for table?
        Allowable privilege types
        are <literal>SELECT</literal>, <literal>INSERT</literal>,
        <literal>UPDATE</literal>, <literal>DELETE</literal>,
        <literal>TRUNCATE</literal>, <literal>REFERENCES</literal>,
        and <literal>TRIGGER</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>has_tablespace_privilege</primary>
        </indexterm>
        <function>has_tablespace_privilege</function> (
          <optional> <parameter>user</parameter> <type>name</type> or <type>oid</type>, </optional>
          <parameter>tablespace</parameter> <type>text</type> or <type>oid</type>,
          <parameter>privilege</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does user have privilege for tablespace?
        The only allowable privilege type is <literal>CREATE</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>has_type_privilege</primary>
        </indexterm>
        <function>has_type_privilege</function> (
          <optional> <parameter>user</parameter> <type>name</type> or <type>oid</type>, </optional>
          <parameter>type</parameter> <type>text</type> or <type>oid</type>,
          <parameter>privilege</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does user have privilege for data type?
        The only allowable privilege type is <literal>USAGE</literal>.
        When specifying a type by name rather than by OID, the allowed input
        is the same as for the <type>regtype</type> data type (see
        <xref linkend="datatype-oid"/>).
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_has_role</primary>
        </indexterm>
        <function>pg_has_role</function> (
          <optional> <parameter>user</parameter> <type>name</type> or <type>oid</type>, </optional>
          <parameter>role</parameter> <type>text</type> or <type>oid</type>,
          <parameter>privilege</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Does user have privilege for role?
        Allowable privilege types are
        <literal>MEMBER</literal> and <literal>USAGE</literal>.
        <literal>MEMBER</literal> denotes direct or indirect membership in
        the role (that is, the right to do <command>SET ROLE</command>), while
        <literal>USAGE</literal> denotes whether the privileges of the role
        are immediately available without doing <command>SET ROLE</command>.
        This function does not allow the special case of
        setting <parameter>user</parameter> to <literal>public</literal>,
        because the PUBLIC pseudo-role can never be a member of real roles.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>row_security_active</primary>
        </indexterm>
        <function>row_security_active</function> (
          <parameter>table</parameter> <type>text</type> or <type>oid</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is row-level security active for the specified table in the context of
        the current user and current environment?
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   <xref linkend="functions-aclitem-op-table"/> shows the operators
   available for the <type>aclitem</type> type, which is the catalog
   representation of access privileges.  See <xref linkend="ddl-priv"/>
   for information about how to read access privilege values.
  </para>

    <table id="functions-aclitem-op-table">
     <title><type>aclitem</type> Operators</title>
     <tgroup cols="1">
      <thead>
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         Operator
        </para>
        <para>
         Description
        </para>
        <para>
         Example(s)
        </para></entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>aclitemeq</primary>
         </indexterm>
         <type>aclitem</type> <literal>=</literal> <type>aclitem</type>
         <returnvalue>boolean</returnvalue>
        </para>
        <para>
         Are <type>aclitem</type>s equal?  (Notice that
         type <type>aclitem</type> lacks the usual set of comparison
         operators; it has only equality.  In turn, <type>aclitem</type>
         arrays can only be compared for equality.)
        </para>
        <para>
         <literal>'calvin=r*w/hobbes'::aclitem = 'calvin=r*w*/hobbes'::aclitem</literal>
         <returnvalue>f</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>aclcontains</primary>
         </indexterm>
         <type>aclitem[]</type> <literal>@&gt;</literal> <type>aclitem</type>
         <returnvalue>boolean</returnvalue>
        </para>
        <para>
         Does array contain the specified privileges?  (This is true if there
         is an array entry that matches the <type>aclitem</type>'s grantee and
         grantor, and has at least the specified set of privileges.)
        </para>
        <para>
         <literal>'{calvin=r*w/hobbes,hobbes=r*w*/postgres}'::aclitem[] @&gt; 'calvin=r*/hobbes'::aclitem</literal>
         <returnvalue>t</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>aclitem[]</type> <literal>~</literal> <type>aclitem</type>
         <returnvalue>boolean</returnvalue>
        </para>
        <para>
         This is a deprecated alias for <literal>@&gt;</literal>.
        </para>
        <para>
         <literal>'{calvin=r*w/hobbes,hobbes=r*w*/postgres}'::aclitem[] ~ 'calvin=r*/hobbes'::aclitem</literal>
         <returnvalue>t</returnvalue>
        </para></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    <xref linkend="functions-aclitem-fn-table"/> shows some additional
    functions to manage the <type>aclitem</type> type.
   </para>

   <table id="functions-aclitem-fn-table">
    <title><type>aclitem</type> Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>acldefault</primary>
        </indexterm>
        <function>acldefault</function> (
          <parameter>type</parameter> <type>"char"</type>,
          <parameter>ownerId</parameter> <type>oid</type> )
        <returnvalue>aclitem[]</returnvalue>
       </para>
       <para>
        Constructs an <type>aclitem</type> array holding the default access
        privileges for an object of type <parameter>type</parameter> belonging
        to the role with OID <parameter>ownerId</parameter>.  This represents
        the access privileges that will be assumed when an object's ACL entry
        is null.  (The default access privileges are described in
        <xref linkend="ddl-priv"/>.)
        The <parameter>type</parameter> parameter must be one of
        'c' for <literal>COLUMN</literal>,
        'r' for <literal>TABLE</literal> and table-like objects,
        's' for <literal>SEQUENCE</literal>,
        'd' for <literal>DATABASE</literal>,
        'f' for <literal>FUNCTION</literal> or <literal>PROCEDURE</literal>,
        'l' for <literal>LANGUAGE</literal>,
        'L' for <literal>LARGE OBJECT</literal>,
        'n' for <literal>SCHEMA</literal>,
        't' for <literal>TABLESPACE</literal>,
        'F' for <literal>FOREIGN DATA WRAPPER</literal>,
        'S' for <literal>FOREIGN SERVER</literal>,
        or
        'T' for <literal>TYPE</literal> or <literal>DOMAIN</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>aclexplode</primary>
        </indexterm>
        <function>aclexplode</function> ( <type>aclitem[]</type> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>grantor</parameter> <type>oid</type>,
        <parameter>grantee</parameter> <type>oid</type>,
        <parameter>privilege_type</parameter> <type>text</type>,
        <parameter>is_grantable</parameter> <type>boolean</type> )
       </para>
       <para>
        Returns the <type>aclitem</type> array as a set of rows.
        If the grantee is the pseudo-role PUBLIC, it is represented by zero in
        the <parameter>grantee</parameter> column.  Each granted privilege is
        represented as <literal>SELECT</literal>, <literal>INSERT</literal>,
        etc.  Note that each privilege is broken out as a separate row, so
        only one keyword appears in the <parameter>privilege_type</parameter>
        column.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>makeaclitem</primary>
        </indexterm>
        <function>makeaclitem</function> (
          <parameter>grantee</parameter> <type>oid</type>,
          <parameter>grantor</parameter> <type>oid</type>,
          <parameter>privileges</parameter> <type>text</type>,
          <parameter>is_grantable</parameter> <type>boolean</type> )
        <returnvalue>aclitem</returnvalue>
       </para>
       <para>
        Constructs an <type>aclitem</type> with the given properties.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   <xref linkend="functions-info-schema-table"/> shows functions that
   determine whether a certain object is <firstterm>visible</firstterm> in the
   current schema search path.
   For example, a table is said to be visible if its
   containing schema is in the search path and no table of the same
   name appears earlier in the search path.  This is equivalent to the
   statement that the table can be referenced by name without explicit
   schema qualification.  Thus, to list the names of all visible tables:
<programlisting>
SELECT relname FROM pg_class WHERE pg_table_is_visible(oid);
</programlisting>
   For functions and operators, an object in the search path is said to be
   visible if there is no object of the same name <emphasis>and argument data
   type(s)</emphasis> earlier in the path.  For operator classes and families,
   both the name and the associated index access method are considered.
  </para>

   <indexterm>
    <primary>search path</primary>
    <secondary>object visibility</secondary>
   </indexterm>

   <table id="functions-info-schema-table">
    <title>Schema Visibility Inquiry Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_collation_is_visible</primary>
        </indexterm>
        <function>pg_collation_is_visible</function> ( <parameter>collation</parameter> <type>oid</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is collation visible in search path?
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_conversion_is_visible</primary>
        </indexterm>
        <function>pg_conversion_is_visible</function> ( <parameter>conversion</parameter> <type>oid</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is conversion visible in search path?
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_function_is_visible</primary>
        </indexterm>
        <function>pg_function_is_visible</function> ( <parameter>function</parameter> <type>oid</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is function visible in search path?
        (This also works for procedures and aggregates.)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_opclass_is_visible</primary>
        </indexterm>
        <function>pg_opclass_is_visible</function> ( <parameter>opclass</parameter> <type>oid</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is operator class visible in search path?
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_operator_is_visible</primary>
        </indexterm>
        <function>pg_operator_is_visible</function> ( <parameter>operator</parameter> <type>oid</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is operator visible in search path?
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_opfamily_is_visible</primary>
        </indexterm>
        <function>pg_opfamily_is_visible</function> ( <parameter>opclass</parameter> <type>oid</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is operator family visible in search path?
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_statistics_obj_is_visible</primary>
        </indexterm>
        <function>pg_statistics_obj_is_visible</function> ( <parameter>stat</parameter> <type>oid</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is statistics object visible in search path?
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_table_is_visible</primary>
        </indexterm>
        <function>pg_table_is_visible</function> ( <parameter>table</parameter> <type>oid</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is table visible in search path?
        (This works for all types of relations, including views, materialized
        views, indexes, sequences and foreign tables.)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_ts_config_is_visible</primary>
        </indexterm>
        <function>pg_ts_config_is_visible</function> ( <parameter>config</parameter> <type>oid</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is text search configuration visible in search path?
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_ts_dict_is_visible</primary>
        </indexterm>
        <function>pg_ts_dict_is_visible</function> ( <parameter>dict</parameter> <type>oid</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is text search dictionary visible in search path?
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_ts_parser_is_visible</primary>
        </indexterm>
        <function>pg_ts_parser_is_visible</function> ( <parameter>parser</parameter> <type>oid</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is text search parser visible in search path?
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_ts_template_is_visible</primary>
        </indexterm>
        <function>pg_ts_template_is_visible</function> ( <parameter>template</parameter> <type>oid</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is text search template visible in search path?
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_type_is_visible</primary>
        </indexterm>
        <function>pg_type_is_visible</function> ( <parameter>type</parameter> <type>oid</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is type (or domain) visible in search path?
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    All these functions require object OIDs to identify the object to be
    checked.  If you want to test an object by name, it is convenient to use
    the OID alias types (<type>regclass</type>, <type>regtype</type>,
    <type>regprocedure</type>, <type>regoperator</type>, <type>regconfig</type>,
    or <type>regdictionary</type>),
    for example:
<programlisting>
SELECT pg_type_is_visible('myschema.widget'::regtype);
</programlisting>
    Note that it would not make much sense to test a non-schema-qualified
    type name in this way &mdash; if the name can be recognized at all, it must be visible.
   </para>

  <para>
   <xref linkend="functions-info-catalog-table"/> lists functions that
   extract information from the system catalogs.
  </para>

   <table id="functions-info-catalog-table">
    <title>System Catalog Information Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>format_type</primary>
        </indexterm>
        <function>format_type</function> ( <parameter>type</parameter> <type>oid</type>, <parameter>typemod</parameter> <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the SQL name for a data type that is identified by its type
        OID and possibly a type modifier.  Pass NULL for the type modifier if
        no specific modifier is known.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_get_constraintdef</primary>
        </indexterm>
        <function>pg_get_constraintdef</function> ( <parameter>constraint</parameter> <type>oid</type> <optional>, <parameter>pretty</parameter> <type>boolean</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Reconstructs the creating command for a constraint.
        (This is a decompiled reconstruction, not the original text
        of the command.)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_get_expr</primary>
        </indexterm>
        <function>pg_get_expr</function> ( <parameter>expr</parameter> <type>pg_node_tree</type>, <parameter>relation</parameter> <type>oid</type> <optional>, <parameter>pretty</parameter> <type>boolean</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Decompiles the internal form of an expression stored in the system
        catalogs, such as the default value for a column.  If the expression
        might contain Vars, specify the OID of the relation they refer to as
        the second parameter; if no Vars are expected, passing zero is
        sufficient.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_get_functiondef</primary>
        </indexterm>
        <function>pg_get_functiondef</function> ( <parameter>func</parameter> <type>oid</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Reconstructs the creating command for a function or procedure.
        (This is a decompiled reconstruction, not the original text
        of the command.)
        The result is a complete <command>CREATE OR REPLACE FUNCTION</command>
        or <command>CREATE OR REPLACE PROCEDURE</command> statement.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_get_function_arguments</primary>
        </indexterm>
        <function>pg_get_function_arguments</function> ( <parameter>func</parameter> <type>oid</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Reconstructs the argument list of a function or procedure, in the form
        it would need to appear in within <command>CREATE FUNCTION</command>
        (including default values).
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_get_function_identity_arguments</primary>
        </indexterm>
        <function>pg_get_function_identity_arguments</function> ( <parameter>func</parameter> <type>oid</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Reconstructs the argument list necessary to identify a function or
        procedure, in the form it would need to appear in within commands such
        as <command>ALTER FUNCTION</command>.  This form omits default values.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_get_function_result</primary>
        </indexterm>
        <function>pg_get_function_result</function> ( <parameter>func</parameter> <type>oid</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Reconstructs the <literal>RETURNS</literal> clause of a function, in
        the form it would need to appear in within <command>CREATE
        FUNCTION</command>.  Returns <literal>NULL</literal> for a procedure.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_get_indexdef</primary>
        </indexterm>
        <function>pg_get_indexdef</function> ( <parameter>index</parameter> <type>oid</type> <optional>, <parameter>column</parameter> <type>integer</type>, <parameter>pretty</parameter> <type>boolean</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Reconstructs the creating command for an index.
        (This is a decompiled reconstruction, not the original text
        of the command.)  If <parameter>column</parameter> is supplied and is
        not zero, only the definition of that column is reconstructed.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_get_keywords</primary>
        </indexterm>
        <function>pg_get_keywords</function> ()
        <returnvalue>setof record</returnvalue>
        ( <parameter>word</parameter> <type>text</type>,
        <parameter>catcode</parameter> <type>"char"</type>,
        <parameter>barelabel</parameter> <type>boolean</type>,
        <parameter>catdesc</parameter> <type>text</type>,
        <parameter>baredesc</parameter> <type>text</type> )
       </para>
       <para>
        Returns a set of records describing the SQL keywords recognized by the
        server.  The <parameter>word</parameter> column contains the
        keyword.  The <parameter>catcode</parameter> column contains a
        category code: <literal>U</literal> for an unreserved
        keyword, <literal>C</literal> for a keyword that can be a column
        name, <literal>T</literal> for a keyword that can be a type or
        function name, or <literal>R</literal> for a fully reserved keyword.
        The <parameter>barelabel</parameter> column
        contains <literal>true</literal> if the keyword can be used as
        a <quote>bare</quote> column label in <command>SELECT</command> lists,
        or <literal>false</literal> if it can only be used
        after <literal>AS</literal>.
        The <parameter>catdesc</parameter> column contains a
        possibly-localized string describing the keyword's category.
        The <parameter>baredesc</parameter> column contains a
        possibly-localized string describing the keyword's column label status.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_get_ruledef</primary>
        </indexterm>
        <function>pg_get_ruledef</function> ( <parameter>rule</parameter> <type>oid</type> <optional>, <parameter>pretty</parameter> <type>boolean</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Reconstructs the creating command for a rule.
        (This is a decompiled reconstruction, not the original text
        of the command.)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_get_serial_sequence</primary>
        </indexterm>
        <function>pg_get_serial_sequence</function> ( <parameter>table</parameter> <type>text</type>, <parameter>column</parameter> <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the name of the sequence associated with a column,
        or NULL if no sequence is associated with the column.
        If the column is an identity column, the associated sequence is the
        sequence internally created for that column.
        For columns created using one of the serial types
        (<type>serial</type>, <type>smallserial</type>, <type>bigserial</type>),
        it is the sequence created for that serial column definition.
        In the latter case, the association can be modified or removed
        with <command>ALTER SEQUENCE OWNED BY</command>.
        (This function probably should have been
        called <function>pg_get_owned_sequence</function>; its current name
        reflects the fact that it has historically been used with serial-type
        columns.)  The first parameter is a table name with optional
        schema, and the second parameter is a column name.  Because the first
        parameter potentially contains both schema and table names, it is
        parsed per usual SQL rules, meaning it is lower-cased by default.
        The second parameter, being just a column name, is treated literally
        and so has its case preserved.  The result is suitably formatted
        for passing to the sequence functions (see
        <xref linkend="functions-sequence"/>).
       </para>
       <para>
        A typical use is in reading the current value of the sequence for an
        identity or serial column, for example:
<programlisting>
SELECT currval(pg_get_serial_sequence('sometable', 'id'));
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_get_statisticsobjdef</primary>
        </indexterm>
        <function>pg_get_statisticsobjdef</function> ( <parameter>statobj</parameter> <type>oid</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Reconstructs the creating command for an extended statistics object.
        (This is a decompiled reconstruction, not the original text
        of the command.)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_get_triggerdef</primary>
        </indexterm>
<function>pg_get_triggerdef</function> ( <parameter>trigger</parameter> <type>oid</type> <optional>, <parameter>pretty</parameter> <type>boolean</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Reconstructs the creating command for a trigger.
        (This is a decompiled reconstruction, not the original text
        of the command.)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_get_userbyid</primary>
        </indexterm>
        <function>pg_get_userbyid</function> ( <parameter>role</parameter> <type>oid</type> )
        <returnvalue>name</returnvalue>
       </para>
       <para>
        Returns a role's name given its OID.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_get_viewdef</primary>
        </indexterm>
        <function>pg_get_viewdef</function> ( <parameter>view</parameter> <type>oid</type> <optional>, <parameter>pretty</parameter> <type>boolean</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Reconstructs the underlying <command>SELECT</command> command for a
        view or materialized view.  (This is a decompiled reconstruction, not
        the original text of the command.)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>pg_get_viewdef</function> ( <parameter>view</parameter> <type>oid</type>, <parameter>wrap_column</parameter> <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Reconstructs the underlying <command>SELECT</command> command for a
        view or materialized view.  (This is a decompiled reconstruction, not
        the original text of the command.)  In this form of the function,
        pretty-printing is always enabled, and long lines are wrapped to try
        to keep them shorter than the specified number of columns.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>pg_get_viewdef</function> ( <parameter>view</parameter> <type>text</type> <optional>, <parameter>pretty</parameter> <type>boolean</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Reconstructs the underlying <command>SELECT</command> command for a
        view or materialized view, working from a textual name for the view
        rather than its OID.  (This is deprecated; use the OID variant
        instead.)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_index_column_has_property</primary>
        </indexterm>
        <function>pg_index_column_has_property</function> ( <parameter>index</parameter> <type>regclass</type>, <parameter>column</parameter> <type>integer</type>, <parameter>property</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Tests whether an index column has the named property.
        Common index column properties are listed in
        <xref linkend="functions-info-index-column-props"/>.
        (Note that extension access methods can define additional property
        names for their indexes.)
        <literal>NULL</literal> is returned if the property name is not known
        or does not apply to the particular object, or if the OID or column
        number does not identify a valid object.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_index_has_property</primary>
        </indexterm>
        <function>pg_index_has_property</function> ( <parameter>index</parameter> <type>regclass</type>, <parameter>property</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Tests whether an index has the named property.
        Common index properties are listed in
        <xref linkend="functions-info-index-props"/>.
        (Note that extension access methods can define additional property
        names for their indexes.)
        <literal>NULL</literal> is returned if the property name is not known
        or does not apply to the particular object, or if the OID does not
        identify a valid object.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_indexam_has_property</primary>
        </indexterm>
        <function>pg_indexam_has_property</function> ( <parameter>am</parameter> <type>oid</type>, <parameter>property</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Tests whether an index access method has the named property.
        Access method properties are listed in
        <xref linkend="functions-info-indexam-props"/>.
        <literal>NULL</literal> is returned if the property name is not known
        or does not apply to the particular object, or if the OID does not
        identify a valid object.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_options_to_table</primary>
        </indexterm>
        <function>pg_options_to_table</function> ( <parameter>options_array</parameter> <type>text[]</type> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>option_name</parameter> <type>text</type>,
        <parameter>option_value</parameter> <type>text</type> )
       </para>
       <para>
        Returns the set of storage options represented by a value from
        <structname>pg_class</structname>.<structfield>reloptions</structfield> or
        <structname>pg_attribute</structname>.<structfield>attoptions</structfield>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_tablespace_databases</primary>
        </indexterm>
        <function>pg_tablespace_databases</function> ( <parameter>tablespace</parameter> <type>oid</type> )
        <returnvalue>setof oid</returnvalue>
       </para>
       <para>
        Returns the set of OIDs of databases that have objects stored in the
        specified tablespace.  If this function returns any rows, the
        tablespace is not empty and cannot be dropped.  To identify the specific
        objects populating the tablespace, you will need to connect to the
        database(s) identified by <function>pg_tablespace_databases</function>
        and query their <structname>pg_class</structname> catalogs.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_tablespace_location</primary>
        </indexterm>
        <function>pg_tablespace_location</function> ( <parameter>tablespace</parameter> <type>oid</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the file system path that this tablespace is located in.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_typeof</primary>
        </indexterm>
        <function>pg_typeof</function> ( <type>"any"</type> )
        <returnvalue>regtype</returnvalue>
       </para>
       <para>
        Returns the OID of the data type of the value that is passed to it.
        This can be helpful for troubleshooting or dynamically constructing
        SQL queries.  The function is declared as
        returning <type>regtype</type>, which is an OID alias type (see
        <xref linkend="datatype-oid"/>); this means that it is the same as an
        OID for comparison purposes but displays as a type name.
       </para>
       <para>
        For example:
<programlisting>
SELECT pg_typeof(33);
 pg_typeof
-----------
 integer

SELECT typlen FROM pg_type WHERE oid = pg_typeof(33);
 typlen
--------
      4
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>COLLATION FOR</primary>
        </indexterm>
        <function>COLLATION FOR</function> ( <type>"any"</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the name of the collation of the value that is passed to it.
        The value is quoted and schema-qualified if necessary.  If no
        collation was derived for the argument expression,
        then <literal>NULL</literal> is returned.  If the argument is not of a
        collatable data type, then an error is raised.
       </para>
       <para>
        For example:
<programlisting>
SELECT collation for (description) FROM pg_description LIMIT 1;
 pg_collation_for
------------------
 "default"

SELECT collation for ('foo' COLLATE "de_DE");
 pg_collation_for
------------------
 "de_DE"
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_regclass</primary>
        </indexterm>
        <function>to_regclass</function> ( <type>text</type> )
        <returnvalue>regclass</returnvalue>
       </para>
       <para>
        Translates a textual relation name to its OID.  A similar result is
        obtained by casting the string to type <type>regclass</type> (see
        <xref linkend="datatype-oid"/>); however, this function will return
        <literal>NULL</literal> rather than throwing an error if the name is
        not found.  Also unlike the cast, this does not accept
        a numeric OID as input.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_regcollation</primary>
        </indexterm>
        <function>to_regcollation</function> ( <type>text</type> )
        <returnvalue>regcollation</returnvalue>
       </para>
       <para>
        Translates a textual collation name to its OID.  A similar result is
        obtained by casting the string to type <type>regcollation</type> (see
        <xref linkend="datatype-oid"/>); however, this function will return
        <literal>NULL</literal> rather than throwing an error if the name is
        not found.  Also unlike the cast, this does not accept
        a numeric OID as input.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_regnamespace</primary>
        </indexterm>
        <function>to_regnamespace</function> ( <type>text</type> )
        <returnvalue>regnamespace</returnvalue>
       </para>
       <para>
        Translates a textual schema name to its OID.  A similar result is
        obtained by casting the string to type <type>regnamespace</type> (see
        <xref linkend="datatype-oid"/>); however, this function will return
        <literal>NULL</literal> rather than throwing an error if the name is
        not found.  Also unlike the cast, this does not accept
        a numeric OID as input.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_regoper</primary>
        </indexterm>
        <function>to_regoper</function> ( <type>text</type> )
        <returnvalue>regoper</returnvalue>
       </para>
       <para>
        Translates a textual operator name to its OID.  A similar result is
        obtained by casting the string to type <type>regoper</type> (see
        <xref linkend="datatype-oid"/>); however, this function will return
        <literal>NULL</literal> rather than throwing an error if the name is
        not found or is ambiguous.  Also unlike the cast, this does not accept
        a numeric OID as input.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_regoperator</primary>
        </indexterm>
        <function>to_regoperator</function> ( <type>text</type> )
        <returnvalue>regoperator</returnvalue>
       </para>
       <para>
        Translates a textual operator name (with parameter types) to its OID.  A similar result is
        obtained by casting the string to type <type>regoperator</type> (see
        <xref linkend="datatype-oid"/>); however, this function will return
        <literal>NULL</literal> rather than throwing an error if the name is
        not found.  Also unlike the cast, this does not accept
        a numeric OID as input.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_regproc</primary>
        </indexterm>
        <function>to_regproc</function> ( <type>text</type> )
        <returnvalue>regproc</returnvalue>
       </para>
       <para>
        Translates a textual function or procedure name to its OID.  A similar result is
        obtained by casting the string to type <type>regproc</type> (see
        <xref linkend="datatype-oid"/>); however, this function will return
        <literal>NULL</literal> rather than throwing an error if the name is
        not found or is ambiguous.  Also unlike the cast, this does not accept
        a numeric OID as input.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_regprocedure</primary>
        </indexterm>
        <function>to_regprocedure</function> ( <type>text</type> )
        <returnvalue>regprocedure</returnvalue>
       </para>
       <para>
        Translates a textual function or procedure name (with argument types) to its OID.  A similar result is
        obtained by casting the string to type <type>regprocedure</type> (see
        <xref linkend="datatype-oid"/>); however, this function will return
        <literal>NULL</literal> rather than throwing an error if the name is
        not found.  Also unlike the cast, this does not accept
        a numeric OID as input.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_regrole</primary>
        </indexterm>
        <function>to_regrole</function> ( <type>text</type> )
        <returnvalue>regrole</returnvalue>
       </para>
       <para>
        Translates a textual role name to its OID.  A similar result is
        obtained by casting the string to type <type>regrole</type> (see
        <xref linkend="datatype-oid"/>); however, this function will return
        <literal>NULL</literal> rather than throwing an error if the name is
        not found.  Also unlike the cast, this does not accept
        a numeric OID as input.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_regtype</primary>
        </indexterm>
        <function>to_regtype</function> ( <type>text</type> )
        <returnvalue>regtype</returnvalue>
       </para>
       <para>
        Translates a textual type name to its OID.  A similar result is
        obtained by casting the string to type <type>regtype</type> (see
        <xref linkend="datatype-oid"/>); however, this function will return
        <literal>NULL</literal> rather than throwing an error if the name is
        not found.  Also unlike the cast, this does not accept
        a numeric OID as input.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   Most of the functions that reconstruct (decompile) database objects
   have an optional <parameter>pretty</parameter> flag, which
   if <literal>true</literal> causes the result to
   be <quote>pretty-printed</quote>.  Pretty-printing suppresses unnecessary
   parentheses and adds whitespace for legibility.
   The pretty-printed format is more readable, but the default format
   is more likely to be interpreted the same way by future versions of
   <productname>PostgreSQL</productname>; so avoid using pretty-printed output
   for dump purposes.  Passing <literal>false</literal> for
   the <parameter>pretty</parameter> parameter yields the same result as
   omitting the parameter.
  </para>

  <table id="functions-info-index-column-props">
   <title>Index Column Properties</title>
   <tgroup cols="2">
    <thead>
     <row><entry>Name</entry><entry>Description</entry></row>
    </thead>
    <tbody>
     <row>
      <entry><literal>asc</literal></entry>
      <entry>Does the column sort in ascending order on a forward scan?
      </entry>
     </row>
     <row>
      <entry><literal>desc</literal></entry>
      <entry>Does the column sort in descending order on a forward scan?
      </entry>
     </row>
     <row>
      <entry><literal>nulls_first</literal></entry>
      <entry>Does the column sort with nulls first on a forward scan?
      </entry>
     </row>
     <row>
      <entry><literal>nulls_last</literal></entry>
      <entry>Does the column sort with nulls last on a forward scan?
      </entry>
     </row>
     <row>
      <entry><literal>orderable</literal></entry>
      <entry>Does the column possess any defined sort ordering?
      </entry>
     </row>
     <row>
      <entry><literal>distance_orderable</literal></entry>
      <entry>Can the column be scanned in order by a <quote>distance</quote>
      operator, for example <literal>ORDER BY col &lt;-&gt; constant</literal> ?
      </entry>
     </row>
     <row>
      <entry><literal>returnable</literal></entry>
      <entry>Can the column value be returned by an index-only scan?
      </entry>
     </row>
     <row>
      <entry><literal>search_array</literal></entry>
      <entry>Does the column natively support <literal>col = ANY(array)</literal>
      searches?
      </entry>
     </row>
     <row>
      <entry><literal>search_nulls</literal></entry>
      <entry>Does the column support <literal>IS NULL</literal> and
      <literal>IS NOT NULL</literal> searches?
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <table id="functions-info-index-props">
   <title>Index Properties</title>
   <tgroup cols="2">
    <thead>
     <row><entry>Name</entry><entry>Description</entry></row>
    </thead>
    <tbody>
     <row>
      <entry><literal>clusterable</literal></entry>
      <entry>Can the index be used in a <literal>CLUSTER</literal> command?
      </entry>
     </row>
     <row>
      <entry><literal>index_scan</literal></entry>
      <entry>Does the index support plain (non-bitmap) scans?
      </entry>
     </row>
     <row>
      <entry><literal>bitmap_scan</literal></entry>
      <entry>Does the index support bitmap scans?
      </entry>
     </row>
     <row>
      <entry><literal>backward_scan</literal></entry>
      <entry>Can the scan direction be changed in mid-scan (to
             support <literal>FETCH BACKWARD</literal> on a cursor without
             needing materialization)?
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <table id="functions-info-indexam-props">
   <title>Index Access Method Properties</title>
   <tgroup cols="2">
    <thead>
     <row><entry>Name</entry><entry>Description</entry></row>
    </thead>
    <tbody>
     <row>
      <entry><literal>can_order</literal></entry>
      <entry>Does the access method support <literal>ASC</literal>,
      <literal>DESC</literal> and related keywords in
      <literal>CREATE INDEX</literal>?
      </entry>
     </row>
     <row>
      <entry><literal>can_unique</literal></entry>
      <entry>Does the access method support unique indexes?
      </entry>
     </row>
     <row>
      <entry><literal>can_multi_col</literal></entry>
      <entry>Does the access method support indexes with multiple columns?
      </entry>
     </row>
     <row>
      <entry><literal>can_exclude</literal></entry>
      <entry>Does the access method support exclusion constraints?
      </entry>
     </row>
     <row>
      <entry><literal>can_include</literal></entry>
      <entry>Does the access method support the <literal>INCLUDE</literal>
        clause of <literal>CREATE INDEX</literal>?
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   <xref linkend="functions-info-object-table"/> lists functions related to
   database object identification and addressing.
  </para>

   <table id="functions-info-object-table">
    <title>Object Information and Addressing Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_describe_object</primary>
        </indexterm>
        <function>pg_describe_object</function> ( <parameter>classid</parameter> <type>oid</type>, <parameter>objid</parameter> <type>oid</type>, <parameter>objsubid</parameter> <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns a textual description of a database object identified by
        catalog OID, object OID, and sub-object ID (such as a column number
        within a table; the sub-object ID is zero when referring to a whole
        object).  This description is intended to be human-readable, and might
        be translated, depending on server configuration.  This is especially
        useful to determine the identity of an object referenced in the
        <structname>pg_depend</structname> catalog. This function returns
        <literal>NULL</literal> values for undefined objects.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_identify_object</primary>
        </indexterm>
        <function>pg_identify_object</function> ( <parameter>classid</parameter> <type>oid</type>, <parameter>objid</parameter> <type>oid</type>, <parameter>objsubid</parameter> <type>integer</type> )
        <returnvalue>record</returnvalue>
        ( <parameter>type</parameter> <type>text</type>,
        <parameter>schema</parameter> <type>text</type>,
        <parameter>name</parameter> <type>text</type>,
        <parameter>identity</parameter> <type>text</type> )
       </para>
       <para>
        Returns a row containing enough information to uniquely identify the
        database object specified by catalog OID, object OID and sub-object
        ID.
        This information is intended to be machine-readable, and is never
        translated.
        <parameter>type</parameter> identifies the type of database object;
        <parameter>schema</parameter> is the schema name that the object
        belongs in, or <literal>NULL</literal> for object types that do not
        belong to schemas;
        <parameter>name</parameter> is the name of the object, quoted if
        necessary, if the name (along with schema name, if pertinent) is
        sufficient to uniquely identify the object,
        otherwise <literal>NULL</literal>;
        <parameter>identity</parameter> is the complete object identity, with
        the precise format depending on object type, and each name within the
        format being schema-qualified and quoted as necessary. Undefined
        objects are identified with <literal>NULL</literal> values.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_identify_object_as_address</primary>
        </indexterm>
        <function>pg_identify_object_as_address</function> ( <parameter>classid</parameter> <type>oid</type>, <parameter>objid</parameter> <type>oid</type>, <parameter>objsubid</parameter> <type>integer</type> )
        <returnvalue>record</returnvalue>
        ( <parameter>type</parameter> <type>text</type>,
        <parameter>object_names</parameter> <type>text[]</type>,
        <parameter>object_args</parameter> <type>text[]</type> )
       </para>
       <para>
        Returns a row containing enough information to uniquely identify the
        database object specified by catalog OID, object OID and sub-object
        ID.
        The returned information is independent of the current server, that
        is, it could be used to identify an identically named object in
        another server.
        <parameter>type</parameter> identifies the type of database object;
        <parameter>object_names</parameter> and
        <parameter>object_args</parameter>
        are text arrays that together form a reference to the object.
        These three values can be passed
        to <function>pg_get_object_address</function> to obtain the internal
        address of the object.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_get_object_address</primary>
        </indexterm>
        <function>pg_get_object_address</function> ( <parameter>type</parameter> <type>text</type>, <parameter>object_names</parameter> <type>text[]</type>, <parameter>object_args</parameter> <type>text[]</type> )
        <returnvalue>record</returnvalue>
        ( <parameter>classid</parameter> <type>oid</type>,
        <parameter>objid</parameter> <type>oid</type>,
        <parameter>objsubid</parameter> <type>integer</type> )
       </para>
       <para>
        Returns a row containing enough information to uniquely identify the
        database object specified by a type code and object name and argument
        arrays.
        The returned values are the ones that would be used in system catalogs
        such as <structname>pg_depend</structname>; they can be passed to
        other system functions such as <function>pg_describe_object</function>
        or <function>pg_identify_object</function>.
        <parameter>classid</parameter> is the OID of the system catalog
        containing the object;
        <parameter>objid</parameter> is the OID of the object itself, and
        <parameter>objsubid</parameter> is the sub-object ID, or zero if none.
        This function is the inverse
        of <function>pg_identify_object_as_address</function>.
        Undefined objects are identified with <literal>NULL</literal> values.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <indexterm>
    <primary>comment</primary>
    <secondary sortas="database objects">about database objects</secondary>
   </indexterm>

   <para>
    The functions shown in <xref linkend="functions-info-comment-table"/>
    extract comments previously stored with the <xref linkend="sql-comment"/>
    command.  A null value is returned if no
    comment could be found for the specified parameters.
   </para>

   <table id="functions-info-comment-table">
    <title>Comment Information Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>col_description</primary>
        </indexterm>
        <function>col_description</function> ( <parameter>table</parameter> <type>oid</type>, <parameter>column</parameter> <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the comment for a table column, which is specified by the OID
        of its table and its column number.
        (<function>obj_description</function> cannot be used for table
        columns, since columns do not have OIDs of their own.)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>obj_description</primary>
        </indexterm>
        <function>obj_description</function> ( <parameter>object</parameter> <type>oid</type>, <parameter>catalog</parameter> <type>name</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the comment for a database object specified by its OID and the
        name of the containing system catalog.  For
        example, <literal>obj_description(123456, 'pg_class')</literal> would
        retrieve the comment for the table with OID 123456.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>obj_description</function> ( <parameter>object</parameter> <type>oid</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the comment for a database object specified by its OID alone.
        This is <emphasis>deprecated</emphasis> since there is no guarantee
        that OIDs are unique across different system catalogs; therefore, the
        wrong comment might be returned.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>shobj_description</primary>
        </indexterm>
        <function>shobj_description</function> ( <parameter>object</parameter> <type>oid</type>, <parameter>catalog</parameter> <type>name</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the comment for a shared database object specified by its OID
        and the name of the containing system catalog.  This is just
        like <function>obj_description</function> except that it is used for
        retrieving comments on shared objects (that is, databases, roles, and
        tablespaces).  Some system catalogs are global to all databases within
        each cluster, and the descriptions for objects in them are stored
        globally as well.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    The functions shown in <xref linkend="functions-pg-snapshot"/>
    provide server transaction information in an exportable form.  The main
    use of these functions is to determine which transactions were committed
    between two snapshots.
   </para>

   <table id="functions-pg-snapshot">
    <title>Transaction ID and Snapshot Information Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_current_xact_id</primary>
        </indexterm>
        <function>pg_current_xact_id</function> ()
        <returnvalue>xid8</returnvalue>
       </para>
       <para>
        Returns the current transaction's ID.  It will assign a new one if the
        current transaction does not have one already (because it has not
        performed any database updates).
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_current_xact_id_if_assigned</primary>
        </indexterm>
        <function>pg_current_xact_id_if_assigned</function> ()
        <returnvalue>xid8</returnvalue>
       </para>
       <para>
        Returns the current transaction's ID, or <literal>NULL</literal> if no
        ID is assigned yet.  (It's best to use this variant if the transaction
        might otherwise be read-only, to avoid unnecessary consumption of an
        XID.)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_xact_status</primary>
        </indexterm>
        <function>pg_xact_status</function> ( <type>xid8</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Reports the commit status of a recent transaction.
        The result is one of <literal>in progress</literal>,
        <literal>committed</literal>, or <literal>aborted</literal>,
        provided that the transaction is recent enough that the system retains
        the commit status of that transaction.
        If it is old enough that no references to the transaction survive in
        the system and the commit status information has been discarded, the
        result is <literal>NULL</literal>.
        Applications might use this function, for example, to determine
        whether their transaction committed or aborted after the application
        and database server become disconnected while
        a <literal>COMMIT</literal> is in progress.
        Note that prepared transactions are reported as <literal>in
        progress</literal>; applications must check <link
        linkend="view-pg-prepared-xacts"><structname>pg_prepared_xacts</structname></link>
        if they need to determine whether a transaction ID belongs to a
        prepared transaction.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_current_snapshot</primary>
        </indexterm>
        <function>pg_current_snapshot</function> ()
        <returnvalue>pg_snapshot</returnvalue>
       </para>
       <para>
        Returns a current <firstterm>snapshot</firstterm>, a data structure
        showing which transaction IDs are now in-progress.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_snapshot_xip</primary>
        </indexterm>
        <function>pg_snapshot_xip</function> ( <type>pg_snapshot</type> )
        <returnvalue>setof xid8</returnvalue>
       </para>
       <para>
        Returns the set of in-progress transaction IDs contained in a snapshot.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_snapshot_xmax</primary>
        </indexterm>
        <function>pg_snapshot_xmax</function> ( <type>pg_snapshot</type> )
        <returnvalue>xid8</returnvalue>
       </para>
       <para>
        Returns the <structfield>xmax</structfield> of a snapshot.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_snapshot_xmin</primary>
        </indexterm>
        <function>pg_snapshot_xmin</function> ( <type>pg_snapshot</type> )
        <returnvalue>xid8</returnvalue>
       </para>
       <para>
        Returns the <structfield>xmin</structfield> of a snapshot.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_visible_in_snapshot</primary>
        </indexterm>
        <function>pg_visible_in_snapshot</function> ( <type>xid8</type>, <type>pg_snapshot</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Is the given transaction ID <firstterm>visible</firstterm> according
        to this snapshot (that is, was it completed before the snapshot was
        taken)?  Note that this function will not give the correct answer for
        a subtransaction ID.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    The internal transaction ID type <type>xid</type> is 32 bits wide and
    wraps around every 4 billion transactions.  However,
    the functions shown in <xref linkend="functions-pg-snapshot"/> use a
    64-bit type <type>xid8</type> that does not wrap around during the life
    of an installation, and can be converted to <type>xid</type> by casting if
    required.  The data type <type>pg_snapshot</type> stores information about
    transaction ID visibility at a particular moment in time.  Its components
    are described in <xref linkend="functions-pg-snapshot-parts"/>.
    <type>pg_snapshot</type>'s textual representation is
    <literal><replaceable>xmin</replaceable>:<replaceable>xmax</replaceable>:<replaceable>xip_list</replaceable></literal>.
    For example <literal>10:20:10,14,15</literal> means
    <literal>xmin=10, xmax=20, xip_list=10, 14, 15</literal>.
   </para>

   <table id="functions-pg-snapshot-parts">
    <title>Snapshot Components</title>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>Name</entry>
       <entry>Description</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry><structfield>xmin</structfield></entry>
       <entry>
         Lowest transaction ID that was still active.  All transaction IDs
         less than <structfield>xmin</structfield> are either committed and visible,
         or rolled back and dead.
       </entry>
      </row>

      <row>
       <entry><structfield>xmax</structfield></entry>
       <entry>
         One past the highest completed transaction ID.  All transaction IDs
         greater than or equal to <structfield>xmax</structfield> had not yet
         completed as of the time of the snapshot, and thus are invisible.
       </entry>
      </row>

      <row>
       <entry><structfield>xip_list</structfield></entry>
       <entry>
        Transactions in progress at the time of the snapshot.  A transaction
        ID that is <literal>xmin &lt;= <replaceable>X</replaceable> &lt;
        xmax</literal> and not in this list was already completed at the time
        of the snapshot, and thus is either visible or dead according to its
        commit status.  This list does not include the transaction IDs of
        subtransactions.
       </entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    In releases of <productname>PostgreSQL</productname> before 13 there was
    no <type>xid8</type> type, so variants of these functions were provided
    that used <type>bigint</type> to represent a 64-bit XID, with a
    correspondingly distinct snapshot data type <type>txid_snapshot</type>.
    These older functions have <literal>txid</literal> in their names.  They
    are still supported for backward compatibility, but may be removed from a
    future release. See <xref linkend="functions-txid-snapshot"/>.
   </para>

   <table id="functions-txid-snapshot">
    <title>Deprecated Transaction ID and Snapshot Information Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>txid_current</primary>
        </indexterm>
        <function>txid_current</function> ()
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        See <function>pg_current_xact_id()</function>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>txid_current_if_assigned</primary>
        </indexterm>
        <function>txid_current_if_assigned</function> ()
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        See <function>pg_current_xact_id_if_assigned()</function>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>txid_current_snapshot</primary>
        </indexterm>
        <function>txid_current_snapshot</function> ()
        <returnvalue>txid_snapshot</returnvalue>
       </para>
       <para>
        See <function>pg_current_snapshot()</function>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>txid_snapshot_xip</primary>
        </indexterm>
        <function>txid_snapshot_xip</function> ( <type>txid_snapshot</type> )
        <returnvalue>setof bigint</returnvalue>
       </para>
       <para>
        See <function>pg_snapshot_xip()</function>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>txid_snapshot_xmax</primary>
        </indexterm>
        <function>txid_snapshot_xmax</function> ( <type>txid_snapshot</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        See <function>pg_snapshot_xmax()</function>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>txid_snapshot_xmin</primary>
        </indexterm>
        <function>txid_snapshot_xmin</function> ( <type>txid_snapshot</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        See <function>pg_snapshot_xmin()</function>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>txid_visible_in_snapshot</primary>
        </indexterm>
        <function>txid_visible_in_snapshot</function> ( <type>bigint</type>, <type>txid_snapshot</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        See <function>pg_visible_in_snapshot()</function>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>txid_status</primary>
        </indexterm>
        <function>txid_status</function> ( <type>bigint</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        See <function>pg_xact_status()</function>.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    The functions shown in <xref linkend="functions-commit-timestamp"/>
    provide information about when past transactions were committed.
    They only provide useful data when the
    <xref linkend="guc-track-commit-timestamp"/> configuration option is
    enabled, and only for transactions that were committed after it was
    enabled.
   </para>

   <table id="functions-commit-timestamp">
    <title>Committed Transaction Information Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_xact_commit_timestamp</primary>
        </indexterm>
        <function>pg_xact_commit_timestamp</function> ( <type>xid</type> )
        <returnvalue>timestamp with time zone</returnvalue>
       </para>
       <para>
        Returns the commit timestamp of a transaction.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_xact_commit_timestamp_origin</primary>
        </indexterm>
        <function>pg_xact_commit_timestamp_origin</function> ( <type>xid</type> )
        <returnvalue>record</returnvalue>
        ( <parameter>timestamp</parameter> <type>timestamp with time zone</type>,
         <parameter>roident</parameter> <type>oid</type>)
       </para>
       <para>
         Returns the commit timestamp and replication origin of a transaction.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_last_committed_xact</primary>
        </indexterm>
        <function>pg_last_committed_xact</function> ()
        <returnvalue>record</returnvalue>
        ( <parameter>xid</parameter> <type>xid</type>,
        <parameter>timestamp</parameter> <type>timestamp with time zone</type>,
        <parameter>roident</parameter> <type>oid</type> )
       </para>
       <para>
        Returns the transaction ID, commit timestamp and replication origin
        of the latest committed transaction.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    The functions shown in <xref linkend="functions-controldata"/>
    print information initialized during <command>initdb</command>, such
    as the catalog version. They also show information about write-ahead
    logging and checkpoint processing. This information is cluster-wide,
    not specific to any one database. These functions provide most of the same
    information, from the same source, as the
    <xref linkend="app-pgcontroldata"/> application.
   </para>

   <table id="functions-controldata">
    <title>Control Data Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_control_checkpoint</primary>
        </indexterm>
        <function>pg_control_checkpoint</function> ()
        <returnvalue>record</returnvalue>
       </para>
       <para>
        Returns information about current checkpoint state, as shown in
        <xref linkend="functions-pg-control-checkpoint"/>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_control_system</primary>
        </indexterm>
        <function>pg_control_system</function> ()
        <returnvalue>record</returnvalue>
       </para>
       <para>
        Returns information about current control file state, as shown in
        <xref linkend="functions-pg-control-system"/>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_control_init</primary>
        </indexterm>
        <function>pg_control_init</function> ()
        <returnvalue>record</returnvalue>
       </para>
       <para>
        Returns information about cluster initialization state, as shown in
        <xref linkend="functions-pg-control-init"/>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_control_recovery</primary>
        </indexterm>
        <function>pg_control_recovery</function> ()
        <returnvalue>record</returnvalue>
       </para>
       <para>
        Returns information about recovery state, as shown in
        <xref linkend="functions-pg-control-recovery"/>.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <table id="functions-pg-control-checkpoint">
    <title><function>pg_control_checkpoint</function> Output Columns</title>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>Column Name</entry>
       <entry>Data Type</entry>
      </row>
     </thead>

     <tbody>

      <row>
       <entry><structfield>checkpoint_lsn</structfield></entry>
       <entry><type>pg_lsn</type></entry>
      </row>

      <row>
       <entry><structfield>redo_lsn</structfield></entry>
       <entry><type>pg_lsn</type></entry>
      </row>

      <row>
       <entry><structfield>redo_wal_file</structfield></entry>
       <entry><type>text</type></entry>
      </row>

      <row>
       <entry><structfield>timeline_id</structfield></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><structfield>prev_timeline_id</structfield></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><structfield>full_page_writes</structfield></entry>
       <entry><type>boolean</type></entry>
      </row>

      <row>
       <entry><structfield>next_xid</structfield></entry>
       <entry><type>text</type></entry>
      </row>

      <row>
       <entry><structfield>next_oid</structfield></entry>
       <entry><type>oid</type></entry>
      </row>

      <row>
       <entry><structfield>next_multixact_id</structfield></entry>
       <entry><type>xid</type></entry>
      </row>

      <row>
       <entry><structfield>next_multi_offset</structfield></entry>
       <entry><type>xid</type></entry>
      </row>

      <row>
       <entry><structfield>oldest_xid</structfield></entry>
       <entry><type>xid</type></entry>
      </row>

      <row>
       <entry><structfield>oldest_xid_dbid</structfield></entry>
       <entry><type>oid</type></entry>
      </row>

      <row>
       <entry><structfield>oldest_active_xid</structfield></entry>
       <entry><type>xid</type></entry>
      </row>

      <row>
       <entry><structfield>oldest_multi_xid</structfield></entry>
       <entry><type>xid</type></entry>
      </row>

      <row>
       <entry><structfield>oldest_multi_dbid</structfield></entry>
       <entry><type>oid</type></entry>
      </row>

      <row>
       <entry><structfield>oldest_commit_ts_xid</structfield></entry>
       <entry><type>xid</type></entry>
      </row>

      <row>
       <entry><structfield>newest_commit_ts_xid</structfield></entry>
       <entry><type>xid</type></entry>
      </row>

      <row>
       <entry><structfield>checkpoint_time</structfield></entry>
       <entry><type>timestamp with time zone</type></entry>
      </row>

     </tbody>
    </tgroup>
   </table>

   <table id="functions-pg-control-system">
    <title><function>pg_control_system</function> Output Columns</title>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>Column Name</entry>
       <entry>Data Type</entry>
      </row>
     </thead>

     <tbody>

      <row>
       <entry><structfield>pg_control_version</structfield></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><structfield>catalog_version_no</structfield></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><structfield>system_identifier</structfield></entry>
       <entry><type>bigint</type></entry>
      </row>

      <row>
       <entry><structfield>pg_control_last_modified</structfield></entry>
       <entry><type>timestamp with time zone</type></entry>
      </row>

     </tbody>
    </tgroup>
   </table>

   <table id="functions-pg-control-init">
    <title><function>pg_control_init</function> Output Columns</title>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>Column Name</entry>
       <entry>Data Type</entry>
      </row>
     </thead>

     <tbody>

      <row>
       <entry><structfield>max_data_alignment</structfield></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><structfield>database_block_size</structfield></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><structfield>blocks_per_segment</structfield></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><structfield>wal_block_size</structfield></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><structfield>bytes_per_wal_segment</structfield></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><structfield>max_identifier_length</structfield></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><structfield>max_index_columns</structfield></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><structfield>max_toast_chunk_size</structfield></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><structfield>large_object_chunk_size</structfield></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><structfield>float8_pass_by_value</structfield></entry>
       <entry><type>boolean</type></entry>
      </row>

      <row>
       <entry><structfield>data_page_checksum_version</structfield></entry>
       <entry><type>integer</type></entry>
      </row>

     </tbody>
    </tgroup>
   </table>

   <table id="functions-pg-control-recovery">
    <title><function>pg_control_recovery</function> Output Columns</title>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>Column Name</entry>
       <entry>Data Type</entry>
      </row>
     </thead>

     <tbody>

      <row>
       <entry><structfield>min_recovery_end_lsn</structfield></entry>
       <entry><type>pg_lsn</type></entry>
      </row>

      <row>
       <entry><structfield>min_recovery_end_timeline</structfield></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><structfield>backup_start_lsn</structfield></entry>
       <entry><type>pg_lsn</type></entry>
      </row>

      <row>
       <entry><structfield>backup_end_lsn</structfield></entry>
       <entry><type>pg_lsn</type></entry>
      </row>

      <row>
       <entry><structfield>end_of_backup_record_required</structfield></entry>
       <entry><type>boolean</type></entry>
      </row>

     </tbody>
    </tgroup>
   </table>

  </sect1>

  <sect1 id="functions-admin">
   <title>System Administration Functions</title>

   <para>
    The functions described in this section are used to control and
    monitor a <productname>PostgreSQL</productname> installation.
   </para>

  <sect2 id="functions-admin-set">
   <title>Configuration Settings Functions</title>

   <indexterm>
    <primary>SET</primary>
   </indexterm>

   <indexterm>
    <primary>SHOW</primary>
   </indexterm>

   <indexterm>
    <primary>configuration</primary>
    <secondary sortas="server">of the server</secondary>
    <tertiary>functions</tertiary>
   </indexterm>

   <para>
    <xref linkend="functions-admin-set-table"/> shows the functions
    available to query and alter run-time configuration parameters.
   </para>

   <table id="functions-admin-set-table">
    <title>Configuration Settings Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>current_setting</primary>
        </indexterm>
        <function>current_setting</function> ( <parameter>setting_name</parameter> <type>text</type> <optional>, <parameter>missing_ok</parameter> <type>boolean</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the current value of the
        setting <parameter>setting_name</parameter>.  If there is no such
        setting, <function>current_setting</function> throws an error
        unless <parameter>missing_ok</parameter> is supplied and
        is <literal>true</literal>.  This function corresponds to
        the <acronym>SQL</acronym> command <command>SHOW</command>.
       </para>
       <para>
        <literal>current_setting('datestyle')</literal>
        <returnvalue>ISO, MDY</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>set_config</primary>
        </indexterm>
        <function>set_config</function> (
          <parameter>setting_name</parameter> <type>text</type>,
          <parameter>new_value</parameter> <type>text</type>,
          <parameter>is_local</parameter> <type>boolean</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Sets the parameter <parameter>setting_name</parameter>
        to <parameter>new_value</parameter>, and returns that value.
        If <parameter>is_local</parameter> is <literal>true</literal>, the new
        value will only apply for the current transaction. If you want the new
        value to apply for the current session, use <literal>false</literal>
        instead. This function corresponds to the SQL
        command <command>SET</command>.
       </para>
       <para>
        <literal>set_config('log_statement_stats', 'off', false)</literal>
        <returnvalue>off</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  </sect2>

  <sect2 id="functions-admin-signal">
   <title>Server Signaling Functions</title>

   <indexterm>
    <primary>signal</primary>
    <secondary sortas="backend">backend processes</secondary>
   </indexterm>

   <para>
    The functions shown in <xref
    linkend="functions-admin-signal-table"/> send control signals to
    other server processes.  Use of these functions is restricted to
    superusers by default but access may be granted to others using
    <command>GRANT</command>, with noted exceptions.
   </para>

   <para>
    Each of these functions returns <literal>true</literal> if
    successful and <literal>false</literal> otherwise.
   </para>

   <table id="functions-admin-signal-table">
    <title>Server Signaling Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_cancel_backend</primary>
        </indexterm>
        <function>pg_cancel_backend</function> ( <parameter>pid</parameter> <type>integer</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Cancels the current query of the session whose backend process has the
        specified process ID.  This is also allowed if the
        calling role is a member of the role whose backend is being canceled or
        the calling role has been granted <literal>pg_signal_backend</literal>,
        however only superusers can cancel superuser backends.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_reload_conf</primary>
        </indexterm>
        <function>pg_reload_conf</function> ()
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Causes all processes of the <productname>PostgreSQL</productname>
        server to reload their configuration files.  (This is initiated by
        sending a <systemitem>SIGHUP</systemitem> signal to the postmaster
        process, which in turn sends <systemitem>SIGHUP</systemitem> to each
        of its children.)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_rotate_logfile</primary>
        </indexterm>
        <function>pg_rotate_logfile</function> ()
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Signals the log-file manager to switch to a new output file
        immediately.  This works only when the built-in log collector is
        running, since otherwise there is no log-file manager subprocess.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_terminate_backend</primary>
        </indexterm>
        <function>pg_terminate_backend</function> ( <parameter>pid</parameter> <type>integer</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Terminates the session whose backend process has the
        specified process ID.  This is also allowed if the calling role
        is a member of the role whose backend is being terminated or the
        calling role has been granted <literal>pg_signal_backend</literal>,
        however only superusers can terminate superuser backends.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    <function>pg_cancel_backend</function> and <function>pg_terminate_backend</function>
    send signals (<systemitem>SIGINT</systemitem> or <systemitem>SIGTERM</systemitem>
    respectively) to backend processes identified by process ID.
    The process ID of an active backend can be found from
    the <structfield>pid</structfield> column of the
    <structname>pg_stat_activity</structname> view, or by listing the
    <command>postgres</command> processes on the server (using
    <application>ps</application> on Unix or the <application>Task
    Manager</application> on <productname>Windows</productname>).
    The role of an active backend can be found from the
    <structfield>usename</structfield> column of the
    <structname>pg_stat_activity</structname> view.
   </para>

  </sect2>

  <sect2 id="functions-admin-backup">
   <title>Backup Control Functions</title>

   <indexterm>
    <primary>backup</primary>
   </indexterm>

   <para>
    The functions shown in <xref
    linkend="functions-admin-backup-table"/> assist in making on-line backups.
    These functions cannot be executed during recovery (except
    non-exclusive <function>pg_start_backup</function>,
    non-exclusive <function>pg_stop_backup</function>,
    <function>pg_is_in_backup</function>, <function>pg_backup_start_time</function>
    and <function>pg_wal_lsn_diff</function>).
   </para>

   <para>
    For details about proper usage of these functions, see
    <xref linkend="continuous-archiving"/>.
   </para>

   <table id="functions-admin-backup-table">
    <title>Backup Control Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_create_restore_point</primary>
        </indexterm>
        <function>pg_create_restore_point</function> ( <parameter>name</parameter> <type>text</type> )
        <returnvalue>pg_lsn</returnvalue>
       </para>
       <para>
        Creates a named marker record in the write-ahead log that can later be
        used as a recovery target, and returns the corresponding write-ahead
        log location.  The given name can then be used with
        <xref linkend="guc-recovery-target-name"/> to specify the point up to
        which recovery will proceed.  Avoid creating multiple restore points
        with the same name, since recovery will stop at the first one whose
        name matches the recovery target.
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_current_wal_flush_lsn</primary>
        </indexterm>
        <function>pg_current_wal_flush_lsn</function> ()
        <returnvalue>pg_lsn</returnvalue>
       </para>
       <para>
        Returns the current write-ahead log flush location (see notes below).
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_current_wal_insert_lsn</primary>
        </indexterm>
        <function>pg_current_wal_insert_lsn</function> ()
        <returnvalue>pg_lsn</returnvalue>
       </para>
       <para>
        Returns the current write-ahead log insert location (see notes below).
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_current_wal_lsn</primary>
        </indexterm>
        <function>pg_current_wal_lsn</function> ()
        <returnvalue>pg_lsn</returnvalue>
       </para>
       <para>
        Returns the current write-ahead log write location (see notes below).
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_start_backup</primary>
        </indexterm>
        <function>pg_start_backup</function> (
          <parameter>label</parameter> <type>text</type>
          <optional>, <parameter>fast</parameter> <type>boolean</type>
          <optional>, <parameter>exclusive</parameter> <type>boolean</type>
          </optional></optional> )
        <returnvalue>pg_lsn</returnvalue>
       </para>
       <para>
        Prepares the server to begin an on-line backup.  The only required
        parameter is an arbitrary user-defined label for the backup.
        (Typically this would be the name under which the backup dump file
        will be stored.)
        If the optional second parameter is given as <literal>true</literal>,
        it specifies executing <function>pg_start_backup</function> as quickly
        as possible.  This forces an immediate checkpoint which will cause a
        spike in I/O operations, slowing any concurrently executing queries.
        The optional third parameter specifies whether to perform an exclusive
        or non-exclusive backup (default is exclusive).
       </para>
       <para>
        When used in exclusive mode, this function writes a backup label file
        (<filename>backup_label</filename>) and, if there are any links in
        the <filename>pg_tblspc/</filename> directory, a tablespace map file
        (<filename>tablespace_map</filename>) into the database cluster's data
        directory, then performs a checkpoint, and then returns the backup's
        starting write-ahead log location.  (The user can ignore this
        result value, but it is provided in case it is useful.)  When used in
        non-exclusive mode, the contents of these files are instead returned
        by the <function>pg_stop_backup</function> function, and should be
        copied to the backup area by the user.
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stop_backup</primary>
        </indexterm>
        <function>pg_stop_backup</function> (
          <parameter>exclusive</parameter> <type>boolean</type>
          <optional>, <parameter>wait_for_archive</parameter> <type>boolean</type>
          </optional> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>lsn</parameter> <type>pg_lsn</type>,
        <parameter>labelfile</parameter> <type>text</type>,
        <parameter>spcmapfile</parameter> <type>text</type> )
       </para>
       <para>
        Finishes performing an exclusive or non-exclusive on-line backup.
        The <parameter>exclusive</parameter> parameter must match the
        previous <function>pg_start_backup</function> call.
        In an exclusive backup, <function>pg_stop_backup</function> removes
        the backup label file and, if it exists, the tablespace map file
        created by <function>pg_start_backup</function>.  In a non-exclusive
        backup, the desired contents of these files are returned as part of
        the result of the function, and should be written to files in the
        backup area (not in the data directory).
       </para>
       <para>
        There is an optional second parameter of type <type>boolean</type>.
        If false, the function will return immediately after the backup is
        completed, without waiting for WAL to be archived.  This behavior is
        only useful with backup software that independently monitors WAL
        archiving.  Otherwise, WAL required to make the backup consistent might
        be missing and make the backup useless.  By default or when this
        parameter is true, <function>pg_stop_backup</function> will wait for
        WAL to be archived when archiving is enabled.  (On a standby, this
        means that it will wait only when <varname>archive_mode</varname> =
        <literal>always</literal>.  If write activity on the primary is low,
        it may be useful to run <function>pg_switch_wal</function> on the
        primary in order to trigger an immediate segment switch.)
       </para>
       <para>
        When executed on a primary, this function also creates a backup
        history file in the write-ahead log archive area.  The history file
        includes the label given to <function>pg_start_backup</function>, the
        starting and ending write-ahead log locations for the backup, and the
        starting and ending times of the backup.  After recording the ending
        location, the current write-ahead log insertion point is automatically
        advanced to the next write-ahead log file, so that the ending
        write-ahead log file can be archived immediately to complete the
        backup.
       </para>
       <para>
        The result of the function is a single record.
        The <parameter>lsn</parameter> column holds the backup's ending
        write-ahead log location (which again can be ignored).  The second and
        third columns are <literal>NULL</literal> when ending an exclusive
        backup; after a non-exclusive backup they hold the desired contents of
        the label and tablespace map files.
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>pg_stop_backup</function> ()
        <returnvalue>pg_lsn</returnvalue>
       </para>
       <para>
        Finishes performing an exclusive on-line backup.  This simplified
        version is equivalent to <literal>pg_stop_backup(true,
        true)</literal>, except that it only returns the <type>pg_lsn</type>
        result.
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_is_in_backup</primary>
        </indexterm>
        <function>pg_is_in_backup</function> ()
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Returns true if an on-line exclusive backup is in progress.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_backup_start_time</primary>
        </indexterm>
        <function>pg_backup_start_time</function> ()
        <returnvalue>timestamp with time zone</returnvalue>
       </para>
       <para>
        Returns the start time of the current on-line exclusive backup if one
        is in progress, otherwise <literal>NULL</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_switch_wal</primary>
        </indexterm>
        <function>pg_switch_wal</function> ()
        <returnvalue>pg_lsn</returnvalue>
       </para>
       <para>
        Forces the server to switch to a new write-ahead log file, which
        allows the current file to be archived (assuming you are using
        continuous archiving).  The result is the ending write-ahead log
        location plus 1 within the just-completed write-ahead log file.  If
        there has been no write-ahead log activity since the last write-ahead
        log switch, <function>pg_switch_wal</function> does nothing and
        returns the start location of the write-ahead log file currently in
        use.
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_walfile_name</primary>
        </indexterm>
        <function>pg_walfile_name</function> ( <parameter>lsn</parameter> <type>pg_lsn</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts a write-ahead log location to the name of the WAL file
        holding that location.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_walfile_name_offset</primary>
        </indexterm>
        <function>pg_walfile_name_offset</function> ( <parameter>lsn</parameter> <type>pg_lsn</type> )
        <returnvalue>record</returnvalue>
        ( <parameter>file_name</parameter> <type>text</type>,
        <parameter>file_offset</parameter> <type>integer</type> )
       </para>
       <para>
        Converts a write-ahead log location to a WAL file name and byte offset
        within that file.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_wal_lsn_diff</primary>
        </indexterm>
        <function>pg_wal_lsn_diff</function> ( <parameter>lsn</parameter> <type>pg_lsn</type>, <parameter>lsn</parameter> <type>pg_lsn</type> )
        <returnvalue>numeric</returnvalue>
       </para>
       <para>
        Calculates the difference in bytes between two write-ahead log
        locations.  This can be used
        with <structname>pg_stat_replication</structname> or some of the
        functions shown in <xref linkend="functions-admin-backup-table"/> to
        get the replication lag.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    <function>pg_current_wal_lsn</function> displays the current write-ahead
    log write location in the same format used by the above functions.
    Similarly, <function>pg_current_wal_insert_lsn</function> displays the
    current write-ahead log insertion location
    and <function>pg_current_wal_flush_lsn</function> displays the current
    write-ahead log flush location. The insertion location is
    the <quote>logical</quote> end of the write-ahead log at any instant,
    while the write location is the end of what has actually been written out
    from the server's internal buffers, and the flush location is the last
    location known to be written to durable storage. The write location is the
    end of what can be examined from outside the server, and is usually what
    you want if you are interested in archiving partially-complete write-ahead
    log files.  The insertion and flush locations are made available primarily
    for server debugging purposes.  These are all read-only operations and do
    not require superuser permissions.
   </para>

   <para>
    You can use <function>pg_walfile_name_offset</function> to extract the
    corresponding write-ahead log file name and byte offset from
    a <type>pg_lsn</type> value.  For example:
<programlisting>
postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
        file_name         | file_offset
--------------------------+-------------
 00000001000000000000000D |     4039624
(1 row)
</programlisting>
    Similarly, <function>pg_walfile_name</function> extracts just the write-ahead log file name.
    When the given write-ahead log location is exactly at a write-ahead log file boundary, both
    these functions return the name of the preceding write-ahead log file.
    This is usually the desired behavior for managing write-ahead log archiving
    behavior, since the preceding file is the last one that currently
    needs to be archived.
   </para>

  </sect2>

  <sect2 id="functions-recovery-control">
   <title>Recovery Control Functions</title>

   <para>
    The functions shown in <xref
    linkend="functions-recovery-info-table"/> provide information
    about the current status of a standby server.
    These functions may be executed both during recovery and in normal running.
   </para>

   <table id="functions-recovery-info-table">
    <title>Recovery Information Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_is_in_recovery</primary>
        </indexterm>
        <function>pg_is_in_recovery</function> ()
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Returns true if recovery is still in progress.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_last_wal_receive_lsn</primary>
        </indexterm>
        <function>pg_last_wal_receive_lsn</function> ()
        <returnvalue>pg_lsn</returnvalue>
       </para>
       <para>
        Returns the last write-ahead log location that has been received and
        synced to disk by streaming replication. While streaming replication
        is in progress this will increase monotonically. If recovery has
        completed then this will remain static at the location of the last WAL
        record received and synced to disk during recovery. If streaming
        replication is disabled, or if it has not yet started, the function
        returns <literal>NULL</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_last_wal_replay_lsn</primary>
        </indexterm>
        <function>pg_last_wal_replay_lsn</function> ()
        <returnvalue>pg_lsn</returnvalue>
       </para>
       <para>
        Returns the last write-ahead log location that has been replayed
        during recovery.  If recovery is still in progress this will increase
        monotonically.  If recovery has completed then this will remain
        static at the location of the last WAL record applied during recovery.
        When the server has been started normally without recovery, the
        function returns <literal>NULL</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_last_xact_replay_timestamp</primary>
        </indexterm>
        <function>pg_last_xact_replay_timestamp</function> ()
        <returnvalue>timestamp with time zone</returnvalue>
       </para>
       <para>
        Returns the time stamp of the last transaction replayed during
        recovery.  This is the time at which the commit or abort WAL record
        for that transaction was generated on the primary.  If no transactions
        have been replayed during recovery, the function
        returns <literal>NULL</literal>.  Otherwise, if recovery is still in
        progress this will increase monotonically.  If recovery has completed
        then this will remain static at the time of the last transaction
        applied during recovery.  When the server has been started normally
        without recovery, the function returns <literal>NULL</literal>.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    The functions shown in <xref
    linkend="functions-recovery-control-table"/> control the progress of recovery.
    These functions may be executed only during recovery.
   </para>

   <table id="functions-recovery-control-table">
    <title>Recovery Control Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_is_wal_replay_paused</primary>
        </indexterm>
        <function>pg_is_wal_replay_paused</function> ()
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Returns true if recovery is paused.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_promote</primary>
        </indexterm>
        <function>pg_promote</function> ( <parameter>wait</parameter> <type>boolean</type> <literal>DEFAULT</literal> <literal>true</literal>, <parameter>wait_seconds</parameter> <type>integer</type> <literal>DEFAULT</literal> <literal>60</literal> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Promotes a standby server to primary status.
        With <parameter>wait</parameter> set to <literal>true</literal> (the
        default), the function waits until promotion is completed
        or <parameter>wait_seconds</parameter> seconds have passed, and
        returns <literal>true</literal> if promotion is successful
        and <literal>false</literal> otherwise.
        If <parameter>wait</parameter> is set to <literal>false</literal>, the
        function returns <literal>true</literal> immediately after sending a
        <literal>SIGUSR1</literal> signal to the postmaster to trigger
        promotion.
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_wal_replay_pause</primary>
        </indexterm>
        <function>pg_wal_replay_pause</function> ()
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Pauses recovery.  While recovery is paused, no further database
        changes are applied.  If hot standby is active, all new queries will
        see the same consistent snapshot of the database, and no further query
        conflicts will be generated until recovery is resumed.
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_wal_replay_resume</primary>
        </indexterm>
        <function>pg_wal_replay_resume</function> ()
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Restarts recovery if it was paused.
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    <function>pg_wal_replay_pause</function> and
    <function>pg_wal_replay_resume</function> cannot be executed while
    a promotion is ongoing. If a promotion is triggered while recovery
    is paused, the paused state ends and promotion continues.
   </para>

   <para>
    If streaming replication is disabled, the paused state may continue
    indefinitely without a problem. If streaming replication is in
    progress then WAL records will continue to be received, which will
    eventually fill available disk space, depending upon the duration of
    the pause, the rate of WAL generation and available disk space.
   </para>

  </sect2>

  <sect2 id="functions-snapshot-synchronization">
   <title>Snapshot Synchronization Functions</title>

   <para>
    <productname>PostgreSQL</productname> allows database sessions to synchronize their
    snapshots. A <firstterm>snapshot</firstterm> determines which data is visible to the
    transaction that is using the snapshot. Synchronized snapshots are
    necessary when two or more sessions need to see identical content in the
    database. If two sessions just start their transactions independently,
    there is always a possibility that some third transaction commits
    between the executions of the two <command>START TRANSACTION</command> commands,
    so that one session sees the effects of that transaction and the other
    does not.
   </para>

   <para>
    To solve this problem, <productname>PostgreSQL</productname> allows a transaction to
    <firstterm>export</firstterm> the snapshot it is using.  As long as the exporting
    transaction remains open, other transactions can <firstterm>import</firstterm> its
    snapshot, and thereby be guaranteed that they see exactly the same view
    of the database that the first transaction sees.  But note that any
    database changes made by any one of these transactions remain invisible
    to the other transactions, as is usual for changes made by uncommitted
    transactions.  So the transactions are synchronized with respect to
    pre-existing data, but act normally for changes they make themselves.
   </para>

   <para>
    Snapshots are exported with the <function>pg_export_snapshot</function> function,
    shown in <xref linkend="functions-snapshot-synchronization-table"/>, and
    imported with the <xref linkend="sql-set-transaction"/> command.
   </para>

   <table id="functions-snapshot-synchronization-table">
    <title>Snapshot Synchronization Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_export_snapshot</primary>
        </indexterm>
        <function>pg_export_snapshot</function> ()
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Saves the transaction's current snapshot and returns
        a <type>text</type> string identifying the snapshot.  This string must
        be passed (outside the database) to clients that want to import the
        snapshot.  The snapshot is available for import only until the end of
        the transaction that exported it.
       </para>
       <para>
        A transaction can export more than one snapshot, if needed.  Note that
        doing so is only useful in <literal>READ COMMITTED</literal>
        transactions, since in <literal>REPEATABLE READ</literal> and higher
        isolation levels, transactions use the same snapshot throughout their
        lifetime.  Once a transaction has exported any snapshots, it cannot be
        prepared with <xref linkend="sql-prepare-transaction"/>.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  </sect2>

  <sect2 id="functions-replication">
   <title>Replication Management Functions</title>

   <para>
    The functions shown
    in <xref linkend="functions-replication-table"/> are for
    controlling and interacting with replication features.
    See <xref linkend="streaming-replication"/>,
    <xref linkend="streaming-replication-slots"/>, and
    <xref linkend="replication-origins"/>
    for information about the underlying features.
    Use of functions for replication origin is only allowed to the
    superuser by default, but may be allowed to other users by using the
    <literal>GRANT</literal> command.
    Use of functions for replication slots is restricted to superusers
    and users having <literal>REPLICATION</literal> privilege.
   </para>

   <para>
    Many of these functions have equivalent commands in the replication
    protocol; see <xref linkend="protocol-replication"/>.
   </para>

   <para>
    The functions described in
    <xref linkend="functions-admin-backup"/>,
    <xref linkend="functions-recovery-control"/>, and
    <xref linkend="functions-snapshot-synchronization"/>
    are also relevant for replication.
   </para>

   <table id="functions-replication-table">
    <title>Replication Management Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_create_physical_replication_slot</primary>
        </indexterm>
        <function>pg_create_physical_replication_slot</function> ( <parameter>slot_name</parameter> <type>name</type> <optional>, <parameter>immediately_reserve</parameter> <type>boolean</type>, <parameter>temporary</parameter> <type>boolean</type> </optional> )
        <returnvalue>record</returnvalue>
        ( <parameter>slot_name</parameter> <type>name</type>,
        <parameter>lsn</parameter> <type>pg_lsn</type> )
       </para>
       <para>
        Creates a new physical replication slot named
        <parameter>slot_name</parameter>. The optional second parameter,
        when <literal>true</literal>, specifies that the <acronym>LSN</acronym> for this
        replication slot be reserved immediately; otherwise
        the <acronym>LSN</acronym> is reserved on first connection from a streaming
        replication client. Streaming changes from a physical slot is only
        possible with the streaming-replication protocol &mdash;
        see <xref linkend="protocol-replication"/>. The optional third
        parameter, <parameter>temporary</parameter>, when set to true, specifies that
        the slot should not be permanently stored to disk and is only meant
        for use by the current session. Temporary slots are also
        released upon any error. This function corresponds
        to the replication protocol command <literal>CREATE_REPLICATION_SLOT
        ... PHYSICAL</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_drop_replication_slot</primary>
        </indexterm>
        <function>pg_drop_replication_slot</function> ( <parameter>slot_name</parameter> <type>name</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Drops the physical or logical replication slot
        named <parameter>slot_name</parameter>. Same as replication protocol
        command <literal>DROP_REPLICATION_SLOT</literal>. For logical slots, this must
        be called while connected to the same database the slot was created on.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_create_logical_replication_slot</primary>
        </indexterm>
        <function>pg_create_logical_replication_slot</function> ( <parameter>slot_name</parameter> <type>name</type>, <parameter>plugin</parameter> <type>name</type> <optional>, <parameter>temporary</parameter> <type>boolean</type> </optional> )
        <returnvalue>record</returnvalue>
        ( <parameter>slot_name</parameter> <type>name</type>,
        <parameter>lsn</parameter> <type>pg_lsn</type> )
       </para>
       <para>
        Creates a new logical (decoding) replication slot named
        <parameter>slot_name</parameter> using the output plugin
        <parameter>plugin</parameter>. The optional third
        parameter, <parameter>temporary</parameter>, when set to true, specifies that
        the slot should not be permanently stored to disk and is only meant
        for use by the current session. Temporary slots are also
        released upon any error. A call to this function has the same
        effect as the replication protocol command
        <literal>CREATE_REPLICATION_SLOT ... LOGICAL</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_copy_physical_replication_slot</primary>
        </indexterm>
        <function>pg_copy_physical_replication_slot</function> ( <parameter>src_slot_name</parameter> <type>name</type>, <parameter>dst_slot_name</parameter> <type>name</type> <optional>, <parameter>temporary</parameter> <type>boolean</type> </optional> )
        <returnvalue>record</returnvalue>
        ( <parameter>slot_name</parameter> <type>name</type>,
        <parameter>lsn</parameter> <type>pg_lsn</type> )
       </para>
       <para>
        Copies an existing physical replication slot named <parameter>src_slot_name</parameter>
        to a physical replication slot named <parameter>dst_slot_name</parameter>.
        The copied physical slot starts to reserve WAL from the same <acronym>LSN</acronym> as the
        source slot.
        <parameter>temporary</parameter> is optional. If <parameter>temporary</parameter>
        is omitted, the same value as the source slot is used.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_copy_logical_replication_slot</primary>
        </indexterm>
        <function>pg_copy_logical_replication_slot</function> ( <parameter>src_slot_name</parameter> <type>name</type>, <parameter>dst_slot_name</parameter> <type>name</type> <optional>, <parameter>temporary</parameter> <type>boolean</type> <optional>, <parameter>plugin</parameter> <type>name</type> </optional></optional> )
        <returnvalue>record</returnvalue>
        ( <parameter>slot_name</parameter> <type>name</type>,
        <parameter>lsn</parameter> <type>pg_lsn</type> )
       </para>
       <para>
        Copies an existing logical replication slot
        named <parameter>src_slot_name</parameter> to a logical replication
        slot named <parameter>dst_slot_name</parameter>, optionally changing
        the output plugin and persistence.  The copied logical slot starts
        from the same <acronym>LSN</acronym> as the source logical slot.  Both
        <parameter>temporary</parameter> and <parameter>plugin</parameter> are
        optional; if they are omitted, the values of the source slot are used.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_logical_slot_get_changes</primary>
        </indexterm>
        <function>pg_logical_slot_get_changes</function> ( <parameter>slot_name</parameter> <type>name</type>, <parameter>upto_lsn</parameter> <type>pg_lsn</type>, <parameter>upto_nchanges</parameter> <type>integer</type>, <literal>VARIADIC</literal> <parameter>options</parameter> <type>text[]</type> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>lsn</parameter> <type>pg_lsn</type>,
        <parameter>xid</parameter> <type>xid</type>,
        <parameter>data</parameter> <type>text</type> )
       </para>
       <para>
        Returns changes in the slot <parameter>slot_name</parameter>, starting
        from the point from which changes have been consumed last.  If
        <parameter>upto_lsn</parameter>
        and <parameter>upto_nchanges</parameter> are NULL,
        logical decoding will continue until end of WAL.  If
        <parameter>upto_lsn</parameter> is non-NULL, decoding will include only
        those transactions which commit prior to the specified LSN.  If
        <parameter>upto_nchanges</parameter> is non-NULL, decoding will
        stop when the number of rows produced by decoding exceeds
        the specified value.  Note, however, that the actual number of
        rows returned may be larger, since this limit is only checked after
        adding the rows produced when decoding each new transaction commit.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_logical_slot_peek_changes</primary>
        </indexterm>
        <function>pg_logical_slot_peek_changes</function> ( <parameter>slot_name</parameter> <type>name</type>, <parameter>upto_lsn</parameter> <type>pg_lsn</type>, <parameter>upto_nchanges</parameter> <type>integer</type>, <literal>VARIADIC</literal> <parameter>options</parameter> <type>text[]</type> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>lsn</parameter> <type>pg_lsn</type>,
        <parameter>xid</parameter> <type>xid</type>,
         <parameter>data</parameter> <type>text</type> )
       </para>
       <para>
        Behaves just like
        the <function>pg_logical_slot_get_changes()</function> function,
        except that changes are not consumed; that is, they will be returned
        again on future calls.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_logical_slot_get_binary_changes</primary>
        </indexterm>
        <function>pg_logical_slot_get_binary_changes</function> ( <parameter>slot_name</parameter> <type>name</type>, <parameter>upto_lsn</parameter> <type>pg_lsn</type>, <parameter>upto_nchanges</parameter> <type>integer</type>, <literal>VARIADIC</literal> <parameter>options</parameter> <type>text[]</type> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>lsn</parameter> <type>pg_lsn</type>,
        <parameter>xid</parameter> <type>xid</type>,
        <parameter>data</parameter> <type>bytea</type> )
       </para>
       <para>
        Behaves just like
        the <function>pg_logical_slot_get_changes()</function> function,
        except that changes are returned as <type>bytea</type>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_logical_slot_peek_binary_changes</primary>
        </indexterm>
        <function>pg_logical_slot_peek_binary_changes</function> ( <parameter>slot_name</parameter> <type>name</type>, <parameter>upto_lsn</parameter> <type>pg_lsn</type>, <parameter>upto_nchanges</parameter> <type>integer</type>, <literal>VARIADIC</literal> <parameter>options</parameter> <type>text[]</type> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>lsn</parameter> <type>pg_lsn</type>,
        <parameter>xid</parameter> <type>xid</type>,
        <parameter>data</parameter> <type>bytea</type> )
       </para>
       <para>
        Behaves just like
        the <function>pg_logical_slot_peek_changes()</function> function,
        except that changes are returned as <type>bytea</type>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_replication_slot_advance</primary>
        </indexterm>
        <function>pg_replication_slot_advance</function> ( <parameter>slot_name</parameter> <type>name</type>, <parameter>upto_lsn</parameter> <type>pg_lsn</type> )
        <returnvalue>record</returnvalue>
        ( <parameter>slot_name</parameter> <type>name</type>,
        <parameter>end_lsn</parameter> <type>pg_lsn</type> )
       </para>
       <para>
        Advances the current confirmed position of a replication slot named
        <parameter>slot_name</parameter>. The slot will not be moved backwards,
        and it will not be moved beyond the current insert location. Returns
        the name of the slot and the actual position that it was advanced to.
        The updated slot position information is written out at the next
        checkpoint if any advancing is done. So in the event of a crash, the
        slot may return to an earlier position.
       </para></entry>
      </row>

      <row>
       <entry id="pg-replication-origin-create" role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_replication_origin_create</primary>
        </indexterm>
        <function>pg_replication_origin_create</function> ( <parameter>node_name</parameter> <type>text</type> )
        <returnvalue>oid</returnvalue>
       </para>
       <para>
        Creates a replication origin with the given external
        name, and returns the internal ID assigned to it.
       </para></entry>
      </row>

      <row>
       <entry id="pg-replication-origin-drop" role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_replication_origin_drop</primary>
        </indexterm>
        <function>pg_replication_origin_drop</function> ( <parameter>node_name</parameter> <type>text</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Deletes a previously-created replication origin, including any
        associated replay progress.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_replication_origin_oid</primary>
        </indexterm>
        <function>pg_replication_origin_oid</function> ( <parameter>node_name</parameter> <type>text</type> )
        <returnvalue>oid</returnvalue>
       </para>
       <para>
        Looks up a replication origin by name and returns the internal ID. If
        no such replication origin is found an error is thrown.
       </para></entry>
      </row>

      <row>
       <entry id="pg-replication-origin-session-setup" role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_replication_origin_session_setup</primary>
        </indexterm>
        <function>pg_replication_origin_session_setup</function> ( <parameter>node_name</parameter> <type>text</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Marks the current session as replaying from the given
        origin, allowing replay progress to be tracked.
        Can only be used if no origin is currently selected.
        Use <function>pg_replication_origin_session_reset</function> to undo.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_replication_origin_session_reset</primary>
        </indexterm>
        <function>pg_replication_origin_session_reset</function> ()
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Cancels the effects
        of <function>pg_replication_origin_session_setup()</function>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_replication_origin_session_is_setup</primary>
        </indexterm>
        <function>pg_replication_origin_session_is_setup</function> ()
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Returns true if a replication origin has been selected in the
        current session.
       </para></entry>
      </row>

      <row>
       <entry id="pg-replication-origin-session-progress" role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_replication_origin_session_progress</primary>
        </indexterm>
        <function>pg_replication_origin_session_progress</function> ( <parameter>flush</parameter> <type>boolean</type> )
        <returnvalue>pg_lsn</returnvalue>
       </para>
       <para>
        Returns the replay location for the replication origin selected in
        the current session. The parameter <parameter>flush</parameter>
        determines whether the corresponding local transaction will be
        guaranteed to have been flushed to disk or not.
       </para></entry>
      </row>

      <row>
       <entry id="pg-replication-origin-xact-setup" role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_replication_origin_xact_setup</primary>
        </indexterm>
        <function>pg_replication_origin_xact_setup</function> ( <parameter>origin_lsn</parameter> <type>pg_lsn</type>, <parameter>origin_timestamp</parameter> <type>timestamp with time zone</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Marks the current transaction as replaying a transaction that has
        committed at the given <acronym>LSN</acronym> and timestamp. Can
        only be called when a replication origin has been selected
        using <function>pg_replication_origin_session_setup</function>.
       </para></entry>
      </row>

      <row>
       <entry id="pg-replication-origin-xact-reset" role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_replication_origin_xact_reset</primary>
        </indexterm>
        <function>pg_replication_origin_xact_reset</function> ()
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Cancels the effects of
        <function>pg_replication_origin_xact_setup()</function>.
       </para></entry>
      </row>

      <row>
       <entry id="pg-replication-origin-advance" role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_replication_origin_advance</primary>
        </indexterm>
        <function>pg_replication_origin_advance</function> ( <parameter>node_name</parameter> <type>text</type>, <parameter>lsn</parameter> <type>pg_lsn</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Sets replication progress for the given node to the given
        location. This is primarily useful for setting up the initial
        location, or setting a new location after configuration changes and
        similar. Be aware that careless use of this function can lead to
        inconsistently replicated data.
       </para></entry>
      </row>

      <row>
       <entry id="pg-replication-origin-progress" role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_replication_origin_progress</primary>
        </indexterm>
        <function>pg_replication_origin_progress</function> ( <parameter>node_name</parameter> <type>text</type>, <parameter>flush</parameter> <type>boolean</type> )
        <returnvalue>pg_lsn</returnvalue>
       </para>
       <para>
        Returns the replay location for the given replication origin. The
        parameter <parameter>flush</parameter> determines whether the
        corresponding local transaction will be guaranteed to have been
        flushed to disk or not.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_logical_emit_message</primary>
        </indexterm>
        <function>pg_logical_emit_message</function> ( <parameter>transactional</parameter> <type>boolean</type>, <parameter>prefix</parameter> <type>text</type>, <parameter>content</parameter> <type>text</type> )
        <returnvalue>pg_lsn</returnvalue>
       </para>
       <para role="func_signature">
        <function>pg_logical_emit_message</function> ( <parameter>transactional</parameter> <type>boolean</type>, <parameter>prefix</parameter> <type>text</type>, <parameter>content</parameter> <type>bytea</type> )
        <returnvalue>pg_lsn</returnvalue>
       </para>
       <para>
        Emits a logical decoding message. This can be used to pass generic
        messages to logical decoding plugins through
        WAL. The <parameter>transactional</parameter> parameter specifies if
        the message should be part of the current transaction, or if it should
        be written immediately and decoded as soon as the logical decoder
        reads the record. The <parameter>prefix</parameter> parameter is a
        textual prefix that can be used by logical decoding plugins to easily
        recognize messages that are interesting for them.
        The <parameter>content</parameter> parameter is the content of the
        message, given either in text or binary form.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  </sect2>

  <sect2 id="functions-admin-dbobject">
   <title>Database Object Management Functions</title>

   <para>
    The functions shown in <xref linkend="functions-admin-dbsize"/> calculate
    the disk space usage of database objects, or assist in presentation
    of usage results.
    All these functions return sizes measured in bytes.  If an OID that does
    not represent an existing object is passed to one of these
    functions, <literal>NULL</literal> is returned.
   </para>

   <table id="functions-admin-dbsize">
    <title>Database Object Size Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_column_size</primary>
        </indexterm>
        <function>pg_column_size</function> ( <type>"any"</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Shows the number of bytes used to store any individual data value.  If
        applied directly to a table column value, this reflects any
        compression that was done.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_database_size</primary>
        </indexterm>
        <function>pg_database_size</function> ( <type>name</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para role="func_signature">
        <function>pg_database_size</function> ( <type>oid</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Computes the total disk space used by the database with the specified
        name or OID.  To use this function, you must
        have <literal>CONNECT</literal> privilege on the specified database
        (which is granted by default) or be a member of
        the <literal>pg_read_all_stats</literal> role.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_indexes_size</primary>
        </indexterm>
        <function>pg_indexes_size</function> ( <type>regclass</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Computes the total disk space used by indexes attached to the
        specified table.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_relation_size</primary>
        </indexterm>
        <function>pg_relation_size</function> ( <parameter>relation</parameter> <type>regclass</type> <optional>, <parameter>fork</parameter> <type>text</type> </optional> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Computes the disk space used by one <quote>fork</quote> of the
        specified relation.  (Note that for most purposes it is more
        convenient to use the higher-level
        functions <function>pg_total_relation_size</function>
        or <function>pg_table_size</function>, which sum the sizes of all
        forks.)  With one argument, this returns the size of the main data
        fork of the relation.  The second argument can be provided to specify
        which fork to examine:
        <itemizedlist spacing="compact">
         <listitem>
          <para>
           <literal>main</literal> returns the size of the main
           data fork of the relation.
          </para>
         </listitem>
         <listitem>
          <para>
           <literal>fsm</literal> returns the size of the Free Space Map
           (see <xref linkend="storage-fsm"/>) associated with the relation.
          </para>
         </listitem>
         <listitem>
          <para>
           <literal>vm</literal> returns the size of the Visibility Map
           (see <xref linkend="storage-vm"/>) associated with the relation.
          </para>
         </listitem>
         <listitem>
          <para>
           <literal>init</literal> returns the size of the initialization
           fork, if any, associated with the relation.
          </para>
         </listitem>
        </itemizedlist>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_size_bytes</primary>
        </indexterm>
        <function>pg_size_bytes</function> ( <type>text</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Converts a size in human-readable format (as returned
        by <function>pg_size_pretty</function>) into bytes.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_size_pretty</primary>
        </indexterm>
        <function>pg_size_pretty</function> ( <type>bigint</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <function>pg_size_pretty</function> ( <type>numeric</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts a size in bytes into a more easily human-readable format with
        size units (bytes, kB, MB, GB or TB as appropriate).  Note that the
        units are powers of 2 rather than powers of 10, so 1kB is 1024 bytes,
        1MB is 1024<superscript>2</superscript> = 1048576 bytes, and so on.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_table_size</primary>
        </indexterm>
        <function>pg_table_size</function> ( <type>regclass</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Computes the disk space used by the specified table, excluding indexes
        (but including its TOAST table if any, free space map, and visibility
        map).
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_tablespace_size</primary>
        </indexterm>
        <function>pg_tablespace_size</function> ( <type>name</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para role="func_signature">
        <function>pg_tablespace_size</function> ( <type>oid</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Computes the total disk space used in the tablespace with the
        specified name or OID. To use this function, you must
        have <literal>CREATE</literal> privilege on the specified tablespace
        or be a member of the <literal>pg_read_all_stats</literal> role,
        unless it is the default tablespace for the current database.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_total_relation_size</primary>
        </indexterm>
        <function>pg_total_relation_size</function> ( <type>regclass</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Computes the total disk space used by the specified table, including
        all indexes and <acronym>TOAST</acronym> data.  The result is
        equivalent to <function>pg_table_size</function>
        <literal>+</literal> <function>pg_indexes_size</function>.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    The functions above that operate on tables or indexes accept a
    <type>regclass</type> argument, which is simply the OID of the table or index
    in the <structname>pg_class</structname> system catalog.  You do not have to look up
    the OID by hand, however, since the <type>regclass</type> data type's input
    converter will do the work for you.  Just write the table name enclosed in
    single quotes so that it looks like a literal constant.  For compatibility
    with the handling of ordinary <acronym>SQL</acronym> names, the string
    will be converted to lower case unless it contains double quotes around
    the table name.
   </para>

   <para>
    The functions shown in <xref linkend="functions-admin-dblocation"/> assist
    in identifying the specific disk files associated with database objects.
   </para>

   <table id="functions-admin-dblocation">
    <title>Database Object Location Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_relation_filenode</primary>
        </indexterm>
        <function>pg_relation_filenode</function> ( <parameter>relation</parameter> <type>regclass</type> )
        <returnvalue>oid</returnvalue>
       </para>
       <para>
        Returns the <quote>filenode</quote> number currently assigned to the
        specified relation.  The filenode is the base component of the file
        name(s) used for the relation (see
        <xref linkend="storage-file-layout"/> for more information).
        For most relations the result is the same as
        <structname>pg_class</structname>.<structfield>relfilenode</structfield>,
        but for certain system catalogs <structfield>relfilenode</structfield>
        is zero and this function must be used to get the correct value.  The
        function returns NULL if passed a relation that does not have storage,
        such as a view.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_relation_filepath</primary>
        </indexterm>
        <function>pg_relation_filepath</function> ( <parameter>relation</parameter> <type>regclass</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the entire file path name (relative to the database cluster's
        data directory, <varname>PGDATA</varname>) of the relation.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_filenode_relation</primary>
        </indexterm>
        <function>pg_filenode_relation</function> ( <parameter>tablespace</parameter> <type>oid</type>, <parameter>filenode</parameter> <type>oid</type> )
        <returnvalue>regclass</returnvalue>
       </para>
       <para>
        Returns a relation's OID given the tablespace OID and filenode it is
        stored under.  This is essentially the inverse mapping of
        <function>pg_relation_filepath</function>.  For a relation in the
        database's default tablespace, the tablespace can be specified as zero.
        Returns <literal>NULL</literal> if no relation in the current database
        is associated with the given values.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    <xref linkend="functions-admin-collation"/> lists functions used to manage
    collations.
   </para>

   <table id="functions-admin-collation">
    <title>Collation Management Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_collation_actual_version</primary>
        </indexterm>
        <function>pg_collation_actual_version</function> ( <type>oid</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the actual version of the collation object as it is currently
        installed in the operating system.  If this is different from the
        value in
        <structname>pg_collation</structname>.<structfield>collversion</structfield>,
        then objects depending on the collation might need to be rebuilt.  See
        also <xref linkend="sql-altercollation"/>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_import_system_collations</primary>
        </indexterm>
        <function>pg_import_system_collations</function> ( <parameter>schema</parameter> <type>regnamespace</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Adds collations to the system
        catalog <structname>pg_collation</structname> based on all the locales
        it finds in the operating system.  This is
        what <command>initdb</command> uses; see
        <xref linkend="collation-managing"/> for more details.  If additional
        locales are installed into the operating system later on, this
        function can be run again to add collations for the new locales.
        Locales that match existing entries
        in <structname>pg_collation</structname> will be skipped.  (But
        collation objects based on locales that are no longer present in the
        operating system are not removed by this function.)
        The <parameter>schema</parameter> parameter would typically
        be <literal>pg_catalog</literal>, but that is not a requirement; the
        collations could be installed into some other schema as well.  The
        function returns the number of new collation objects it created.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    <xref linkend="functions-info-partition"/> lists functions that provide
    information about the structure of partitioned tables.
   </para>

   <table id="functions-info-partition">
    <title>Partitioning Information Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_partition_tree</primary>
        </indexterm>
        <function>pg_partition_tree</function> ( <type>regclass</type> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>relid</parameter> <type>regclass</type>,
        <parameter>parentrelid</parameter> <type>regclass</type>,
        <parameter>isleaf</parameter> <type>boolean</type>,
        <parameter>level</parameter> <type>integer</type> )
       </para>
       <para>
        Lists the tables or indexes in the partition tree of the
        given partitioned table or partitioned index, with one row for each
        partition.  Information provided includes the OID of the partition,
        the OID of its immediate parent, a boolean value telling if the
        partition is a leaf, and an integer telling its level in the hierarchy.
        The level value is 0 for the input table or index, 1 for its
        immediate child partitions, 2 for their partitions, and so on.
        Returns no rows if the relation does not exist or is not a partition
        or partitioned table.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_partition_ancestors</primary>
        </indexterm>
        <function>pg_partition_ancestors</function> ( <type>regclass</type> )
        <returnvalue>setof regclass</returnvalue>
       </para>
       <para>
        Lists the ancestor relations of the given partition,
        including the relation itself.  Returns no rows if the relation
        does not exist or is not a partition or partitioned table.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_partition_root</primary>
        </indexterm>
        <function>pg_partition_root</function> ( <type>regclass</type> )
        <returnvalue>regclass</returnvalue>
       </para>
       <para>
        Returns the top-most parent of the partition tree to which the given
        relation belongs.  Returns <literal>NULL</literal> if the relation
        does not exist or is not a partition or partitioned table.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    For example, to check the total size of the data contained in a
    partitioned table <structname>measurement</structname>, one could use the
    following query:
<programlisting>
SELECT pg_size_pretty(sum(pg_relation_size(relid))) AS total_size
  FROM pg_partition_tree('measurement');
</programlisting>
   </para>

  </sect2>

  <sect2 id="functions-admin-index">
   <title>Index Maintenance Functions</title>

   <para>
    <xref linkend="functions-admin-index-table"/> shows the functions
    available for index maintenance tasks.  (Note that these maintenance
    tasks are normally done automatically by autovacuum; use of these
    functions is only required in special cases.)
    These functions cannot be executed during recovery.
    Use of these functions is restricted to superusers and the owner
    of the given index.
   </para>

   <table id="functions-admin-index-table">
    <title>Index Maintenance Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>brin_summarize_new_values</primary>
        </indexterm>
        <function>brin_summarize_new_values</function> ( <parameter>index</parameter> <type>regclass</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Scans the specified BRIN index to find page ranges in the base table
        that are not currently summarized by the index; for any such range it
        creates a new summary index tuple by scanning those table pages.
        Returns the number of new page range summaries that were inserted
        into the index.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>brin_summarize_range</primary>
        </indexterm>
        <function>brin_summarize_range</function> ( <parameter>index</parameter> <type>regclass</type>, <parameter>blockNumber</parameter> <type>bigint</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Summarizes the page range covering the given block, if not already
        summarized.  This is
        like <function>brin_summarize_new_values</function> except that it
        only processes the page range that covers the given table block number.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>brin_desummarize_range</primary>
        </indexterm>
        <function>brin_desummarize_range</function> ( <parameter>index</parameter> <type>regclass</type>, <parameter>blockNumber</parameter> <type>bigint</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Removes the BRIN index tuple that summarizes the page range covering
        the given table block, if there is one.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>gin_clean_pending_list</primary>
        </indexterm>
        <function>gin_clean_pending_list</function> ( <parameter>index</parameter> <type>regclass</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Cleans up the <quote>pending</quote> list of the specified GIN index
        by moving entries in it, in bulk, to the main GIN data structure.
        Returns the number of pages removed from the pending list.
        If the argument is a GIN index built with
        the <literal>fastupdate</literal> option disabled, no cleanup happens
        and the result is zero, because the index doesn't have a pending list.
        See <xref linkend="gin-fast-update"/> and <xref linkend="gin-tips"/>
        for details about the pending list and <literal>fastupdate</literal>
        option.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  </sect2>

  <sect2 id="functions-admin-genfile">
   <title>Generic File Access Functions</title>

   <para>
    The functions shown in <xref
    linkend="functions-admin-genfile-table"/> provide native access to
    files on the machine hosting the server. Only files within the
    database cluster directory and the <varname>log_directory</varname> can be
    accessed, unless the user is a superuser or is granted the role
    <literal>pg_read_server_files</literal>.  Use a relative path for files in
    the cluster directory, and a path matching the <varname>log_directory</varname>
    configuration setting for log files.
   </para>

   <para>
    Note that granting users the EXECUTE privilege on
    <function>pg_read_file()</function>, or related functions, allows them the
    ability to read any file on the server that the database server process can
    read; these functions bypass all in-database privilege checks.  This means
    that, for example, a user with such access is able to read the contents of
    the <structname>pg_authid</structname> table where authentication
    information is stored, as well as read any table data in the database.
    Therefore, granting access to these functions should be carefully
    considered.
   </para>

   <para>
    Some of these functions take an optional <parameter>missing_ok</parameter>
    parameter, which specifies the behavior when the file or directory does
    not exist.  If <literal>true</literal>, the function
    returns <literal>NULL</literal> or an empty result set, as appropriate.
    If <literal>false</literal>, an error is raised.  The default
    is <literal>false</literal>.
   </para>

   <table id="functions-admin-genfile-table">
    <title>Generic File Access Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_ls_dir</primary>
        </indexterm>
        <function>pg_ls_dir</function> ( <parameter>dirname</parameter> <type>text</type> <optional>, <parameter>missing_ok</parameter> <type>boolean</type>, <parameter>include_dot_dirs</parameter> <type>boolean</type> </optional> )
        <returnvalue>setof text</returnvalue>
       </para>
       <para>
        Returns the names of all files (and directories and other special
        files) in the specified
        directory. The <parameter>include_dot_dirs</parameter> parameter
        indicates whether <quote>.</quote> and <quote>..</quote> are to be
        included in the result set; the default is to exclude them.  Including
        them can be useful when <parameter>missing_ok</parameter>
        is <literal>true</literal>, to distinguish an empty directory from a
        non-existent directory.
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_ls_logdir</primary>
        </indexterm>
        <function>pg_ls_logdir</function> ()
        <returnvalue>setof record</returnvalue>
        ( <parameter>name</parameter> <type>text</type>,
        <parameter>size</parameter> <type>bigint</type>,
        <parameter>modification</parameter> <type>timestamp with time zone</type> )
       </para>
       <para>
        Returns the name, size, and last modification time (mtime) of each
        ordinary file in the server's log directory.  Filenames beginning with
        a dot, directories, and other special files are excluded.
       </para>
       <para>
        This function is restricted to superusers and members of
        the <literal>pg_monitor</literal> role by default, but other users can
        be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_ls_waldir</primary>
        </indexterm>
        <function>pg_ls_waldir</function> ()
        <returnvalue>setof record</returnvalue>
        ( <parameter>name</parameter> <type>text</type>,
        <parameter>size</parameter> <type>bigint</type>,
        <parameter>modification</parameter> <type>timestamp with time zone</type> )
       </para>
       <para>
        Returns the name, size, and last modification time (mtime) of each
        ordinary file in the server's write-ahead log (WAL) directory.
        Filenames beginning with a dot, directories, and other special files
        are excluded.
       </para>
       <para>
        This function is restricted to superusers and members of
        the <literal>pg_monitor</literal> role by default, but other users can
        be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_ls_archive_statusdir</primary>
        </indexterm>
        <function>pg_ls_archive_statusdir</function> ()
        <returnvalue>setof record</returnvalue>
        ( <parameter>name</parameter> <type>text</type>,
        <parameter>size</parameter> <type>bigint</type>,
        <parameter>modification</parameter> <type>timestamp with time zone</type> )
       </para>
       <para>
        Returns the name, size, and last modification time (mtime) of each
        ordinary file in the server's WAL archive status directory
        (<filename>pg_wal/archive_status</filename>).  Filenames beginning
        with a dot, directories, and other special files are excluded.
       </para>
       <para>
        This function is restricted to superusers and members of
        the <literal>pg_monitor</literal> role by default, but other users can
        be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">

        <indexterm>
         <primary>pg_ls_tmpdir</primary>
        </indexterm>
        <function>pg_ls_tmpdir</function> ( <optional> <parameter>tablespace</parameter> <type>oid</type> </optional> )
        <returnvalue>setof record</returnvalue>
        ( <parameter>name</parameter> <type>text</type>,
        <parameter>size</parameter> <type>bigint</type>,
        <parameter>modification</parameter> <type>timestamp with time zone</type> )
       </para>
       <para>
        Returns the name, size, and last modification time (mtime) of each
        ordinary file in the temporary file directory for the
        specified <parameter>tablespace</parameter>.
        If <parameter>tablespace</parameter> is not provided,
        the <literal>pg_default</literal> tablespace is examined.  Filenames
        beginning with a dot, directories, and other special files are
        excluded.
       </para>
       <para>
        This function is restricted to superusers and members of
        the <literal>pg_monitor</literal> role by default, but other users can
        be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_read_file</primary>
        </indexterm>
        <function>pg_read_file</function> ( <parameter>filename</parameter> <type>text</type> <optional>, <parameter>offset</parameter> <type>bigint</type>, <parameter>length</parameter> <type>bigint</type> <optional>, <parameter>missing_ok</parameter> <type>boolean</type> </optional></optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns all or part of a text file, starting at the
        given byte <parameter>offset</parameter>, returning at
        most <parameter>length</parameter> bytes (less if the end of file is
        reached first).  If <parameter>offset</parameter> is negative, it is
        relative to the end of the file.  If <parameter>offset</parameter>
        and <parameter>length</parameter> are omitted, the entire file is
        returned.  The bytes read from the file are interpreted as a string in
        the database's encoding; an error is thrown if they are not valid in
        that encoding.
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_read_binary_file</primary>
        </indexterm>
        <function>pg_read_binary_file</function> ( <parameter>filename</parameter> <type>text</type> <optional>, <parameter>offset</parameter> <type>bigint</type>, <parameter>length</parameter> <type>bigint</type> <optional>, <parameter>missing_ok</parameter> <type>boolean</type> </optional></optional> )
        <returnvalue>bytea</returnvalue>
       </para>
       <para>
        Returns all or part of a file.  This function is identical to
        <function>pg_read_file</function> except that it can read arbitrary
        binary data, returning the result as <type>bytea</type>
        not <type>text</type>; accordingly, no encoding checks are performed.
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para>
       <para>
        In combination with the <function>convert_from</function> function,
        this function can be used to read a text file in a specified encoding
        and convert to the database's encoding:
<programlisting>
SELECT convert_from(pg_read_binary_file('file_in_utf8.txt'), 'UTF8');
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_file</primary>
        </indexterm>
        <function>pg_stat_file</function> ( <parameter>filename</parameter> <type>text</type> <optional>, <parameter>missing_ok</parameter> <type>boolean</type> </optional> )
        <returnvalue>record</returnvalue>
        ( <parameter>size</parameter> <type>bigint</type>,
        <parameter>access</parameter> <type>timestamp with time zone</type>,
        <parameter>modification</parameter> <type>timestamp with time zone</type>,
        <parameter>change</parameter> <type>timestamp with time zone</type>,
        <parameter>creation</parameter> <type>timestamp with time zone</type>,
        <parameter>isdir</parameter> <type>boolean</type> )
       </para>
       <para>
        Returns a record containing the file's size, last access time stamp,
        last modification time stamp, last file status change time stamp (Unix
        platforms only), file creation time stamp (Windows only), and a flag
        indicating if it is a directory.
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  </sect2>

  <sect2 id="functions-advisory-locks">
   <title>Advisory Lock Functions</title>

   <para>
    The functions shown in <xref linkend="functions-advisory-locks-table"/>
    manage advisory locks.  For details about proper use of these functions,
    see <xref linkend="advisory-locks"/>.
   </para>

   <para>
    All these functions are intended to be used to lock application-defined
    resources, which can be identified either by a single 64-bit key value or
    two 32-bit key values (note that these two key spaces do not overlap).
    If another session already holds a conflicting lock on the same resource
    identifier, the functions will either wait until the resource becomes
    available, or return a <literal>false</literal> result, as appropriate for
    the function.
    Locks can be either shared or exclusive: a shared lock does not conflict
    with other shared locks on the same resource, only with exclusive locks.
    Locks can be taken at session level (so that they are held until released
    or the session ends) or at transaction level (so that they are held until
    the current transaction ends; there is no provision for manual release).
    Multiple session-level lock requests stack, so that if the same resource
    identifier is locked three times there must then be three unlock requests
    to release the resource in advance of session end.
   </para>

   <table id="functions-advisory-locks-table">
    <title>Advisory Lock Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_advisory_lock</primary>
        </indexterm>
        <function>pg_advisory_lock</function> ( <parameter>key</parameter> <type>bigint</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para role="func_signature">
        <function>pg_advisory_lock</function> ( <parameter>key1</parameter> <type>integer</type>, <parameter>key2</parameter> <type>integer</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Obtains an exclusive session-level advisory lock, waiting if necessary.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_advisory_lock_shared</primary>
        </indexterm>
        <function>pg_advisory_lock_shared</function> ( <parameter>key</parameter> <type>bigint</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para role="func_signature">
        <function>pg_advisory_lock_shared</function> ( <parameter>key1</parameter> <type>integer</type>, <parameter>key2</parameter> <type>integer</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Obtains a shared session-level advisory lock, waiting if necessary.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_advisory_unlock</primary>
        </indexterm>
        <function>pg_advisory_unlock</function> ( <parameter>key</parameter> <type>bigint</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para role="func_signature">
        <function>pg_advisory_unlock</function> ( <parameter>key1</parameter> <type>integer</type>, <parameter>key2</parameter> <type>integer</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Releases a previously-acquired exclusive session-level advisory lock.
        Returns <literal>true</literal> if the lock is successfully released.
        If the lock was not held, <literal>false</literal> is returned, and in
        addition, an SQL warning will be reported by the server.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_advisory_unlock_all</primary>
        </indexterm>
        <function>pg_advisory_unlock_all</function> ()
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Releases all session-level advisory locks held by the current session.
        (This function is implicitly invoked at session end, even if the
        client disconnects ungracefully.)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_advisory_unlock_shared</primary>
        </indexterm>
        <function>pg_advisory_unlock_shared</function> ( <parameter>key</parameter> <type>bigint</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para role="func_signature">
        <function>pg_advisory_unlock_shared</function> ( <parameter>key1</parameter> <type>integer</type>, <parameter>key2</parameter> <type>integer</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Releases a previously-acquired shared session-level advisory lock.
        Returns <literal>true</literal> if the lock is successfully released.
        If the lock was not held, <literal>false</literal> is returned, and in
        addition, an SQL warning will be reported by the server.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_advisory_xact_lock</primary>
        </indexterm>
        <function>pg_advisory_xact_lock</function> ( <parameter>key</parameter> <type>bigint</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para role="func_signature">
        <function>pg_advisory_xact_lock</function> ( <parameter>key1</parameter> <type>integer</type>, <parameter>key2</parameter> <type>integer</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Obtains an exclusive transaction-level advisory lock, waiting if
        necessary.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_advisory_xact_lock_shared</primary>
        </indexterm>
        <function>pg_advisory_xact_lock_shared</function> ( <parameter>key</parameter> <type>bigint</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para role="func_signature">
        <function>pg_advisory_xact_lock_shared</function> ( <parameter>key1</parameter> <type>integer</type>, <parameter>key2</parameter> <type>integer</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Obtains a shared transaction-level advisory lock, waiting if
        necessary.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_try_advisory_lock</primary>
        </indexterm>
        <function>pg_try_advisory_lock</function> ( <parameter>key</parameter> <type>bigint</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para role="func_signature">
        <function>pg_try_advisory_lock</function> ( <parameter>key1</parameter> <type>integer</type>, <parameter>key2</parameter> <type>integer</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Obtains an exclusive session-level advisory lock if available.
        This will either obtain the lock immediately and
        return <literal>true</literal>, or return <literal>false</literal>
        without waiting if the lock cannot be acquired immediately.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_try_advisory_lock_shared</primary>
        </indexterm>
        <function>pg_try_advisory_lock_shared</function> ( <parameter>key</parameter> <type>bigint</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para role="func_signature">
        <function>pg_try_advisory_lock_shared</function> ( <parameter>key1</parameter> <type>integer</type>, <parameter>key2</parameter> <type>integer</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Obtains a shared session-level advisory lock if available.
        This will either obtain the lock immediately and
        return <literal>true</literal>, or return <literal>false</literal>
        without waiting if the lock cannot be acquired immediately.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_try_advisory_xact_lock</primary>
        </indexterm>
        <function>pg_try_advisory_xact_lock</function> ( <parameter>key</parameter> <type>bigint</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para role="func_signature">
        <function>pg_try_advisory_xact_lock</function> ( <parameter>key1</parameter> <type>integer</type>, <parameter>key2</parameter> <type>integer</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Obtains an exclusive transaction-level advisory lock if available.
        This will either obtain the lock immediately and
        return <literal>true</literal>, or return <literal>false</literal>
        without waiting if the lock cannot be acquired immediately.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_try_advisory_xact_lock_shared</primary>
        </indexterm>
        <function>pg_try_advisory_xact_lock_shared</function> ( <parameter>key</parameter> <type>bigint</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para role="func_signature">
        <function>pg_try_advisory_xact_lock_shared</function> ( <parameter>key1</parameter> <type>integer</type>, <parameter>key2</parameter> <type>integer</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Obtains a shared transaction-level advisory lock if available.
        This will either obtain the lock immediately and
        return <literal>true</literal>, or return <literal>false</literal>
        without waiting if the lock cannot be acquired immediately.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  </sect2>

  </sect1>

  <sect1 id="functions-trigger">
   <title>Trigger Functions</title>

  <para>
   While many uses of triggers involve user-written trigger functions,
   <productname>PostgreSQL</productname> provides a few built-in trigger
   functions that can be used directly in user-defined triggers.  These
   are summarized in <xref linkend="builtin-triggers-table"/>.
   (Additional built-in trigger functions exist, which implement foreign
   key constraints and deferred index constraints.  Those are not documented
   here since users need not use them directly.)
  </para>

  <para>
   For more information about creating triggers, see
   <xref linkend="sql-createtrigger"/>.
  </para>

   <table id="builtin-triggers-table">
    <title>Built-In Trigger Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para>
       <para>
        Example Usage
       </para></entry>
      </row>
     </thead>

     <tbody>
       <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>suppress_redundant_updates_trigger</primary>
        </indexterm>
        <function>suppress_redundant_updates_trigger</function> ( )
        <returnvalue>trigger</returnvalue>
       </para>
       <para>
        Suppresses do-nothing update operations.  See below for details.
       </para>
       <para>
        <literal>CREATE TRIGGER ... suppress_redundant_updates_trigger()</literal>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>tsvector_update_trigger</primary>
        </indexterm>
        <function>tsvector_update_trigger</function> ( )
        <returnvalue>trigger</returnvalue>
       </para>
       <para>
        Automatically updates a <type>tsvector</type> column from associated
        plain-text document column(s).  The text search configuration to use
        is specified by name as a trigger argument.  See
        <xref linkend="textsearch-update-triggers"/> for details.
       </para>
       <para>
        <literal>CREATE TRIGGER ... tsvector_update_trigger(tsvcol, 'pg_catalog.swedish', title, body)</literal>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>tsvector_update_trigger_column</primary>
        </indexterm>
        <function>tsvector_update_trigger_column</function> ( )
        <returnvalue>trigger</returnvalue>
       </para>
       <para>
        Automatically updates a <type>tsvector</type> column from associated
        plain-text document column(s).  The text search configuration to use
        is taken from a <type>regconfig</type> column of the table.  See
        <xref linkend="textsearch-update-triggers"/> for details.
       </para>
       <para>
        <literal>CREATE TRIGGER ... tsvector_update_trigger_column(tsvcol, tsconfigcol, title, body)</literal>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
      The <function>suppress_redundant_updates_trigger</function> function,
      when applied as a row-level <literal>BEFORE UPDATE</literal> trigger,
      will prevent any update that does not actually change the data in the
      row from taking place.  This overrides the normal behavior which always
      performs a physical row update
      regardless of whether or not the data has changed. (This normal behavior
      makes updates run faster, since no checking is required, and is also
      useful in certain cases.)
    </para>

    <para>
      Ideally, you should avoid running updates that don't actually
      change the data in the record. Redundant updates can cost considerable
      unnecessary time, especially if there are lots of indexes to alter,
      and space in dead rows that will eventually have to be vacuumed.
      However, detecting such situations in client code is not
      always easy, or even possible, and writing expressions to detect
      them can be error-prone. An alternative is to use
      <function>suppress_redundant_updates_trigger</function>, which will skip
      updates that don't change the data. You should use this with care,
      however. The trigger takes a small but non-trivial time for each record,
      so if most of the records affected by updates do actually change,
      use of this trigger will make updates run slower on average.
    </para>

    <para>
      The <function>suppress_redundant_updates_trigger</function> function can be
      added to a table like this:
<programlisting>
CREATE TRIGGER z_min_update
BEFORE UPDATE ON tablename
FOR EACH ROW EXECUTE FUNCTION suppress_redundant_updates_trigger();
</programlisting>
      In most cases, you need to fire this trigger last for each row, so that
      it does not override other triggers that might wish to alter the row.
      Bearing in mind that triggers fire in name order, you would therefore
      choose a trigger name that comes after the name of any other trigger
      you might have on the table.  (Hence the <quote>z</quote> prefix in the
      example.)
    </para>
  </sect1>

  <sect1 id="functions-event-triggers">
   <title>Event Trigger Functions</title>

   <para>
    <productname>PostgreSQL</productname> provides these helper functions
    to retrieve information from event triggers.
   </para>

   <para>
    For more information about event triggers,
    see <xref linkend="event-triggers"/>.
   </para>

  <sect2 id="pg-event-trigger-ddl-command-end-functions">
   <title>Capturing Changes at Command End</title>

   <indexterm>
    <primary>pg_event_trigger_ddl_commands</primary>
   </indexterm>

<synopsis>
<function>pg_event_trigger_ddl_commands</function> () <returnvalue>setof record</returnvalue>
</synopsis>

   <para>
    <function>pg_event_trigger_ddl_commands</function> returns a list of
    <acronym>DDL</acronym> commands executed by each user action,
    when invoked in a function attached to a
    <literal>ddl_command_end</literal> event trigger.  If called in any other
    context, an error is raised.
    <function>pg_event_trigger_ddl_commands</function> returns one row for each
    base command executed; some commands that are a single SQL sentence
    may return more than one row.  This function returns the following
    columns:

    <informaltable>
     <tgroup cols="3">
      <thead>
       <row>
        <entry>Name</entry>
        <entry>Type</entry>
        <entry>Description</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><literal>classid</literal></entry>
        <entry><type>oid</type></entry>
        <entry>OID of catalog the object belongs in</entry>
       </row>
       <row>
        <entry><literal>objid</literal></entry>
        <entry><type>oid</type></entry>
        <entry>OID of the object itself</entry>
       </row>
       <row>
        <entry><literal>objsubid</literal></entry>
        <entry><type>integer</type></entry>
        <entry>Sub-object ID (e.g., attribute number for a column)</entry>
       </row>
       <row>
        <entry><literal>command_tag</literal></entry>
        <entry><type>text</type></entry>
        <entry>Command tag</entry>
       </row>
       <row>
        <entry><literal>object_type</literal></entry>
        <entry><type>text</type></entry>
        <entry>Type of the object</entry>
       </row>
       <row>
        <entry><literal>schema_name</literal></entry>
        <entry><type>text</type></entry>
        <entry>
         Name of the schema the object belongs in, if any; otherwise <literal>NULL</literal>.
         No quoting is applied.
        </entry>
       </row>
       <row>
        <entry><literal>object_identity</literal></entry>
        <entry><type>text</type></entry>
        <entry>
         Text rendering of the object identity, schema-qualified. Each
         identifier included in the identity is quoted if necessary.
        </entry>
       </row>
       <row>
        <entry><literal>in_extension</literal></entry>
        <entry><type>boolean</type></entry>
        <entry>True if the command is part of an extension script</entry>
       </row>
       <row>
        <entry><literal>command</literal></entry>
        <entry><type>pg_ddl_command</type></entry>
        <entry>
         A complete representation of the command, in internal format.
         This cannot be output directly, but it can be passed to other
         functions to obtain different pieces of information about the
         command.
        </entry>
       </row>
      </tbody>
     </tgroup>
    </informaltable>
   </para>
  </sect2>

  <sect2 id="pg-event-trigger-sql-drop-functions">
   <title>Processing Objects Dropped by a DDL Command</title>

   <indexterm>
     <primary>pg_event_trigger_dropped_objects</primary>
   </indexterm>

<synopsis>
<function>pg_event_trigger_dropped_objects</function> () <returnvalue>setof record</returnvalue>
</synopsis>

   <para>
    <function>pg_event_trigger_dropped_objects</function> returns a list of all objects
    dropped by the command in whose <literal>sql_drop</literal> event it is called.
    If called in any other context, an error is raised.
    This function returns the following columns:

    <informaltable>
     <tgroup cols="3">
      <thead>
       <row>
        <entry>Name</entry>
        <entry>Type</entry>
        <entry>Description</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><literal>classid</literal></entry>
        <entry><type>oid</type></entry>
        <entry>OID of catalog the object belonged in</entry>
       </row>
       <row>
        <entry><literal>objid</literal></entry>
        <entry><type>oid</type></entry>
        <entry>OID of the object itself</entry>
       </row>
       <row>
        <entry><literal>objsubid</literal></entry>
        <entry><type>integer</type></entry>
        <entry>Sub-object ID (e.g., attribute number for a column)</entry>
       </row>
       <row>
        <entry><literal>original</literal></entry>
        <entry><type>boolean</type></entry>
        <entry>True if this was one of the root object(s) of the deletion</entry>
       </row>
       <row>
        <entry><literal>normal</literal></entry>
        <entry><type>boolean</type></entry>
        <entry>
         True if there was a normal dependency relationship
         in the dependency graph leading to this object
        </entry>
       </row>
       <row>
        <entry><literal>is_temporary</literal></entry>
        <entry><type>boolean</type></entry>
        <entry>
         True if this was a temporary object
        </entry>
       </row>
       <row>
        <entry><literal>object_type</literal></entry>
        <entry><type>text</type></entry>
        <entry>Type of the object</entry>
       </row>
       <row>
        <entry><literal>schema_name</literal></entry>
        <entry><type>text</type></entry>
        <entry>
         Name of the schema the object belonged in, if any; otherwise <literal>NULL</literal>.
         No quoting is applied.
        </entry>
       </row>
       <row>
        <entry><literal>object_name</literal></entry>
        <entry><type>text</type></entry>
        <entry>
         Name of the object, if the combination of schema and name can be
         used as a unique identifier for the object; otherwise <literal>NULL</literal>.
         No quoting is applied, and name is never schema-qualified.
        </entry>
       </row>
       <row>
        <entry><literal>object_identity</literal></entry>
        <entry><type>text</type></entry>
        <entry>
         Text rendering of the object identity, schema-qualified. Each
         identifier included in the identity is quoted if necessary.
        </entry>
       </row>
       <row>
        <entry><literal>address_names</literal></entry>
        <entry><type>text[]</type></entry>
        <entry>
         An array that, together with <literal>object_type</literal> and
         <literal>address_args</literal>, can be used by
         the <function>pg_get_object_address</function> function to
         recreate the object address in a remote server containing an
         identically named object of the same kind.
        </entry>
       </row>
       <row>
        <entry><literal>address_args</literal></entry>
        <entry><type>text[]</type></entry>
        <entry>
         Complement for <literal>address_names</literal>
        </entry>
       </row>
      </tbody>
     </tgroup>
    </informaltable>
   </para>

   <para>
    The <function>pg_event_trigger_dropped_objects</function> function can be used
    in an event trigger like this:
<programlisting>
CREATE FUNCTION test_event_trigger_for_drops()
        RETURNS event_trigger LANGUAGE plpgsql AS $$
DECLARE
    obj record;
BEGIN
    FOR obj IN SELECT * FROM pg_event_trigger_dropped_objects()
    LOOP
        RAISE NOTICE '% dropped object: % %.% %',
                     tg_tag,
                     obj.object_type,
                     obj.schema_name,
                     obj.object_name,
                     obj.object_identity;
    END LOOP;
END
$$;
CREATE EVENT TRIGGER test_event_trigger_for_drops
   ON sql_drop
   EXECUTE FUNCTION test_event_trigger_for_drops();
</programlisting>
    </para>
  </sect2>

  <sect2 id="pg-event-trigger-table-rewrite-functions">
   <title>Handling a Table Rewrite Event</title>

   <para>
    The functions shown in
    <xref linkend="functions-event-trigger-table-rewrite"/>
    provide information about a table for which a
    <literal>table_rewrite</literal> event has just been called.
    If called in any other context, an error is raised.
   </para>

   <table id="functions-event-trigger-table-rewrite">
    <title>Table Rewrite Information Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_event_trigger_table_rewrite_oid</primary>
        </indexterm>
        <function>pg_event_trigger_table_rewrite_oid</function> ()
        <returnvalue>oid</returnvalue>
       </para>
       <para>
        Returns the OID of the table about to be rewritten.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_event_trigger_table_rewrite_reason</primary>
        </indexterm>
        <function>pg_event_trigger_table_rewrite_reason</function> ()
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns a code explaining the reason(s) for rewriting.  The exact
        meaning of the codes is release dependent.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    These functions can be used in an event trigger like this:
<programlisting>
CREATE FUNCTION test_event_trigger_table_rewrite_oid()
 RETURNS event_trigger
 LANGUAGE plpgsql AS
$$
BEGIN
  RAISE NOTICE 'rewriting table % for reason %',
                pg_event_trigger_table_rewrite_oid()::regclass,
                pg_event_trigger_table_rewrite_reason();
END;
$$;

CREATE EVENT TRIGGER test_table_rewrite_oid
                  ON table_rewrite
   EXECUTE FUNCTION test_event_trigger_table_rewrite_oid();
</programlisting>
    </para>
  </sect2>
  </sect1>

  <sect1 id="functions-statistics">
   <title>Statistics Information Functions</title>

   <indexterm zone="functions-statistics">
    <primary>function</primary>
    <secondary>statistics</secondary>
   </indexterm>

   <para>
    <productname>PostgreSQL</productname> provides a function to inspect complex
    statistics defined using the <command>CREATE STATISTICS</command> command.
   </para>

  <sect2 id="functions-statistics-mcv">
   <title>Inspecting MCV Lists</title>

   <indexterm>
    <primary>pg_mcv_list_items</primary>
   </indexterm>

<synopsis>
<function>pg_mcv_list_items</function> ( <type>pg_mcv_list</type> ) <returnvalue>setof record</returnvalue>
</synopsis>

   <para>
    <function>pg_mcv_list_items</function> returns a set of records describing
    all items stored in a multi-column <acronym>MCV</acronym> list.  It
    returns the following columns:

    <informaltable>
     <tgroup cols="3">
      <thead>
       <row>
        <entry>Name</entry>
        <entry>Type</entry>
        <entry>Description</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><literal>index</literal></entry>
        <entry><type>integer</type></entry>
        <entry>index of the item in the <acronym>MCV</acronym> list</entry>
       </row>
       <row>
        <entry><literal>values</literal></entry>
        <entry><type>text[]</type></entry>
        <entry>values stored in the MCV item</entry>
       </row>
       <row>
        <entry><literal>nulls</literal></entry>
        <entry><type>boolean[]</type></entry>
        <entry>flags identifying <literal>NULL</literal> values</entry>
       </row>
       <row>
        <entry><literal>frequency</literal></entry>
        <entry><type>double precision</type></entry>
        <entry>frequency of this <acronym>MCV</acronym> item</entry>
       </row>
       <row>
        <entry><literal>base_frequency</literal></entry>
        <entry><type>double precision</type></entry>
        <entry>base frequency of this <acronym>MCV</acronym> item</entry>
       </row>
      </tbody>
     </tgroup>
    </informaltable>
   </para>

   <para>
    The <function>pg_mcv_list_items</function> function can be used like this:

<programlisting>
SELECT m.* FROM pg_statistic_ext join pg_statistic_ext_data on (oid = stxoid),
                pg_mcv_list_items(stxdmcv) m WHERE stxname = 'stts';
</programlisting>

    Values of the <type>pg_mcv_list</type> type can be obtained only from the
    <structname>pg_statistic_ext_data</structname>.<structfield>stxdmcv</structfield>
    column.
   </para>
  </sect2>

  </sect1>

</chapter>
