<!-- 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.  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>
   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.  This chapter is also
   not exhaustive;  additional functions appear in relevant sections of
   the manual.
  </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>

    <simplelist>
     <member><literal>AND</literal></member>
     <member><literal>OR</literal></member>
     <member><literal>NOT</literal></member>
    </simplelist>

    <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 operand
    without affecting the result.  But 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> <literal>&lt;</literal> </entry>
       <entry>less than</entry>
      </row>

      <row>
       <entry> <literal>&gt;</literal> </entry>
       <entry>greater than</entry>
      </row>

      <row>
       <entry> <literal>&lt;=</literal> </entry>
       <entry>less than or equal to</entry>
      </row>

      <row>
       <entry> <literal>&gt;=</literal> </entry>
       <entry>greater than or equal to</entry>
      </row>

      <row>
       <entry> <literal>=</literal> </entry>
       <entry>equal</entry>
      </row>

      <row>
       <entry> <literal>&lt;&gt;</literal> or <literal>!=</literal> </entry>
       <entry>not equal</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <note>
    <para>
     The <literal>!=</literal> operator is converted to
     <literal>&lt;&gt;</literal> in the parser stage.  It is not
     possible to implement <literal>!=</literal> and
     <literal>&lt;&gt;</literal> operators that do different things.
    </para>
   </note>

   <para>
    Comparison operators are available for all relevant data types.
    All comparison operators are binary operators that
    return values of type <type>boolean</type>; 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>).
   </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="2">
     <thead>
      <row>
       <entry>Predicate</entry>
       <entry>Description</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry> <replaceable>a</replaceable> <literal>BETWEEN</literal> <replaceable>x</replaceable> <literal>AND</literal> <replaceable>y</replaceable> </entry>
       <entry>between</entry>
      </row>

      <row>
       <entry> <replaceable>a</replaceable> <literal>NOT BETWEEN</literal> <replaceable>x</replaceable> <literal>AND</literal> <replaceable>y</replaceable> </entry>
       <entry>not between</entry>
      </row>

      <row>
       <entry> <replaceable>a</replaceable> <literal>BETWEEN SYMMETRIC</literal> <replaceable>x</replaceable> <literal>AND</literal> <replaceable>y</replaceable> </entry>
       <entry>between, after sorting the comparison values</entry>
      </row>

      <row>
       <entry> <replaceable>a</replaceable> <literal>NOT BETWEEN SYMMETRIC</literal> <replaceable>x</replaceable> <literal>AND</literal> <replaceable>y</replaceable> </entry>
       <entry>not between, after sorting the comparison values</entry>
      </row>

      <row>
       <entry> <replaceable>a</replaceable> <literal>IS DISTINCT FROM</literal> <replaceable>b</replaceable> </entry>
       <entry>not equal, treating null like an ordinary value</entry>
      </row>

      <row>
       <entry><replaceable>a</replaceable> <literal>IS NOT DISTINCT FROM</literal> <replaceable>b</replaceable></entry>
       <entry>equal, treating null like an ordinary value</entry>
      </row>

      <row>
       <entry> <replaceable>expression</replaceable> <literal>IS NULL</literal> </entry>
       <entry>is null</entry>
      </row>

      <row>
       <entry> <replaceable>expression</replaceable> <literal>IS NOT NULL</literal> </entry>
       <entry>is not null</entry>
      </row>

      <row>
       <entry> <replaceable>expression</replaceable> <literal>ISNULL</literal> </entry>
       <entry>is null (nonstandard syntax)</entry>
      </row>

      <row>
       <entry> <replaceable>expression</replaceable> <literal>NOTNULL</literal> </entry>
       <entry>is not null (nonstandard syntax)</entry>
      </row>

      <row>
       <entry> <replaceable>boolean_expression</replaceable> <literal>IS TRUE</literal> </entry>
       <entry>is true</entry>
      </row>

      <row>
       <entry> <replaceable>boolean_expression</replaceable> <literal>IS NOT TRUE</literal> </entry>
       <entry>is false or unknown</entry>
      </row>

      <row>
       <entry> <replaceable>boolean_expression</replaceable> <literal>IS FALSE</literal> </entry>
       <entry>is false</entry>
      </row>

      <row>
       <entry> <replaceable>boolean_expression</replaceable> <literal>IS NOT FALSE</literal> </entry>
       <entry>is true or unknown</entry>
      </row>

      <row>
       <entry> <replaceable>boolean_expression</replaceable> <literal>IS UNKNOWN</literal> </entry>
       <entry>is unknown</entry>
      </row>

      <row>
       <entry> <replaceable>boolean_expression</replaceable> <literal>IS NOT UNKNOWN</literal> </entry>
       <entry>is true or false</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    <indexterm>
     <primary>BETWEEN</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>NOT BETWEEN</literal> does the opposite comparison:
<synopsis>
<replaceable>a</replaceable> NOT BETWEEN <replaceable>x</replaceable> AND <replaceable>y</replaceable>
</synopsis>
    is equivalent to
<synopsis>
<replaceable>a</replaceable> &lt; <replaceable>x</replaceable> OR <replaceable>a</replaceable> &gt; <replaceable>y</replaceable>
</synopsis>
    <indexterm>
     <primary>BETWEEN SYMMETRIC</primary>
    </indexterm>
    <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>
    <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="4">
     <thead>
      <row>
       <entry>Function</entry>
       <entry>Description</entry>
       <entry>Example</entry>
       <entry>Example Result</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>
         <indexterm>
          <primary>num_nonnulls</primary>
         </indexterm>
         <literal>num_nonnulls(VARIADIC "any")</literal>
       </entry>
       <entry>returns the number of non-null arguments</entry>
       <entry><literal>num_nonnulls(1, NULL, 2)</literal></entry>
       <entry><literal>2</literal></entry>
      </row>
      <row>
       <entry>
         <indexterm>
          <primary>num_nulls</primary>
         </indexterm>
         <literal>num_nulls(VARIADIC "any")</literal>
       </entry>
       <entry>returns the number of null arguments</entry>
       <entry><literal>num_nulls(1, NULL, 2)</literal></entry>
       <entry><literal>1</literal></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 available mathematical operators.
   </para>

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

    <tgroup cols="4">
     <thead>
      <row>
       <entry>Operator</entry>
       <entry>Description</entry>
       <entry>Example</entry>
       <entry>Result</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry> <literal>+</literal> </entry>
       <entry>addition</entry>
       <entry><literal>2 + 3</literal></entry>
       <entry><literal>5</literal></entry>
      </row>

      <row>
       <entry> <literal>-</literal> </entry>
       <entry>subtraction</entry>
       <entry><literal>2 - 3</literal></entry>
       <entry><literal>-1</literal></entry>
      </row>

      <row>
       <entry> <literal>*</literal> </entry>
       <entry>multiplication</entry>
       <entry><literal>2 * 3</literal></entry>
       <entry><literal>6</literal></entry>
      </row>

      <row>
       <entry> <literal>/</literal> </entry>
       <entry>division (integer division truncates the result)</entry>
       <entry><literal>4 / 2</literal></entry>
       <entry><literal>2</literal></entry>
      </row>

      <row>
       <entry> <literal>%</literal> </entry>
       <entry>modulo (remainder)</entry>
       <entry><literal>5 % 4</literal></entry>
       <entry><literal>1</literal></entry>
      </row>

      <row>
       <entry> <literal>^</literal> </entry>
       <entry>exponentiation (associates left to right)</entry>
       <entry><literal>2.0 ^ 3.0</literal></entry>
       <entry><literal>8</literal></entry>
      </row>

      <row>
       <entry> <literal>|/</literal> </entry>
       <entry>square root</entry>
       <entry><literal>|/ 25.0</literal></entry>
       <entry><literal>5</literal></entry>
      </row>

      <row>
       <entry> <literal>||/</literal> </entry>
       <entry>cube root</entry>
       <entry><literal>||/ 27.0</literal></entry>
       <entry><literal>3</literal></entry>
      </row>

      <row>
       <entry> <literal>!</literal> </entry>
       <entry>factorial</entry>
       <entry><literal>5 !</literal></entry>
       <entry><literal>120</literal></entry>
      </row>

      <row>
       <entry> <literal>!!</literal> </entry>
       <entry>factorial (prefix operator)</entry>
       <entry><literal>!! 5</literal></entry>
       <entry><literal>120</literal></entry>
      </row>

      <row>
       <entry> <literal>@</literal> </entry>
       <entry>absolute value</entry>
       <entry><literal>@ -5.0</literal></entry>
       <entry><literal>5</literal></entry>
      </row>

      <row>
       <entry> <literal>&amp;</literal> </entry>
       <entry>bitwise AND</entry>
       <entry><literal>91 &amp; 15</literal></entry>
       <entry><literal>11</literal></entry>
      </row>

      <row>
       <entry> <literal>|</literal> </entry>
       <entry>bitwise OR</entry>
       <entry><literal>32 | 3</literal></entry>
       <entry><literal>35</literal></entry>
      </row>

      <row>
       <entry> <literal>#</literal> </entry>
       <entry>bitwise XOR</entry>
       <entry><literal>17 # 5</literal></entry>
       <entry><literal>20</literal></entry>
      </row>

      <row>
       <entry> <literal>~</literal> </entry>
       <entry>bitwise NOT</entry>
       <entry><literal>~1</literal></entry>
       <entry><literal>-2</literal></entry>
      </row>

      <row>
       <entry> <literal>&lt;&lt;</literal> </entry>
       <entry>bitwise shift left</entry>
       <entry><literal>1 &lt;&lt; 4</literal></entry>
       <entry><literal>16</literal></entry>
      </row>

      <row>
       <entry> <literal>&gt;&gt;</literal> </entry>
       <entry>bitwise shift right</entry>
       <entry><literal>8 &gt;&gt; 2</literal></entry>
       <entry><literal>2</literal></entry>
      </row>

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

   <para>
    The bitwise operators work only on integral data types, whereas
    the others are available for all numeric data types.  The bitwise
    operators are also available for the bit
    string types <type>bit</type> and <type>bit varying</type>, as
    shown in <xref linkend="functions-bit-string-op-table"/>.
   </para>

  <para>
   <xref linkend="functions-math-func-table"/> shows the available
   mathematical functions.  In the table, <literal>dp</literal>
   indicates <type>double precision</type>.  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.
   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="5">
     <thead>
      <row>
       <entry>Function</entry>
       <entry>Return Type</entry>
       <entry>Description</entry>
       <entry>Example</entry>
       <entry>Result</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>
        <indexterm>
         <primary>abs</primary>
        </indexterm>
        <literal><function>abs(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>(same as input)</entry>
       <entry>absolute value</entry>
       <entry><literal>abs(-17.4)</literal></entry>
       <entry><literal>17.4</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>cbrt</primary>
        </indexterm>
        <literal><function>cbrt(<type>dp</type>)</function></literal>
       </entry>
       <entry><type>dp</type></entry>
       <entry>cube root</entry>
       <entry><literal>cbrt(27.0)</literal></entry>
       <entry><literal>3</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>ceil</primary>
        </indexterm>
        <literal><function>ceil(<type>dp</type> or <type>numeric</type>)</function></literal>
       </entry>
       <entry>(same as input)</entry>
       <entry>nearest integer greater than or equal to argument</entry>
       <entry><literal>ceil(-42.8)</literal></entry>
       <entry><literal>-42</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>ceiling</primary>
        </indexterm>
        <literal><function>ceiling(<type>dp</type> or <type>numeric</type>)</function></literal>
       </entry>
       <entry>(same as input)</entry>
       <entry>nearest integer greater than or equal to argument (same as <function>ceil</function>)</entry>
       <entry><literal>ceiling(-95.3)</literal></entry>
       <entry><literal>-95</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>degrees</primary>
        </indexterm>
        <literal><function>degrees(<type>dp</type>)</function></literal>
       </entry>
       <entry><type>dp</type></entry>
       <entry>radians to degrees</entry>
       <entry><literal>degrees(0.5)</literal></entry>
       <entry><literal>28.6478897565412</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>div</primary>
        </indexterm>
        <literal><function>div(<parameter>y</parameter> <type>numeric</type>,
         <parameter>x</parameter> <type>numeric</type>)</function></literal>
       </entry>
       <entry><type>numeric</type></entry>
       <entry>integer quotient of <parameter>y</parameter>/<parameter>x</parameter></entry>
       <entry><literal>div(9,4)</literal></entry>
       <entry><literal>2</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>exp</primary>
        </indexterm>
        <literal><function>exp(<type>dp</type> or <type>numeric</type>)</function></literal>
       </entry>
       <entry>(same as input)</entry>
       <entry>exponential</entry>
       <entry><literal>exp(1.0)</literal></entry>
       <entry><literal>2.71828182845905</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>floor</primary>
        </indexterm>
        <literal><function>floor(<type>dp</type> or <type>numeric</type>)</function></literal>
       </entry>
       <entry>(same as input)</entry>
       <entry>nearest integer less than or equal to argument</entry>
       <entry><literal>floor(-42.8)</literal></entry>
       <entry><literal>-43</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>ln</primary>
        </indexterm>
        <literal><function>ln(<type>dp</type> or <type>numeric</type>)</function></literal>
       </entry>
       <entry>(same as input)</entry>
       <entry>natural logarithm</entry>
       <entry><literal>ln(2.0)</literal></entry>
       <entry><literal>0.693147180559945</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>log</primary>
        </indexterm>
        <literal><function>log(<type>dp</type> or <type>numeric</type>)</function></literal>
       </entry>
       <entry>(same as input)</entry>
       <entry>base 10 logarithm</entry>
       <entry><literal>log(100.0)</literal></entry>
       <entry><literal>2</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>log10</primary>
        </indexterm>
        <literal><function>log10(<type>dp</type> or <type>numeric</type>)</function></literal>
       </entry>
       <entry>(same as input)</entry>
       <entry>base 10 logarithm</entry>
       <entry><literal>log10(100.0)</literal></entry>
       <entry><literal>2</literal></entry>
      </row>

      <row>
       <entry><literal><function>log(<parameter>b</parameter> <type>numeric</type>,
        <parameter>x</parameter> <type>numeric</type>)</function></literal></entry>
       <entry><type>numeric</type></entry>
       <entry>logarithm to base <parameter>b</parameter></entry>
       <entry><literal>log(2.0, 64.0)</literal></entry>
       <entry><literal>6.0000000000</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>min_scale</primary>
        </indexterm>
        <literal><function>min_scale(<type>numeric</type>)</function></literal>
       </entry>
       <entry><type>integer</type></entry>
       <entry>minimum scale (number of fractional decimal digits) needed
        to represent the supplied value</entry>
       <entry><literal>min_scale(8.4100)</literal></entry>
       <entry><literal>2</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>mod</primary>
        </indexterm>
        <literal><function>mod(<parameter>y</parameter>,
         <parameter>x</parameter>)</function></literal>
       </entry>
       <entry>(same as argument types)</entry>
       <entry>remainder of <parameter>y</parameter>/<parameter>x</parameter></entry>
       <entry><literal>mod(9,4)</literal></entry>
       <entry><literal>1</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>pi</primary>
        </indexterm>
        <literal><function>pi()</function></literal>
       </entry>
       <entry><type>dp</type></entry>
       <entry><quote>&pi;</quote> constant</entry>
       <entry><literal>pi()</literal></entry>
       <entry><literal>3.14159265358979</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>power</primary>
        </indexterm>
        <literal><function>power(<parameter>a</parameter> <type>dp</type>,
        <parameter>b</parameter> <type>dp</type>)</function></literal>
       </entry>
       <entry><type>dp</type></entry>
       <entry><parameter>a</parameter> raised to the power of <parameter>b</parameter></entry>
       <entry><literal>power(9.0, 3.0)</literal></entry>
       <entry><literal>729</literal></entry>
      </row>

      <row>
       <entry><literal><function>power(<parameter>a</parameter> <type>numeric</type>,
        <parameter>b</parameter> <type>numeric</type>)</function></literal></entry>
       <entry><type>numeric</type></entry>
       <entry><parameter>a</parameter> raised to the power of <parameter>b</parameter></entry>
       <entry><literal>power(9.0, 3.0)</literal></entry>
       <entry><literal>729</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>radians</primary>
        </indexterm>
        <literal><function>radians(<type>dp</type>)</function></literal>
       </entry>
       <entry><type>dp</type></entry>
       <entry>degrees to radians</entry>
       <entry><literal>radians(45.0)</literal></entry>
       <entry><literal>0.785398163397448</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>round</primary>
        </indexterm>
        <literal><function>round(<type>dp</type> or <type>numeric</type>)</function></literal>
       </entry>
       <entry>(same as input)</entry>
       <entry>round to nearest integer</entry>
       <entry><literal>round(42.4)</literal></entry>
       <entry><literal>42</literal></entry>
      </row>

      <row>
       <entry><literal><function>round(<parameter>v</parameter> <type>numeric</type>, <parameter>s</parameter> <type>int</type>)</function></literal></entry>
       <entry><type>numeric</type></entry>
       <entry>round to <parameter>s</parameter> decimal places</entry>
       <entry><literal>round(42.4382, 2)</literal></entry>
       <entry><literal>42.44</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>scale</primary>
        </indexterm>
        <literal><function>scale(<type>numeric</type>)</function></literal>
       </entry>
       <entry><type>integer</type></entry>
       <entry>scale of the argument (the number of decimal digits in the fractional part)</entry>
       <entry><literal>scale(8.4100)</literal></entry>
       <entry><literal>4</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>sign</primary>
        </indexterm>
        <literal><function>sign(<type>dp</type> or <type>numeric</type>)</function></literal>
       </entry>
       <entry>(same as input)</entry>
       <entry>sign of the argument (-1, 0, +1)</entry>
       <entry><literal>sign(-8.4)</literal></entry>
       <entry><literal>-1</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>sqrt</primary>
        </indexterm>
        <literal><function>sqrt(<type>dp</type> or <type>numeric</type>)</function></literal>
       </entry>
       <entry>(same as input)</entry>
       <entry>square root</entry>
       <entry><literal>sqrt(2.0)</literal></entry>
       <entry><literal>1.4142135623731</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>trim_scale</primary>
        </indexterm>
        <literal><function>trim_scale(<type>numeric</type>)</function></literal>
       </entry>
       <entry><type>numeric</type></entry>
       <entry>reduce the scale (number of fractional decimal digits) by
        removing trailing zeroes</entry>
       <entry><literal>trim_scale(8.4100)</literal></entry>
       <entry><literal>8.41</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>trunc</primary>
        </indexterm>
        <literal><function>trunc(<type>dp</type> or <type>numeric</type>)</function></literal>
       </entry>
       <entry>(same as input)</entry>
       <entry>truncate toward zero</entry>
       <entry><literal>trunc(42.8)</literal></entry>
       <entry><literal>42</literal></entry>
      </row>

      <row>
       <entry><literal><function>trunc(<parameter>v</parameter> <type>numeric</type>, <parameter>s</parameter> <type>int</type>)</function></literal></entry>
       <entry><type>numeric</type></entry>
       <entry>truncate to <parameter>s</parameter> decimal places</entry>
       <entry><literal>trunc(42.4382, 2)</literal></entry>
       <entry><literal>42.43</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>width_bucket</primary>
        </indexterm>
        <literal><function>width_bucket(<parameter>operand</parameter> <type>dp</type>, <parameter>b1</parameter> <type>dp</type>, <parameter>b2</parameter> <type>dp</type>, <parameter>count</parameter> <type>int</type>)</function></literal></entry>
       <entry><type>int</type></entry>
       <entry>return the bucket number to which <parameter>operand</parameter> would
       be assigned in a histogram having <parameter>count</parameter> equal-width
       buckets spanning the range <parameter>b1</parameter> to <parameter>b2</parameter>;
       returns <literal>0</literal> or <literal><parameter>count</parameter>+1</literal> for
       an input outside the range</entry>
       <entry><literal>width_bucket(5.35, 0.024, 10.06, 5)</literal></entry>
       <entry><literal>3</literal></entry>
      </row>

      <row>
       <entry><literal><function>width_bucket(<parameter>operand</parameter> <type>numeric</type>, <parameter>b1</parameter> <type>numeric</type>, <parameter>b2</parameter> <type>numeric</type>, <parameter>count</parameter> <type>int</type>)</function></literal></entry>
       <entry><type>int</type></entry>
       <entry>return the bucket number to which <parameter>operand</parameter> would
       be assigned in a histogram having <parameter>count</parameter> equal-width
       buckets spanning the range <parameter>b1</parameter> to <parameter>b2</parameter>;
       returns <literal>0</literal> or <literal><parameter>count</parameter>+1</literal> for
       an input outside the range</entry>
       <entry><literal>width_bucket(5.35, 0.024, 10.06, 5)</literal></entry>
       <entry><literal>3</literal></entry>
      </row>

      <row>
       <entry><literal><function>width_bucket(<parameter>operand</parameter> <type>anyelement</type>, <parameter>thresholds</parameter> <type>anyarray</type>)</function></literal></entry>
       <entry><type>int</type></entry>
       <entry>return the bucket number to which <parameter>operand</parameter> would
       be assigned given an array listing the lower bounds of the buckets;
       returns <literal>0</literal> for an input less than the first lower bound;
       the <parameter>thresholds</parameter> array <emphasis>must be sorted</emphasis>,
       smallest first, or unexpected results will be obtained</entry>
       <entry><literal>width_bucket(now(), array['yesterday', 'today', 'tomorrow']::timestamptz[])</literal></entry>
       <entry><literal>2</literal></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="3">
     <thead>
      <row>
       <entry>Function</entry>
       <entry>Return Type</entry>
       <entry>Description</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>
        <indexterm>
         <primary>random</primary>
        </indexterm>
        <literal><function>random()</function></literal>
       </entry>
       <entry><type>dp</type></entry>
       <entry>random value in the range 0.0 &lt;= x &lt; 1.0</entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>setseed</primary>
        </indexterm>
        <literal><function>setseed(<type>dp</type>)</function></literal>
       </entry>
       <entry><type>void</type></entry>
       <entry>set seed for subsequent <literal>random()</literal> calls (value between -1.0 and
       1.0, inclusive)</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 results of
   subsequent <function>random()</function> calls in the current session are
   repeatable by re-issuing <function>setseed()</function> with the same
   argument.
  </para>

  <para>
   <xref linkend="functions-math-trig-table"/> shows the
   available trigonometric functions.  All these functions
   take arguments and return values of type <type>double
   precision</type>.  Each of the trigonometric 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="3">
     <thead>
      <row>
       <entry>Function (radians)</entry>
       <entry>Function (degrees)</entry>
       <entry>Description</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>
        <indexterm>
         <primary>acos</primary>
        </indexterm><literal><function>acos(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>
        <indexterm>
         <primary>acosd</primary>
        </indexterm><literal><function>acosd(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>inverse cosine</entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>asin</primary>
        </indexterm>
        <literal><function>asin(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>
        <indexterm>
         <primary>asind</primary>
        </indexterm>
        <literal><function>asind(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>inverse sine</entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>atan</primary>
        </indexterm>
        <literal><function>atan(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>
        <indexterm>
         <primary>atand</primary>
        </indexterm>
        <literal><function>atand(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>inverse tangent</entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>atan2</primary>
        </indexterm>
        <literal><function>atan2(<replaceable>y</replaceable>,
        <replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>
        <indexterm>
         <primary>atan2d</primary>
        </indexterm>
        <literal><function>atan2d(<replaceable>y</replaceable>,
        <replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>inverse tangent of
        <literal><replaceable>y</replaceable>/<replaceable>x</replaceable></literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>cos</primary>
        </indexterm>
        <literal><function>cos(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>
        <indexterm>
         <primary>cosd</primary>
        </indexterm>
        <literal><function>cosd(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>cosine</entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>cot</primary>
        </indexterm>
        <literal><function>cot(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>
        <indexterm>
         <primary>cotd</primary>
        </indexterm>
        <literal><function>cotd(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>cotangent</entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>sin</primary>
        </indexterm>
        <literal><function>sin(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>
        <indexterm>
         <primary>sind</primary>
        </indexterm>
        <literal><function>sind(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>sine</entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>tan</primary>
        </indexterm>
        <literal><function>tan(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>
        <indexterm>
         <primary>tand</primary>
        </indexterm>
        <literal><function>tand(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>tangent</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.  All these functions
   take arguments and return values of type <type>double
   precision</type>.
  </para>

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

    <tgroup cols="4">
     <thead>
      <row>
       <entry>Function</entry>
       <entry>Description</entry>
       <entry>Example</entry>
       <entry>Result</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>
        <indexterm>
         <primary>sinh</primary>
        </indexterm>
        <literal><function>sinh(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>hyperbolic sine</entry>
       <entry><literal>sinh(0)</literal></entry>
       <entry><literal>0</literal></entry>
      </row>
      <row>
       <entry>
        <indexterm>
         <primary>cosh</primary>
        </indexterm>
        <literal><function>cosh(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>hyperbolic cosine</entry>
       <entry><literal>cosh(0)</literal></entry>
       <entry><literal>1</literal></entry>
      </row>
      <row>
       <entry>
        <indexterm>
         <primary>tanh</primary>
        </indexterm>
        <literal><function>tanh(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>hyperbolic tangent</entry>
       <entry><literal>tanh(0)</literal></entry>
       <entry><literal>0</literal></entry>
      </row>
      <row>
       <entry>
        <indexterm>
         <primary>asinh</primary>
        </indexterm>
        <literal><function>asinh(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>inverse hyperbolic sine</entry>
       <entry><literal>asinh(0)</literal></entry>
       <entry><literal>0</literal></entry>
      </row>
      <row>
       <entry>
        <indexterm>
         <primary>acosh</primary>
        </indexterm>
        <literal><function>acosh(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>inverse hyperbolic cosine</entry>
       <entry><literal>acosh(1)</literal></entry>
       <entry><literal>0</literal></entry>
      </row>
      <row>
       <entry>
        <indexterm>
         <primary>atanh</primary>
        </indexterm>
        <literal><function>atanh(<replaceable>x</replaceable>)</function></literal>
       </entry>
       <entry>inverse hyperbolic tangent</entry>
       <entry><literal>atanh(0)</literal></entry>
       <entry><literal>0</literal></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>.  Unless otherwise noted, all
    of the functions listed below work on all of these types, but be
    wary of potential effects of automatic space-padding when using the
    <type>character</type> type.  Some functions also exist
    natively for the bit-string types.
   </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="5">
     <thead>
      <row>
       <entry>Function</entry>
       <entry>Return Type</entry>
       <entry>Description</entry>
       <entry>Example</entry>
       <entry>Result</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry><literal><parameter>string</parameter> <literal>||</literal>
        <parameter>string</parameter></literal></entry>
       <entry> <type>text</type> </entry>
       <entry>
        String concatenation
        <indexterm>
         <primary>character string</primary>
         <secondary>concatenation</secondary>
        </indexterm>
       </entry>
       <entry><literal>'Post' || 'greSQL'</literal></entry>
       <entry><literal>PostgreSQL</literal></entry>
      </row>

      <row>
       <entry>
        <literal><parameter>string</parameter> <literal>||</literal>
        <parameter>non-string</parameter></literal>
        or
        <literal><parameter>non-string</parameter> <literal>||</literal>
        <parameter>string</parameter></literal>
       </entry>
       <entry> <type>text</type> </entry>
       <entry>
        String concatenation with one non-string input
       </entry>
       <entry><literal>'Value: ' || 42</literal></entry>
       <entry><literal>Value: 42</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>bit_length</primary>
        </indexterm>
        <literal><function>bit_length(<parameter>string</parameter>)</function></literal>
       </entry>
       <entry><type>int</type></entry>
       <entry>Number of bits in string</entry>
       <entry><literal>bit_length('jose')</literal></entry>
       <entry><literal>32</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>char_length</primary>
        </indexterm>
        <literal><function>char_length(<parameter>string</parameter>)</function></literal> or <literal><function>character_length(<parameter>string</parameter>)</function></literal>
       </entry>
       <entry><type>int</type></entry>
       <entry>
        Number of characters in string
        <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>
       </entry>
       <entry><literal>char_length('jose')</literal></entry>
       <entry><literal>4</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>lower</primary>
        </indexterm>
        <literal><function>lower(<parameter>string</parameter>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>Convert string to lower case</entry>
       <entry><literal>lower('TOM')</literal></entry>
       <entry><literal>tom</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>octet_length</primary>
        </indexterm>
        <literal><function>octet_length(<parameter>string</parameter>)</function></literal>
       </entry>
       <entry><type>int</type></entry>
       <entry>Number of bytes in string</entry>
       <entry><literal>octet_length('jose')</literal></entry>
       <entry><literal>4</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>overlay</primary>
        </indexterm>
        <literal><function>overlay(<parameter>string</parameter> placing <parameter>string</parameter> from <type>int</type> <optional>for <type>int</type></optional>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Replace substring
       </entry>
       <entry><literal>overlay('Txxxxas' placing 'hom' from 2 for 4)</literal></entry>
       <entry><literal>Thomas</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>position</primary>
        </indexterm>
        <literal><function>position(<parameter>substring</parameter> in <parameter>string</parameter>)</function></literal>
       </entry>
       <entry><type>int</type></entry>
       <entry>Location of specified substring</entry>
       <entry><literal>position('om' in 'Thomas')</literal></entry>
       <entry><literal>3</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>substring</primary>
        </indexterm>
        <literal><function>substring(<parameter>string</parameter> <optional>from <type>int</type></optional> <optional>for <type>int</type></optional>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Extract substring
       </entry>
       <entry><literal>substring('Thomas' from 2 for 3)</literal></entry>
       <entry><literal>hom</literal></entry>
      </row>

      <row>
       <entry><literal><function>substring(<parameter>string</parameter> from <replaceable>pattern</replaceable>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>
        Extract substring matching POSIX regular expression. See
        <xref linkend="functions-matching"/> for more information on pattern
        matching.
       </entry>
       <entry><literal>substring('Thomas' from '...$')</literal></entry>
       <entry><literal>mas</literal></entry>
      </row>

      <row>
       <entry><literal><function>substring(<parameter>string</parameter> from <replaceable>pattern</replaceable> for <replaceable>escape</replaceable>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>
        Extract substring matching <acronym>SQL</acronym> regular expression.
        See <xref linkend="functions-matching"/> for more information on
        pattern matching.
       </entry>
       <entry><literal>substring('Thomas' from '%#"o_a#"_' for '#')</literal></entry>
       <entry><literal>oma</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>trim</primary>
        </indexterm>
        <literal><function>trim(<optional>leading | trailing | both</optional>
        <optional><parameter>characters</parameter></optional> from
        <parameter>string</parameter>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Remove the longest string containing only characters from
        <parameter>characters</parameter> (a space by default) from the
        start, end, or both ends (<literal>both</literal> is the default)
        of <parameter>string</parameter>
       </entry>
       <entry><literal>trim(both 'xyz' from 'yxTomxx')</literal></entry>
       <entry><literal>Tom</literal></entry>
      </row>

      <row>
       <entry>
        <literal><function>trim(<optional>leading | trailing
        | both</optional> <optional>from</optional>
        <parameter>string</parameter>
        <optional>, <parameter>characters</parameter></optional>
        )</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Non-standard syntax for <function>trim()</function>
       </entry>
       <entry><literal>trim(both from 'yxTomxx', 'xyz')</literal></entry>
       <entry><literal>Tom</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>upper</primary>
        </indexterm>
        <literal><function>upper(<parameter>string</parameter>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>Convert string to upper case</entry>
       <entry><literal>upper('tom')</literal></entry>
       <entry><literal>TOM</literal></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="5">
     <thead>
      <row>
       <entry>Function</entry>
       <entry>Return Type</entry>
       <entry>Description</entry>
       <entry>Example</entry>
       <entry>Result</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>
        <indexterm>
         <primary>ascii</primary>
        </indexterm>
        <literal><function>ascii(<parameter>string</parameter>)</function></literal>
       </entry>
       <entry><type>int</type></entry>
       <entry>
        <acronym>ASCII</acronym> code of the first character of the
        argument.  For <acronym>UTF8</acronym> returns the Unicode code
        point of the character.  For other multibyte encodings, the
        argument must be an <acronym>ASCII</acronym> character.
       </entry>
       <entry><literal>ascii('x')</literal></entry>
       <entry><literal>120</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>btrim</primary>
        </indexterm>
        <literal><function>btrim(<parameter>string</parameter> <type>text</type>
        <optional>, <parameter>characters</parameter> <type>text</type></optional>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Remove the longest string consisting only of characters
        in <parameter>characters</parameter> (a space by default)
        from the start and end of <parameter>string</parameter>
       </entry>
       <entry><literal>btrim('xyxtrimyyx', 'xyz')</literal></entry>
       <entry><literal>trim</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>chr</primary>
        </indexterm>
        <literal><function>chr(<type>int</type>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Character with the given code. For <acronym>UTF8</acronym> the
        argument is treated as a Unicode code point. For other multibyte
        encodings the argument must designate an
        <acronym>ASCII</acronym> character.  The NULL (0) character is not
        allowed because text data types cannot store such bytes.
       </entry>
       <entry><literal>chr(65)</literal></entry>
       <entry><literal>A</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>concat</primary>
        </indexterm>
        <literal><function>concat(<parameter>str</parameter> <type>"any"</type>
         [, <parameter>str</parameter> <type>"any"</type> [, ...] ])</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Concatenate the text representations of all the arguments.
        NULL arguments are ignored.
       </entry>
       <entry><literal>concat('abcde', 2, NULL, 22)</literal></entry>
       <entry><literal>abcde222</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>concat_ws</primary>
        </indexterm>
        <literal><function>concat_ws(<parameter>sep</parameter> <type>text</type>,
        <parameter>str</parameter> <type>"any"</type>
        [, <parameter>str</parameter> <type>"any"</type> [, ...] ])</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Concatenate all but the first argument with separators. The first
        argument is used as the separator string. NULL arguments are ignored.
       </entry>
       <entry><literal>concat_ws(',', 'abcde', 2, NULL, 22)</literal></entry>
       <entry><literal>abcde,2,22</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>convert</primary>
        </indexterm>
        <literal><function>convert(<parameter>string</parameter> <type>bytea</type>,
        <parameter>src_encoding</parameter> <type>name</type>,
        <parameter>dest_encoding</parameter> <type>name</type>)</function></literal>
       </entry>
       <entry><type>bytea</type></entry>
       <entry>
        Convert string to <parameter>dest_encoding</parameter>.  The
        original encoding is specified by
        <parameter>src_encoding</parameter>. The
        <parameter>string</parameter> must be valid in this encoding.
        Conversions can be defined by <command>CREATE CONVERSION</command>.
        Also there are some predefined conversions. See <xref
        linkend="conversion-names"/> for available conversions.
       </entry>
       <entry><literal>convert('text_in_utf8', 'UTF8', 'LATIN1')</literal></entry>
       <entry><literal>text_in_utf8</literal> represented in Latin-1
       encoding (ISO 8859-1)</entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>convert_from</primary>
        </indexterm>
        <literal><function>convert_from(<parameter>string</parameter> <type>bytea</type>,
        <parameter>src_encoding</parameter> <type>name</type>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Convert string to the database encoding.  The original encoding
        is specified by <parameter>src_encoding</parameter>. The
        <parameter>string</parameter> must be valid in this encoding.
       </entry>
       <entry><literal>convert_from('text_in_utf8', 'UTF8')</literal></entry>
       <entry><literal>text_in_utf8</literal> represented in the current database encoding</entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>convert_to</primary>
        </indexterm>
        <literal><function>convert_to(<parameter>string</parameter> <type>text</type>,
        <parameter>dest_encoding</parameter> <type>name</type>)</function></literal>
       </entry>
       <entry><type>bytea</type></entry>
       <entry>
        Convert string to <parameter>dest_encoding</parameter>.
       </entry>
       <entry><literal>convert_to('some text', 'UTF8')</literal></entry>
       <entry><literal>some text</literal> represented in the UTF8 encoding</entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>decode</primary>
        </indexterm>
        <literal><function>decode(<parameter>string</parameter> <type>text</type>,
        <parameter>format</parameter> <type>text</type>)</function></literal>
       </entry>
       <entry><type>bytea</type></entry>
       <entry>
        Decode binary data from textual representation in <parameter>string</parameter>.
        Options for <parameter>format</parameter> are same as in <function>encode</function>.
       </entry>
       <entry><literal>decode('MTIzAAE=', 'base64')</literal></entry>
       <entry><literal>\x3132330001</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>encode</primary>
        </indexterm>
        <literal><function>encode(<parameter>data</parameter> <type>bytea</type>,
        <parameter>format</parameter> <type>text</type>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Encode binary data into a textual representation.  Supported
        formats are: <literal>base64</literal>, <literal>hex</literal>, <literal>escape</literal>.
        <literal>escape</literal> converts zero bytes and high-bit-set bytes to
        octal sequences (<literal>\</literal><replaceable>nnn</replaceable>) and
        doubles backslashes.
       </entry>
       <entry><literal>encode('123\000\001', 'base64')</literal></entry>
       <entry><literal>MTIzAAE=</literal></entry>
      </row>

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

      <row>
       <entry>
        <indexterm>
         <primary>initcap</primary>
        </indexterm>
        <literal><function>initcap(<parameter>string</parameter>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Convert 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.
       </entry>
       <entry><literal>initcap('hi THOMAS')</literal></entry>
       <entry><literal>Hi Thomas</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>left</primary>
        </indexterm>
        <literal><function>left(<parameter>str</parameter> <type>text</type>,
        <parameter>n</parameter> <type>int</type>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Return first <replaceable>n</replaceable> characters in the string. When <replaceable>n</replaceable>
        is negative, return all but last |<replaceable>n</replaceable>| characters.
        </entry>
       <entry><literal>left('abcde', 2)</literal></entry>
       <entry><literal>ab</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>length</primary>
        </indexterm>
        <literal><function>length(<parameter>string</parameter>)</function></literal>
       </entry>
       <entry><type>int</type></entry>
       <entry>
        Number of characters in <parameter>string</parameter>
       </entry>
       <entry><literal>length('jose')</literal></entry>
       <entry><literal>4</literal></entry>
      </row>

      <row>
       <entry><literal><function>length(<parameter>string</parameter> <type>bytea</type>,
        <parameter>encoding</parameter> <type>name</type> )</function></literal></entry>
       <entry><type>int</type></entry>
       <entry>
        Number of characters in <parameter>string</parameter> in the given
        <parameter>encoding</parameter>. The <parameter>string</parameter>
        must be valid in this encoding.
       </entry>
       <entry><literal>length('jose', 'UTF8')</literal></entry>
       <entry><literal>4</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>lpad</primary>
        </indexterm>
        <literal><function>lpad(<parameter>string</parameter> <type>text</type>,
        <parameter>length</parameter> <type>int</type>
        <optional>, <parameter>fill</parameter> <type>text</type></optional>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Fill up 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).
       </entry>
       <entry><literal>lpad('hi', 5, 'xy')</literal></entry>
       <entry><literal>xyxhi</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>ltrim</primary>
        </indexterm>
        <literal><function>ltrim(<parameter>string</parameter> <type>text</type>
        <optional>, <parameter>characters</parameter> <type>text</type></optional>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Remove the longest string containing only characters from
        <parameter>characters</parameter> (a space by default) from the start of
        <parameter>string</parameter>
       </entry>
       <entry><literal>ltrim('zzzytest', 'xyz')</literal></entry>
       <entry><literal>test</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>md5</primary>
        </indexterm>
        <literal><function>md5(<parameter>string</parameter>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Calculates the MD5 hash of <parameter>string</parameter>,
        returning the result in hexadecimal
       </entry>
       <entry><literal>md5('abc')</literal></entry>
       <entry><literal>900150983cd24fb0 d6963f7d28e17f72</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>parse_ident</primary>
        </indexterm>
        <literal><function>parse_ident(<parameter>qualified_identifier</parameter> <type>text</type>
           [, <parameter>strictmode</parameter> <type>boolean</type> DEFAULT true ] )</function></literal>
       </entry>
       <entry><type>text[]</type></entry>
       <entry>
        Split <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>.
       </entry>
       <entry><literal>parse_ident('"SomeSchema".someTable')</literal></entry>
       <entry><literal>{SomeSchema,sometable}</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>pg_client_encoding</primary>
        </indexterm>
        <literal><function>pg_client_encoding()</function></literal>
       </entry>
       <entry><type>name</type></entry>
       <entry>
        Current client encoding name
       </entry>
       <entry><literal>pg_client_encoding()</literal></entry>
       <entry><literal>SQL_ASCII</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>quote_ident</primary>
        </indexterm>
        <literal><function>quote_ident(<parameter>string</parameter> <type>text</type>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Return 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"/>.
       </entry>
       <entry><literal>quote_ident('Foo bar')</literal></entry>
       <entry><literal>"Foo bar"</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>quote_literal</primary>
        </indexterm>
        <literal><function>quote_literal(<parameter>string</parameter> <type>text</type>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Return 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"/>.
       </entry>
       <entry><literal>quote_literal(E'O\'Reilly')</literal></entry>
       <entry><literal>'O''Reilly'</literal></entry>
      </row>

      <row>
       <entry><literal><function>quote_literal(<parameter>value</parameter> <type>anyelement</type>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>
        Coerce the given value to text and then quote it as a literal.
        Embedded single-quotes and backslashes are properly doubled.
       </entry>
       <entry><literal>quote_literal(42.5)</literal></entry>
       <entry><literal>'42.5'</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>quote_nullable</primary>
        </indexterm>
        <literal><function>quote_nullable(<parameter>string</parameter> <type>text</type>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Return 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, return <literal>NULL</literal>.
        Embedded single-quotes and backslashes are properly doubled.
        See also <xref linkend="plpgsql-quote-literal-example"/>.
       </entry>
       <entry><literal>quote_nullable(NULL)</literal></entry>
       <entry><literal>NULL</literal></entry>
      </row>

      <row>
       <entry><literal><function>quote_nullable(<parameter>value</parameter> <type>anyelement</type>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>
        Coerce the given value to text and then quote it as a literal;
        or, if the argument is null, return <literal>NULL</literal>.
        Embedded single-quotes and backslashes are properly doubled.
       </entry>
       <entry><literal>quote_nullable(42.5)</literal></entry>
       <entry><literal>'42.5'</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>regexp_match</primary>
        </indexterm>
        <literal><function>regexp_match(<parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type> [, <parameter>flags</parameter> <type>text</type>])</function></literal>
       </entry>
       <entry><type>text[]</type></entry>
       <entry>
        Return captured substring(s) resulting from the first match of a POSIX
        regular expression to the <parameter>string</parameter>. See
        <xref linkend="functions-posix-regexp"/> for more information.
       </entry>
       <entry><literal>regexp_match('foobarbequebaz', '(bar)(beque)')</literal></entry>
       <entry><literal>{bar,beque}</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>regexp_matches</primary>
        </indexterm>
        <literal><function>regexp_matches(<parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type> [, <parameter>flags</parameter> <type>text</type>])</function></literal>
       </entry>
       <entry><type>setof text[]</type></entry>
       <entry>
        Return captured substring(s) resulting from matching a POSIX regular
        expression to the <parameter>string</parameter>. See
        <xref linkend="functions-posix-regexp"/> for more information.
       </entry>
       <entry><literal>regexp_matches('foobarbequebaz', 'ba.', 'g')</literal></entry>
       <entry><literal>{bar}</literal><para><literal>{baz}</literal></para> (2 rows)</entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>regexp_replace</primary>
        </indexterm>
        <literal><function>regexp_replace(<parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type>, <parameter>replacement</parameter> <type>text</type> [, <parameter>flags</parameter> <type>text</type>])</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Replace substring(s) matching a POSIX regular expression. See
        <xref linkend="functions-posix-regexp"/> for more information.
       </entry>
       <entry><literal>regexp_replace('Thomas', '.[mN]a.', 'M')</literal></entry>
       <entry><literal>ThM</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>regexp_split_to_array</primary>
        </indexterm>
        <literal><function>regexp_split_to_array(<parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type> [, <parameter>flags</parameter> <type>text</type> ])</function></literal>
       </entry>
       <entry><type>text[]</type></entry>
       <entry>
        Split <parameter>string</parameter> using a POSIX regular expression as
        the delimiter.  See <xref linkend="functions-posix-regexp"/> for more
        information.
       </entry>
       <entry><literal>regexp_split_to_array('hello world', '\s+')</literal></entry>
       <entry><literal>{hello,world}</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>regexp_split_to_table</primary>
        </indexterm>
        <literal><function>regexp_split_to_table(<parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type> [, <parameter>flags</parameter> <type>text</type>])</function></literal>
       </entry>
       <entry><type>setof text</type></entry>
       <entry>
        Split <parameter>string</parameter> using a POSIX regular expression as
        the delimiter.  See <xref linkend="functions-posix-regexp"/> for more
        information.
       </entry>
       <entry><literal>regexp_split_to_table('hello world', '\s+')</literal></entry>
       <entry><literal>hello</literal><para><literal>world</literal></para> (2 rows)</entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>repeat</primary>
        </indexterm>
        <literal><function>repeat(<parameter>string</parameter> <type>text</type>, <parameter>number</parameter> <type>int</type>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>Repeat <parameter>string</parameter> the specified
       <parameter>number</parameter> of times</entry>
       <entry><literal>repeat('Pg', 4)</literal></entry>
       <entry><literal>PgPgPgPg</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>replace</primary>
        </indexterm>
        <literal><function>replace(<parameter>string</parameter> <type>text</type>,
        <parameter>from</parameter> <type>text</type>,
        <parameter>to</parameter> <type>text</type>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>Replace all occurrences in <parameter>string</parameter> of substring
        <parameter>from</parameter> with substring <parameter>to</parameter>
       </entry>
       <entry><literal>replace('abcdefabcdef', 'cd', 'XX')</literal></entry>
       <entry><literal>abXXefabXXef</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>reverse</primary>
        </indexterm>
        <literal><function>reverse(<parameter>str</parameter>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Return reversed string.
       </entry>
       <entry><literal>reverse('abcde')</literal></entry>
       <entry><literal>edcba</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>right</primary>
        </indexterm>
        <literal><function>right(<parameter>str</parameter> <type>text</type>,
         <parameter>n</parameter> <type>int</type>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Return last <replaceable>n</replaceable> characters in the string. When <replaceable>n</replaceable>
        is negative, return all but first |<replaceable>n</replaceable>| characters.
       </entry>
       <entry><literal>right('abcde', 2)</literal></entry>
       <entry><literal>de</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>rpad</primary>
        </indexterm>
        <literal><function>rpad(<parameter>string</parameter> <type>text</type>,
        <parameter>length</parameter> <type>int</type>
        <optional>, <parameter>fill</parameter> <type>text</type></optional>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Fill up 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.
       </entry>
       <entry><literal>rpad('hi', 5, 'xy')</literal></entry>
       <entry><literal>hixyx</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>rtrim</primary>
        </indexterm>
        <literal><function>rtrim(<parameter>string</parameter> <type>text</type>
         <optional>, <parameter>characters</parameter> <type>text</type></optional>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Remove the longest string containing only characters from
        <parameter>characters</parameter> (a space by default) from the end of
        <parameter>string</parameter>
       </entry>
       <entry><literal>rtrim('testxxzx', 'xyz')</literal></entry>
       <entry><literal>test</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>split_part</primary>
        </indexterm>
        <literal><function>split_part(<parameter>string</parameter> <type>text</type>,
        <parameter>delimiter</parameter> <type>text</type>,
        <parameter>field</parameter> <type>int</type>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>Split <parameter>string</parameter> on <parameter>delimiter</parameter>
        and return the given field (counting from one)
       </entry>
       <entry><literal>split_part('abc~@~def~@~ghi', '~@~', 2)</literal></entry>
       <entry><literal>def</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>strpos</primary>
        </indexterm>
        <literal><function>strpos(<parameter>string</parameter>, <parameter>substring</parameter>)</function></literal>
       </entry>
       <entry><type>int</type></entry>
       <entry>
        Location of specified substring (same as
        <literal>position(<parameter>substring</parameter> in
         <parameter>string</parameter>)</literal>, but note the reversed
        argument order)
       </entry>
       <entry><literal>strpos('high', 'ig')</literal></entry>
       <entry><literal>2</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>substr</primary>
        </indexterm>
        <literal><function>substr(<parameter>string</parameter>, <parameter>from</parameter> <optional>, <parameter>count</parameter></optional>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Extract substring (same as
        <literal>substring(<parameter>string</parameter> from <parameter>from</parameter> for <parameter>count</parameter>)</literal>)
       </entry>
       <entry><literal>substr('alphabet', 3, 2)</literal></entry>
       <entry><literal>ph</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>starts_with</primary>
        </indexterm>
        <literal><function>starts_with(<parameter>string</parameter>, <parameter>prefix</parameter>)</function></literal>
       </entry>
       <entry><type>bool</type></entry>
       <entry>
        Returns true if <parameter>string</parameter> starts with <parameter>prefix</parameter>.
       </entry>
       <entry><literal>starts_with('alphabet', 'alph')</literal></entry>
       <entry><literal>t</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>to_ascii</primary>
        </indexterm>
        <literal><function>to_ascii(<parameter>string</parameter> <type>text</type>
        <optional>, <parameter>encoding</parameter> <type>text</type></optional>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
       Convert <parameter>string</parameter> to <acronym>ASCII</acronym> from another encoding
       (only supports conversion from  <literal>LATIN1</literal>, <literal>LATIN2</literal>, <literal>LATIN9</literal>,
       and <literal>WIN1250</literal> encodings)
       </entry>
       <entry><literal>to_ascii('Karel')</literal></entry>
       <entry><literal>Karel</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>to_hex</primary>
        </indexterm>
        <literal><function>to_hex(<parameter>number</parameter> <type>int</type>
        or <type>bigint</type>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>Convert <parameter>number</parameter> to its equivalent hexadecimal
        representation
       </entry>
       <entry><literal>to_hex(2147483647)</literal></entry>
       <entry><literal>7fffffff</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>translate</primary>
        </indexterm>
        <literal><function>translate(<parameter>string</parameter> <type>text</type>,
        <parameter>from</parameter> <type>text</type>,
        <parameter>to</parameter> <type>text</type>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Any character in <parameter>string</parameter> that matches a
        character in the <parameter>from</parameter> set is replaced by
        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 removed.
       </entry>
       <entry><literal>translate('12345', '143', 'ax')</literal></entry>
       <entry><literal>a2x5</literal></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"/>.
   </para>

   <table id="conversion-names">
    <title>Built-in Conversions</title>
    <tgroup cols="3">
     <thead>
      <row>
       <entry>Conversion Name
        <footnote>
         <para>
          The conversion names follow a standard naming scheme: The
          official name of the source encoding with all
          non-alphanumeric characters replaced by underscores, followed
          by <literal>_to_</literal>, followed by the similarly processed
          destination encoding name. Therefore, the names might deviate
          from the customary encoding names.
         </para>
        </footnote>
       </entry>
       <entry>Source Encoding</entry>
       <entry>Destination Encoding</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry><literal>big5_to_euc_tw</literal></entry>
       <entry><literal>BIG5</literal></entry>
       <entry><literal>EUC_TW</literal></entry>
      </row>

      <row>
       <entry><literal>big5_to_mic</literal></entry>
       <entry><literal>BIG5</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
      </row>

      <row>
       <entry><literal>big5_to_utf8</literal></entry>
       <entry><literal>BIG5</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>euc_cn_to_mic</literal></entry>
       <entry><literal>EUC_CN</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
      </row>

      <row>
       <entry><literal>euc_cn_to_utf8</literal></entry>
       <entry><literal>EUC_CN</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>euc_jp_to_mic</literal></entry>
       <entry><literal>EUC_JP</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
      </row>

      <row>
       <entry><literal>euc_jp_to_sjis</literal></entry>
       <entry><literal>EUC_JP</literal></entry>
       <entry><literal>SJIS</literal></entry>
      </row>

      <row>
       <entry><literal>euc_jp_to_utf8</literal></entry>
       <entry><literal>EUC_JP</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>euc_kr_to_mic</literal></entry>
       <entry><literal>EUC_KR</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
      </row>

      <row>
       <entry><literal>euc_kr_to_utf8</literal></entry>
       <entry><literal>EUC_KR</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>euc_tw_to_big5</literal></entry>
       <entry><literal>EUC_TW</literal></entry>
       <entry><literal>BIG5</literal></entry>
      </row>

      <row>
       <entry><literal>euc_tw_to_mic</literal></entry>
       <entry><literal>EUC_TW</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
      </row>

      <row>
       <entry><literal>euc_tw_to_utf8</literal></entry>
       <entry><literal>EUC_TW</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>gb18030_to_utf8</literal></entry>
       <entry><literal>GB18030</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>gbk_to_utf8</literal></entry>
       <entry><literal>GBK</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_10_to_utf8</literal></entry>
       <entry><literal>LATIN6</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_13_to_utf8</literal></entry>
       <entry><literal>LATIN7</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_14_to_utf8</literal></entry>
       <entry><literal>LATIN8</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_15_to_utf8</literal></entry>
       <entry><literal>LATIN9</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_16_to_utf8</literal></entry>
       <entry><literal>LATIN10</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_1_to_mic</literal></entry>
       <entry><literal>LATIN1</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_1_to_utf8</literal></entry>
       <entry><literal>LATIN1</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_2_to_mic</literal></entry>
       <entry><literal>LATIN2</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_2_to_utf8</literal></entry>
       <entry><literal>LATIN2</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_2_to_windows_1250</literal></entry>
       <entry><literal>LATIN2</literal></entry>
       <entry><literal>WIN1250</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_3_to_mic</literal></entry>
       <entry><literal>LATIN3</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_3_to_utf8</literal></entry>
       <entry><literal>LATIN3</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_4_to_mic</literal></entry>
       <entry><literal>LATIN4</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_4_to_utf8</literal></entry>
       <entry><literal>LATIN4</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_5_to_koi8_r</literal></entry>
       <entry><literal>ISO_8859_5</literal></entry>
       <entry><literal>KOI8R</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_5_to_mic</literal></entry>
       <entry><literal>ISO_8859_5</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_5_to_utf8</literal></entry>
       <entry><literal>ISO_8859_5</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_5_to_windows_1251</literal></entry>
       <entry><literal>ISO_8859_5</literal></entry>
       <entry><literal>WIN1251</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_5_to_windows_866</literal></entry>
       <entry><literal>ISO_8859_5</literal></entry>
       <entry><literal>WIN866</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_6_to_utf8</literal></entry>
       <entry><literal>ISO_8859_6</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_7_to_utf8</literal></entry>
       <entry><literal>ISO_8859_7</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_8_to_utf8</literal></entry>
       <entry><literal>ISO_8859_8</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>iso_8859_9_to_utf8</literal></entry>
       <entry><literal>LATIN5</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>johab_to_utf8</literal></entry>
       <entry><literal>JOHAB</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>koi8_r_to_iso_8859_5</literal></entry>
       <entry><literal>KOI8R</literal></entry>
       <entry><literal>ISO_8859_5</literal></entry>
      </row>

      <row>
       <entry><literal>koi8_r_to_mic</literal></entry>
       <entry><literal>KOI8R</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
      </row>

      <row>
       <entry><literal>koi8_r_to_utf8</literal></entry>
       <entry><literal>KOI8R</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>koi8_r_to_windows_1251</literal></entry>
       <entry><literal>KOI8R</literal></entry>
       <entry><literal>WIN1251</literal></entry>
      </row>

      <row>
       <entry><literal>koi8_r_to_windows_866</literal></entry>
       <entry><literal>KOI8R</literal></entry>
       <entry><literal>WIN866</literal></entry>
      </row>

      <row>
       <entry><literal>koi8_u_to_utf8</literal></entry>
       <entry><literal>KOI8U</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>mic_to_big5</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
       <entry><literal>BIG5</literal></entry>
      </row>

      <row>
       <entry><literal>mic_to_euc_cn</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
       <entry><literal>EUC_CN</literal></entry>
      </row>

      <row>
       <entry><literal>mic_to_euc_jp</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
       <entry><literal>EUC_JP</literal></entry>
      </row>

      <row>
       <entry><literal>mic_to_euc_kr</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
       <entry><literal>EUC_KR</literal></entry>
      </row>

      <row>
       <entry><literal>mic_to_euc_tw</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
       <entry><literal>EUC_TW</literal></entry>
      </row>

      <row>
       <entry><literal>mic_to_iso_8859_1</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
       <entry><literal>LATIN1</literal></entry>
      </row>

      <row>
       <entry><literal>mic_to_iso_8859_2</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
       <entry><literal>LATIN2</literal></entry>
      </row>

      <row>
       <entry><literal>mic_to_iso_8859_3</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
       <entry><literal>LATIN3</literal></entry>
      </row>

      <row>
       <entry><literal>mic_to_iso_8859_4</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
       <entry><literal>LATIN4</literal></entry>
      </row>

      <row>
       <entry><literal>mic_to_iso_8859_5</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
       <entry><literal>ISO_8859_5</literal></entry>
      </row>

      <row>
       <entry><literal>mic_to_koi8_r</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
       <entry><literal>KOI8R</literal></entry>
      </row>

      <row>
       <entry><literal>mic_to_sjis</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
       <entry><literal>SJIS</literal></entry>
      </row>

      <row>
       <entry><literal>mic_to_windows_1250</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
       <entry><literal>WIN1250</literal></entry>
      </row>

      <row>
       <entry><literal>mic_to_windows_1251</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
       <entry><literal>WIN1251</literal></entry>
      </row>

      <row>
       <entry><literal>mic_to_windows_866</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
       <entry><literal>WIN866</literal></entry>
      </row>

      <row>
       <entry><literal>sjis_to_euc_jp</literal></entry>
       <entry><literal>SJIS</literal></entry>
       <entry><literal>EUC_JP</literal></entry>
      </row>

      <row>
       <entry><literal>sjis_to_mic</literal></entry>
       <entry><literal>SJIS</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
      </row>

      <row>
       <entry><literal>sjis_to_utf8</literal></entry>
       <entry><literal>SJIS</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>windows_1258_to_utf8</literal></entry>
       <entry><literal>WIN1258</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>uhc_to_utf8</literal></entry>
       <entry><literal>UHC</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_big5</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>BIG5</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_euc_cn</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>EUC_CN</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_euc_jp</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>EUC_JP</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_euc_kr</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>EUC_KR</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_euc_tw</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>EUC_TW</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_gb18030</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>GB18030</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_gbk</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>GBK</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_iso_8859_1</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>LATIN1</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_iso_8859_10</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>LATIN6</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_iso_8859_13</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>LATIN7</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_iso_8859_14</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>LATIN8</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_iso_8859_15</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>LATIN9</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_iso_8859_16</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>LATIN10</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_iso_8859_2</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>LATIN2</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_iso_8859_3</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>LATIN3</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_iso_8859_4</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>LATIN4</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_iso_8859_5</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>ISO_8859_5</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_iso_8859_6</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>ISO_8859_6</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_iso_8859_7</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>ISO_8859_7</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_iso_8859_8</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>ISO_8859_8</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_iso_8859_9</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>LATIN5</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_johab</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>JOHAB</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_koi8_r</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>KOI8R</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_koi8_u</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>KOI8U</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_sjis</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>SJIS</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_windows_1258</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>WIN1258</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_uhc</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>UHC</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_windows_1250</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>WIN1250</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_windows_1251</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>WIN1251</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_windows_1252</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>WIN1252</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_windows_1253</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>WIN1253</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_windows_1254</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>WIN1254</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_windows_1255</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>WIN1255</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_windows_1256</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>WIN1256</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_windows_1257</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>WIN1257</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_windows_866</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>WIN866</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_windows_874</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>WIN874</literal></entry>
      </row>

      <row>
       <entry><literal>windows_1250_to_iso_8859_2</literal></entry>
       <entry><literal>WIN1250</literal></entry>
       <entry><literal>LATIN2</literal></entry>
      </row>

      <row>
       <entry><literal>windows_1250_to_mic</literal></entry>
       <entry><literal>WIN1250</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
      </row>

      <row>
       <entry><literal>windows_1250_to_utf8</literal></entry>
       <entry><literal>WIN1250</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>windows_1251_to_iso_8859_5</literal></entry>
       <entry><literal>WIN1251</literal></entry>
       <entry><literal>ISO_8859_5</literal></entry>
      </row>

      <row>
       <entry><literal>windows_1251_to_koi8_r</literal></entry>
       <entry><literal>WIN1251</literal></entry>
       <entry><literal>KOI8R</literal></entry>
      </row>

      <row>
       <entry><literal>windows_1251_to_mic</literal></entry>
       <entry><literal>WIN1251</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
      </row>

      <row>
       <entry><literal>windows_1251_to_utf8</literal></entry>
       <entry><literal>WIN1251</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>windows_1251_to_windows_866</literal></entry>
       <entry><literal>WIN1251</literal></entry>
       <entry><literal>WIN866</literal></entry>
      </row>

      <row>
       <entry><literal>windows_1252_to_utf8</literal></entry>
       <entry><literal>WIN1252</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>windows_1256_to_utf8</literal></entry>
       <entry><literal>WIN1256</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>windows_866_to_iso_8859_5</literal></entry>
       <entry><literal>WIN866</literal></entry>
       <entry><literal>ISO_8859_5</literal></entry>
      </row>

      <row>
       <entry><literal>windows_866_to_koi8_r</literal></entry>
       <entry><literal>WIN866</literal></entry>
       <entry><literal>KOI8R</literal></entry>
      </row>

      <row>
       <entry><literal>windows_866_to_mic</literal></entry>
       <entry><literal>WIN866</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
      </row>

      <row>
       <entry><literal>windows_866_to_utf8</literal></entry>
       <entry><literal>WIN866</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>windows_866_to_windows_1251</literal></entry>
       <entry><literal>WIN866</literal></entry>
       <entry><literal>WIN</literal></entry>
      </row>

      <row>
       <entry><literal>windows_874_to_utf8</literal></entry>
       <entry><literal>WIN874</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>euc_jis_2004_to_utf8</literal></entry>
       <entry><literal>EUC_JIS_2004</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_euc_jis_2004</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>EUC_JIS_2004</literal></entry>
      </row>

      <row>
       <entry><literal>shift_jis_2004_to_utf8</literal></entry>
       <entry><literal>SHIFT_JIS_2004</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>utf8_to_shift_jis_2004</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>SHIFT_JIS_2004</literal></entry>
      </row>

      <row>
       <entry><literal>euc_jis_2004_to_shift_jis_2004</literal></entry>
       <entry><literal>EUC_JIS_2004</literal></entry>
       <entry><literal>SHIFT_JIS_2004</literal></entry>
      </row>

      <row>
       <entry><literal>shift_jis_2004_to_euc_jis_2004</literal></entry>
       <entry><literal>SHIFT_JIS_2004</literal></entry>
       <entry><literal>EUC_JIS_2004</literal></entry>
      </row>

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

   <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>
     <replaceable>formatstr</replaceable> 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 <replaceable>formatarg</replaceable> 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>
%[<replaceable>position</replaceable>][<replaceable>flags</replaceable>][<replaceable>width</replaceable>]<replaceable>type</replaceable>
</synopsis>
     where the component fields are:

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

      <varlistentry>
       <term><replaceable>flags</replaceable> (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 <replaceable>width</replaceable>
         field is also specified.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><replaceable>width</replaceable> (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>*<replaceable>n</replaceable>$</literal> to
         use the <replaceable>n</replaceable>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>(<replaceable>width</replaceable>).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><replaceable>type</replaceable> (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 <replaceable>width</replaceable> 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 <replaceable>position</replaceable> 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 <replaceable>position</replaceable> fields to be mixed
     in the same format string.  A format specifier without a
     <replaceable>position</replaceable> 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 values of type <type>bytea</type>.
   </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>

   <note>
    <para>
     The sample results shown on this page assume that the server parameter
     <link linkend="guc-bytea-output"><varname>bytea_output</varname></link> is set
     to <literal>escape</literal> (the traditional PostgreSQL format).
    </para>
   </note>

   <table id="functions-binarystring-sql">
    <title><acronym>SQL</acronym> Binary String Functions and Operators</title>
    <tgroup cols="5">
     <thead>
      <row>
       <entry>Function</entry>
       <entry>Return Type</entry>
       <entry>Description</entry>
       <entry>Example</entry>
       <entry>Result</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry><literal><parameter>string</parameter> <literal>||</literal>
        <parameter>string</parameter></literal></entry>
       <entry> <type>bytea</type> </entry>
       <entry>
        String concatenation
        <indexterm>
         <primary>binary string</primary>
         <secondary>concatenation</secondary>
        </indexterm>
       </entry>
       <entry><literal>'\\Post'::bytea || '\047gres\000'::bytea</literal></entry>
       <entry><literal>\\Post'gres\000</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>octet_length</primary>
        </indexterm>
        <literal><function>octet_length(<parameter>string</parameter>)</function></literal>
       </entry>
       <entry><type>int</type></entry>
       <entry>Number of bytes in binary string</entry>
       <entry><literal>octet_length('jo\000se'::bytea)</literal></entry>
       <entry><literal>5</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>overlay</primary>
        </indexterm>
        <literal><function>overlay(<parameter>string</parameter> placing <parameter>string</parameter> from <type>int</type> <optional>for <type>int</type></optional>)</function></literal>
       </entry>
       <entry><type>bytea</type></entry>
       <entry>
        Replace substring
       </entry>
       <entry><literal>overlay('Th\000omas'::bytea placing '\002\003'::bytea from 2 for 3)</literal></entry>
       <entry><literal>T\\002\\003mas</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>position</primary>
        </indexterm>
        <literal><function>position(<parameter>substring</parameter> in <parameter>string</parameter>)</function></literal>
       </entry>
       <entry><type>int</type></entry>
       <entry>Location of specified substring</entry>
      <entry><literal>position('\000om'::bytea in 'Th\000omas'::bytea)</literal></entry>
       <entry><literal>3</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>substring</primary>
        </indexterm>
        <literal><function>substring(<parameter>string</parameter> <optional>from <type>int</type></optional> <optional>for <type>int</type></optional>)</function></literal>
       </entry>
       <entry><type>bytea</type></entry>
       <entry>
        Extract substring
       </entry>
       <entry><literal>substring('Th\000omas'::bytea from 2 for 3)</literal></entry>
       <entry><literal>h\000o</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>trim</primary>
        </indexterm>
        <literal><function>trim(<optional>both</optional>
        <parameter>bytes</parameter> from
        <parameter>string</parameter>)</function></literal>
       </entry>
       <entry><type>bytea</type></entry>
       <entry>
        Remove the longest string containing only bytes appearing in
        <parameter>bytes</parameter> from the start
        and end of <parameter>string</parameter>
       </entry>
       <entry><literal>trim('\000\001'::bytea from '\000Tom\001'::bytea)</literal></entry>
       <entry><literal>Tom</literal></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="5">
     <thead>
      <row>
       <entry>Function</entry>
       <entry>Return Type</entry>
       <entry>Description</entry>
       <entry>Example</entry>
       <entry>Result</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>
        <indexterm>
         <primary>btrim</primary>
        </indexterm>
        <literal><function>btrim(<parameter>string</parameter>
        <type>bytea</type>, <parameter>bytes</parameter> <type>bytea</type>)</function></literal>
       </entry>
       <entry><type>bytea</type></entry>
       <entry>
        Remove the longest string containing only bytes appearing in
        <parameter>bytes</parameter> from the start and end of
        <parameter>string</parameter>
      </entry>
      <entry><literal>btrim('\000trim\001'::bytea, '\000\001'::bytea)</literal></entry>
      <entry><literal>trim</literal></entry>
     </row>

     <row>
      <entry>
        <indexterm>
         <primary>decode</primary>
        </indexterm>
       <literal><function>decode(<parameter>string</parameter> <type>text</type>,
       <parameter>format</parameter> <type>text</type>)</function></literal>
      </entry>
      <entry><type>bytea</type></entry>
      <entry>
       Decode binary data from textual representation in <parameter>string</parameter>.
       Options for <parameter>format</parameter> are same as in <function>encode</function>.
      </entry>
      <entry><literal>decode('123\000456', 'escape')</literal></entry>
      <entry><literal>123\000456</literal></entry>
     </row>

     <row>
      <entry>
        <indexterm>
         <primary>encode</primary>
        </indexterm>
       <literal><function>encode(<parameter>data</parameter> <type>bytea</type>,
       <parameter>format</parameter> <type>text</type>)</function></literal>
      </entry>
      <entry><type>text</type></entry>
      <entry>
       Encode binary data into a textual representation.  Supported
       formats are: <literal>base64</literal>, <literal>hex</literal>, <literal>escape</literal>.
       <literal>escape</literal> converts zero bytes and high-bit-set bytes to
       octal sequences (<literal>\</literal><replaceable>nnn</replaceable>) and
       doubles backslashes.
      </entry>
      <entry><literal>encode('123\000456'::bytea, 'escape')</literal></entry>
      <entry><literal>123\000456</literal></entry>
     </row>

      <row>
       <entry>
        <indexterm>
         <primary>get_bit</primary>
        </indexterm>
        <literal><function>get_bit(<parameter>string</parameter>, <parameter>offset</parameter>)</function></literal>
       </entry>
       <entry><type>int</type></entry>
       <entry>
        Extract bit from string
       </entry>
       <entry><literal>get_bit('Th\000omas'::bytea, 45)</literal></entry>
       <entry><literal>1</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>get_byte</primary>
        </indexterm>
        <literal><function>get_byte(<parameter>string</parameter>, <parameter>offset</parameter>)</function></literal>
       </entry>
       <entry><type>int</type></entry>
       <entry>
        Extract byte from string
       </entry>
       <entry><literal>get_byte('Th\000omas'::bytea, 4)</literal></entry>
       <entry><literal>109</literal></entry>
      </row>

     <row>
      <entry>
       <indexterm>
        <primary>length</primary>
       </indexterm>
       <literal><function>length(<parameter>string</parameter>)</function></literal>
      </entry>
      <entry><type>int</type></entry>
      <entry>
       Length of binary string
       <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>
      </entry>
      <entry><literal>length('jo\000se'::bytea)</literal></entry>
      <entry><literal>5</literal></entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>md5</primary>
       </indexterm>
       <literal><function>md5(<parameter>string</parameter>)</function></literal>
      </entry>
      <entry><type>text</type></entry>
      <entry>
       Calculates the MD5 hash of <parameter>string</parameter>,
       returning the result in hexadecimal
      </entry>
      <entry><literal>md5('Th\000omas'::bytea)</literal></entry>
      <entry><literal>8ab2d3c9689aaf18&#x200B;b4958c334c82d8b1</literal></entry>
     </row>

      <row>
       <entry>
        <indexterm>
         <primary>set_bit</primary>
        </indexterm>
        <literal><function>set_bit(<parameter>string</parameter>,
        <parameter>offset</parameter>, <parameter>newvalue</parameter>)</function></literal>
       </entry>
       <entry><type>bytea</type></entry>
       <entry>
        Set bit in string
       </entry>
       <entry><literal>set_bit('Th\000omas'::bytea, 45, 0)</literal></entry>
       <entry><literal>Th\000omAs</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>set_byte</primary>
        </indexterm>
        <literal><function>set_byte(<parameter>string</parameter>,
        <parameter>offset</parameter>, <parameter>newvalue</parameter>)</function></literal>
       </entry>
       <entry><type>bytea</type></entry>
       <entry>
        Set byte in string
       </entry>
       <entry><literal>set_byte('Th\000omas'::bytea, 4, 64)</literal></entry>
       <entry><literal>Th\000o@as</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>sha224</primary>
        </indexterm>
        <literal><function>sha224(<type>bytea</type>)</function></literal>
       </entry>
       <entry><type>bytea</type></entry>
       <entry>
        SHA-224 hash
       </entry>
       <entry><literal>sha224('abc')</literal></entry>
       <entry><literal>\x23097d223405d8228642a477bda2&#x200B;55b32aadbce4bda0b3f7e36c9da7</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>sha256</primary>
        </indexterm>
        <literal><function>sha256(<type>bytea</type>)</function></literal>
       </entry>
       <entry><type>bytea</type></entry>
       <entry>
        SHA-256 hash
       </entry>
       <entry><literal>sha256('abc')</literal></entry>
       <entry><literal>\xba7816bf8f01cfea414140de5dae2223&#x200B;b00361a396177a9cb410ff61f20015ad</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>sha384</primary>
        </indexterm>
        <literal><function>sha384(<type>bytea</type>)</function></literal>
       </entry>
       <entry><type>bytea</type></entry>
       <entry>
        SHA-384 hash
       </entry>
       <entry><literal>sha384('abc')</literal></entry>
       <entry><literal>\xcb00753f45a35e8bb5a03d699ac65007&#x200B;272c32ab0eded1631a8b605a43ff5bed&#x200B;8086072ba1e7cc2358baeca134c825a7</literal></entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>sha512</primary>
        </indexterm>
        <literal><function>sha512(<type>bytea</type>)</function></literal>
       </entry>
       <entry><type>bytea</type></entry>
       <entry>
        SHA-512 hash
       </entry>
       <entry><literal>sha512('abc')</literal></entry>
       <entry><literal>\xddaf35a193617abacc417349ae204131&#x200B;12e6fa4e89a97ea20a9eeee64b55d39a&#x200B;2192992a274fc1a836ba3c23a3feebbd&#x200B;454d4423643ce80e2a9ac94fa54ca49f</literal></entry>
      </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   <function>get_byte</function> and <function>set_byte</function> number the first byte
   of a binary string as byte 0.
   <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>
   Note that for historic 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
   <function>encode</function> and <function>decode</function> to convert
   between the two, for example <literal>encode(sha256('abc'),
   'hex')</literal> to get a hex-encoded text representation.
  </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>.  Aside from the
    usual comparison operators, the operators
    shown in <xref linkend="functions-bit-string-op-table"/> can be used.
    Bit string operands of <literal>&amp;</literal>, <literal>|</literal>,
    and <literal>#</literal> must be of equal length.  When bit
    shifting, the original length of the string is preserved, as shown
    in the examples.
   </para>

   <table id="functions-bit-string-op-table">
    <title>Bit String Operators</title>

    <tgroup cols="4">
     <thead>
      <row>
       <entry>Operator</entry>
       <entry>Description</entry>
       <entry>Example</entry>
       <entry>Result</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry> <literal>||</literal> </entry>
       <entry>concatenation</entry>
       <entry><literal>B'10001' || B'011'</literal></entry>
       <entry><literal>10001011</literal></entry>
      </row>

      <row>
       <entry> <literal>&amp;</literal> </entry>
       <entry>bitwise AND</entry>
       <entry><literal>B'10001' &amp; B'01101'</literal></entry>
       <entry><literal>00001</literal></entry>
      </row>

      <row>
       <entry> <literal>|</literal> </entry>
       <entry>bitwise OR</entry>
       <entry><literal>B'10001' | B'01101'</literal></entry>
       <entry><literal>11101</literal></entry>
      </row>

      <row>
       <entry> <literal>#</literal> </entry>
       <entry>bitwise XOR</entry>
       <entry><literal>B'10001' # B'01101'</literal></entry>
       <entry><literal>11100</literal></entry>
      </row>

      <row>
       <entry> <literal>~</literal> </entry>
       <entry>bitwise NOT</entry>
       <entry><literal>~ B'10001'</literal></entry>
       <entry><literal>01110</literal></entry>
      </row>

      <row>
       <entry> <literal>&lt;&lt;</literal> </entry>
       <entry>bitwise shift left</entry>
       <entry><literal>B'10001' &lt;&lt; 3</literal></entry>
       <entry><literal>01000</literal></entry>
      </row>

      <row>
       <entry> <literal>&gt;&gt;</literal> </entry>
       <entry>bitwise shift right</entry>
       <entry><literal>B'10001' &gt;&gt; 2</literal></entry>
       <entry><literal>00100</literal></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    The following <acronym>SQL</acronym>-standard functions work on bit
    strings as well as character strings:
    <literal><function>length</function></literal>,
    <literal><function>bit_length</function></literal>,
    <literal><function>octet_length</function></literal>,
    <literal><function>position</function></literal>,
    <literal><function>substring</function></literal>,
    <literal><function>overlay</function></literal>.
   </para>

   <para>
    The following functions work on bit strings as well as binary
    strings:
    <literal><function>get_bit</function></literal>,
    <literal><function>set_bit</function></literal>.
    When working with a bit string, these functions number the first
    (leftmost) bit of the string as bit 0.
   </para>

   <para>
    In addition, it is possible to cast integral values to and from type
    <type>bit</type>.
    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>

    <note>
     <para>
      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.
     </para>
    </note>

  </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.
   </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 SQL99 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' from '%#"o_b#"%' for '#')   <lineannotation>oob</lineannotation>
substring('foobar' from '#"o_b#"%' for '#')    <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="3">
     <thead>
      <row>
       <entry>Operator</entry>
       <entry>Description</entry>
       <entry>Example</entry>
      </row>
     </thead>

      <tbody>
       <row>
        <entry> <literal>~</literal> </entry>
        <entry>Matches regular expression, case sensitive</entry>
        <entry><literal>'thomas' ~ '.*thomas.*'</literal></entry>
       </row>

       <row>
        <entry> <literal>~*</literal> </entry>
        <entry>Matches regular expression, case insensitive</entry>
        <entry><literal>'thomas' ~* '.*Thomas.*'</literal></entry>
       </row>

       <row>
        <entry> <literal>!~</literal> </entry>
        <entry>Does not match regular expression, case sensitive</entry>
        <entry><literal>'thomas' !~ '.*Thomas.*'</literal></entry>
       </row>

       <row>
        <entry> <literal>!~*</literal> </entry>
        <entry>Does not match regular expression, case insensitive</entry>
        <entry><literal>'thomas' !~* '.*vadim.*'</literal></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="4">
      <thead>
       <row>
        <entry>Function</entry>
        <entry>Return Type</entry>
        <entry>Description</entry>
        <entry>Example</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry>
         <indexterm>
          <primary>to_char</primary>
         </indexterm>
         <literal><function>to_char(<type>timestamp</type>, <type>text</type>)</function></literal>
        </entry>
        <entry><type>text</type></entry>
        <entry>convert time stamp to string</entry>
        <entry><literal>to_char(current_timestamp, 'HH12:MI:SS')</literal></entry>
       </row>
       <row>
        <entry><literal><function>to_char(<type>interval</type>, <type>text</type>)</function></literal></entry>
        <entry><type>text</type></entry>
        <entry>convert interval to string</entry>
        <entry><literal>to_char(interval '15h&nbsp;2m&nbsp;12s', 'HH24:MI:SS')</literal></entry>
       </row>
       <row>
        <entry><literal><function>to_char(<type>int</type>, <type>text</type>)</function></literal></entry>
        <entry><type>text</type></entry>
        <entry>convert integer to string</entry>
        <entry><literal>to_char(125, '999')</literal></entry>
       </row>
       <row>
        <entry><literal><function>to_char</function>(<type>double precision</type>,
        <type>text</type>)</literal></entry>
        <entry><type>text</type></entry>
        <entry>convert real/double precision to string</entry>
        <entry><literal>to_char(125.8::real, '999D9')</literal></entry>
       </row>
       <row>
        <entry><literal><function>to_char(<type>numeric</type>, <type>text</type>)</function></literal></entry>
        <entry><type>text</type></entry>
        <entry>convert numeric to string</entry>
        <entry><literal>to_char(-125.8, '999D99S')</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>to_date</primary>
         </indexterm>
         <literal><function>to_date(<type>text</type>, <type>text</type>)</function></literal>
        </entry>
        <entry><type>date</type></entry>
        <entry>convert string to date</entry>
        <entry><literal>to_date('05&nbsp;Dec&nbsp;2000', 'DD&nbsp;Mon&nbsp;YYYY')</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>to_number</primary>
         </indexterm>
         <literal><function>to_number(<type>text</type>, <type>text</type>)</function></literal>
        </entry>
        <entry><type>numeric</type></entry>
        <entry>convert string to numeric</entry>
        <entry><literal>to_number('12,454.8-', '99G999D9S')</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>to_timestamp</primary>
         </indexterm>
         <literal><function>to_timestamp(<type>text</type>, <type>text</type>)</function></literal>
        </entry>
        <entry><type>timestamp with time zone</type></entry>
        <entry>convert string to time stamp</entry>
        <entry><literal>to_timestamp('05&nbsp;Dec&nbsp;2000', 'DD&nbsp;Mon&nbsp;YYYY')</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <note>
    <para>
     There is also a single-argument <function>to_timestamp</function>
     function; see <xref linkend="functions-datetime-table"/>.
    </para>
   </note>

   <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 (print 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> does not include trailing blanks.
       <function>to_timestamp</function> and <function>to_date</function> ignore
       the <literal>TM</literal> modifier.
      </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>,
       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="3">
      <thead>
       <row>
        <entry>Operator</entry>
        <entry>Example</entry>
        <entry>Result</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry> <literal>+</literal> </entry>
        <entry><literal>date '2001-09-28' + integer '7'</literal></entry>
        <entry><literal>date '2001-10-05'</literal></entry>
       </row>

       <row>
        <entry> <literal>+</literal> </entry>
        <entry><literal>date '2001-09-28' + interval '1 hour'</literal></entry>
        <entry><literal>timestamp '2001-09-28 01:00:00'</literal></entry>
       </row>

       <row>
        <entry> <literal>+</literal> </entry>
        <entry><literal>date '2001-09-28' + time '03:00'</literal></entry>
        <entry><literal>timestamp '2001-09-28 03:00:00'</literal></entry>
       </row>

       <row>
        <entry> <literal>+</literal> </entry>
        <entry><literal>interval '1 day' + interval '1 hour'</literal></entry>
        <entry><literal>interval '1 day 01:00:00'</literal></entry>
       </row>

       <row>
        <entry> <literal>+</literal> </entry>
        <entry><literal>timestamp '2001-09-28 01:00' + interval '23 hours'</literal></entry>
        <entry><literal>timestamp '2001-09-29 00:00:00'</literal></entry>
       </row>

       <row>
        <entry> <literal>+</literal> </entry>
        <entry><literal>time '01:00' + interval '3 hours'</literal></entry>
        <entry><literal>time '04:00:00'</literal></entry>
       </row>

       <row>
        <entry> <literal>-</literal> </entry>
        <entry><literal>- interval '23 hours'</literal></entry>
        <entry><literal>interval '-23:00:00'</literal></entry>
       </row>

       <row>
        <entry> <literal>-</literal> </entry>
        <entry><literal>date '2001-10-01' - date '2001-09-28'</literal></entry>
        <entry><literal>integer '3'</literal> (days)</entry>
       </row>

       <row>
        <entry> <literal>-</literal> </entry>
        <entry><literal>date '2001-10-01' - integer '7'</literal></entry>
        <entry><literal>date '2001-09-24'</literal></entry>
       </row>

       <row>
        <entry> <literal>-</literal> </entry>
        <entry><literal>date '2001-09-28' - interval '1 hour'</literal></entry>
        <entry><literal>timestamp '2001-09-27 23:00:00'</literal></entry>
       </row>

       <row>
        <entry> <literal>-</literal> </entry>
        <entry><literal>time '05:00' - time '03:00'</literal></entry>
        <entry><literal>interval '02:00:00'</literal></entry>
       </row>

       <row>
        <entry> <literal>-</literal> </entry>
        <entry><literal>time '05:00' - interval '2 hours'</literal></entry>
        <entry><literal>time '03:00:00'</literal></entry>
       </row>

       <row>
        <entry> <literal>-</literal> </entry>
        <entry><literal>timestamp '2001-09-28 23:00' - interval '23 hours'</literal></entry>
        <entry><literal>timestamp '2001-09-28 00:00:00'</literal></entry>
       </row>

       <row>
        <entry> <literal>-</literal> </entry>
        <entry><literal>interval '1 day' - interval '1 hour'</literal></entry>
        <entry><literal>interval '1 day -01:00:00'</literal></entry>
       </row>

       <row>
        <entry> <literal>-</literal> </entry>
        <entry><literal>timestamp '2001-09-29 03:00' - timestamp '2001-09-27 12:00'</literal></entry>
        <entry><literal>interval '1 day 15:00:00'</literal></entry>
       </row>

       <row>
        <entry> <literal>*</literal> </entry>
        <entry><literal>900 * interval '1 second'</literal></entry>
        <entry><literal>interval '00:15:00'</literal></entry>
       </row>

       <row>
        <entry> <literal>*</literal> </entry>
        <entry><literal>21 * interval '1 day'</literal></entry>
        <entry><literal>interval '21 days'</literal></entry>
       </row>

       <row>
        <entry> <literal>*</literal> </entry>
        <entry><literal>double precision '3.5' * interval '1 hour'</literal></entry>
        <entry><literal>interval '03:30:00'</literal></entry>
       </row>

       <row>
        <entry> <literal>/</literal> </entry>
        <entry><literal>interval '1 hour' / double precision '1.5'</literal></entry>
        <entry><literal>interval '00:40:00'</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <table id="functions-datetime-table">
     <title>Date/Time Functions</title>
     <tgroup cols="5">
      <thead>
       <row>
        <entry>Function</entry>
        <entry>Return Type</entry>
        <entry>Description</entry>
        <entry>Example</entry>
        <entry>Result</entry>
       </row>
      </thead>

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

       <row>
        <entry><literal><function>age(<type>timestamp</type>)</function></literal></entry>
        <entry><type>interval</type></entry>
        <entry>Subtract from <function>current_date</function> (at midnight)</entry>
        <entry><literal>age(timestamp '1957-06-13')</literal></entry>
        <entry><literal>43 years 8 mons 3 days</literal></entry>
       </row>

       <row>
        <entry>
         <indexterm>
          <primary>clock_timestamp</primary>
         </indexterm>
         <literal><function>clock_timestamp()</function></literal>
        </entry>
        <entry><type>timestamp with time zone</type></entry>
        <entry>Current date and time (changes during statement execution);
         see <xref linkend="functions-datetime-current"/>
        </entry>
        <entry></entry>
        <entry></entry>
       </row>

       <row>
        <entry>
         <indexterm>
          <primary>current_date</primary>
         </indexterm>
         <literal><function>current_date</function></literal>
        </entry>
        <entry><type>date</type></entry>
        <entry>Current date;
         see <xref linkend="functions-datetime-current"/>
        </entry>
        <entry></entry>
        <entry></entry>
       </row>

       <row>
        <entry>
         <indexterm>
          <primary>current_time</primary>
         </indexterm>
         <literal><function>current_time</function></literal>
        </entry>
        <entry><type>time with time zone</type></entry>
        <entry>Current time of day;
         see <xref linkend="functions-datetime-current"/>
        </entry>
        <entry></entry>
        <entry></entry>
       </row>

       <row>
        <entry>
         <indexterm>
          <primary>current_timestamp</primary>
         </indexterm>
         <literal><function>current_timestamp</function></literal>
        </entry>
        <entry><type>timestamp with time zone</type></entry>
        <entry>Current date and time (start of current transaction);
         see <xref linkend="functions-datetime-current"/>
        </entry>
        <entry></entry>
        <entry></entry>
       </row>

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

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

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

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

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

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

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

       <row>
        <entry>
         <indexterm>
          <primary>isfinite</primary>
         </indexterm>
         <literal><function>isfinite(<type>date</type>)</function></literal>
        </entry>
        <entry><type>boolean</type></entry>
        <entry>Test for finite date (not +/-infinity)</entry>
        <entry><literal>isfinite(date '2001-02-16')</literal></entry>
        <entry><literal>true</literal></entry>
       </row>

       <row>
        <entry><literal><function>isfinite(<type>timestamp</type>)</function></literal></entry>
        <entry><type>boolean</type></entry>
        <entry>Test for finite time stamp (not +/-infinity)</entry>
        <entry><literal>isfinite(timestamp '2001-02-16 21:28:30')</literal></entry>
        <entry><literal>true</literal></entry>
       </row>

       <row>
        <entry><literal><function>isfinite(<type>interval</type>)</function></literal></entry>
        <entry><type>boolean</type></entry>
        <entry>Test for finite interval</entry>
        <entry><literal>isfinite(interval '4 hours')</literal></entry>
        <entry><literal>true</literal></entry>
       </row>

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

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

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

       <row>
        <entry>
         <indexterm>
          <primary>localtime</primary>
         </indexterm>
         <literal><function>localtime</function></literal>
        </entry>
        <entry><type>time</type></entry>
        <entry>Current time of day;
         see <xref linkend="functions-datetime-current"/>
        </entry>
        <entry></entry>
        <entry></entry>
       </row>

       <row>
        <entry>
         <indexterm>
          <primary>localtimestamp</primary>
         </indexterm>
         <literal><function>localtimestamp</function></literal>
        </entry>
        <entry><type>timestamp</type></entry>
        <entry>Current date and time (start of current transaction);
         see <xref linkend="functions-datetime-current"/>
        </entry>
        <entry></entry>
        <entry></entry>
       </row>

       <row>
        <entry>
         <indexterm>
          <primary>make_date</primary>
         </indexterm>
         <literal>
            <function>
             make_date(<parameter>year</parameter> <type>int</type>,
             <parameter>month</parameter> <type>int</type>,
             <parameter>day</parameter> <type>int</type>)
            </function>
         </literal>
        </entry>
        <entry><type>date</type></entry>
        <entry>
         Create date from year, month and day fields
        </entry>
        <entry><literal>make_date(2013, 7, 15)</literal></entry>
        <entry><literal>2013-07-15</literal></entry>
       </row>

       <row>
        <entry>
         <indexterm>
          <primary>make_interval</primary>
         </indexterm>
         <literal>
          <function>
           make_interval(<parameter>years</parameter> <type>int</type> DEFAULT 0,
           <parameter>months</parameter> <type>int</type> DEFAULT 0,
           <parameter>weeks</parameter> <type>int</type> DEFAULT 0,
           <parameter>days</parameter> <type>int</type> DEFAULT 0,
           <parameter>hours</parameter> <type>int</type> DEFAULT 0,
           <parameter>mins</parameter> <type>int</type> DEFAULT 0,
           <parameter>secs</parameter> <type>double precision</type> DEFAULT 0.0)
          </function>
         </literal>
        </entry>
        <entry><type>interval</type></entry>
        <entry>
         Create interval from years, months, weeks, days, hours, minutes and
         seconds fields
        </entry>
        <entry><literal>make_interval(days =&gt; 10)</literal></entry>
        <entry><literal>10 days</literal></entry>
       </row>

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

       <row>
        <entry>
         <indexterm>
          <primary>make_timestamp</primary>
         </indexterm>
         <literal>
          <function>
           make_timestamp(<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>)
          </function>
         </literal>
        </entry>
        <entry><type>timestamp</type></entry>
        <entry>
         Create timestamp from year, month, day, hour, minute and seconds fields
        </entry>
        <entry><literal>make_timestamp(2013, 7, 15, 8, 15, 23.5)</literal></entry>
        <entry><literal>2013-07-15 08:15:23.5</literal></entry>
       </row>

       <row>
        <entry>
         <indexterm>
          <primary>make_timestamptz</primary>
         </indexterm>
         <literal>
          <function>
           make_timestamptz(<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>)
          </function>
         </literal>
        </entry>
        <entry><type>timestamp with time zone</type></entry>
        <entry>
         Create timestamp with time zone from year, month, day, hour, minute
         and seconds fields; if <parameter>timezone</parameter> is not
         specified, the current time zone is used
        </entry>
        <entry><literal>make_timestamptz(2013, 7, 15, 8, 15, 23.5)</literal></entry>
        <entry><literal>2013-07-15 08:15:23.5+01</literal></entry>
       </row>

       <row>
        <entry>
         <indexterm>
          <primary>now</primary>
         </indexterm>
         <literal><function>now()</function></literal>
        </entry>
        <entry><type>timestamp with time zone</type></entry>
        <entry>Current date and time (start of current transaction);
         see <xref linkend="functions-datetime-current"/>
        </entry>
        <entry></entry>
        <entry></entry>
       </row>

       <row>
        <entry>
         <indexterm>
          <primary>statement_timestamp</primary>
         </indexterm>
         <literal><function>statement_timestamp()</function></literal>
        </entry>
        <entry><type>timestamp with time zone</type></entry>
        <entry>Current date and time (start of current statement);
         see <xref linkend="functions-datetime-current"/>
        </entry>
        <entry></entry>
        <entry></entry>
       </row>

       <row>
        <entry>
         <indexterm>
          <primary>timeofday</primary>
         </indexterm>
         <literal><function>timeofday()</function></literal>
        </entry>
        <entry><type>text</type></entry>
        <entry>Current date and time
         (like <function>clock_timestamp</function>, but as a <type>text</type> string);
         see <xref linkend="functions-datetime-current"/>
        </entry>
        <entry></entry>
        <entry></entry>
       </row>

       <row>
        <entry>
         <indexterm>
          <primary>transaction_timestamp</primary>
         </indexterm>
         <literal><function>transaction_timestamp()</function></literal>
        </entry>
        <entry><type>timestamp with time zone</type></entry>
        <entry>Current date and time (start of current transaction);
         see <xref linkend="functions-datetime-current"/>
        </entry>
        <entry></entry>
        <entry></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>to_timestamp</primary>
         </indexterm>
         <literal><function>to_timestamp(<type>double precision</type>)</function></literal>
        </entry>
        <entry><type>timestamp with time zone</type></entry>
        <entry>Convert Unix epoch (seconds since 1970-01-01 00:00:00+00) to
         timestamp</entry>
        <entry><literal>to_timestamp(1284352323)</literal></entry>
        <entry><literal>2010-09-13 04:32:03+00</literal></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.
   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>CST7CDT</literal>,
   <literal>timestamp with time zone '2005-04-02 12:00-07' + interval '1 day'</literal>
   will produce <literal>timestamp with time zone '2005-04-03 12:00-06'</literal>,
   while adding <literal>interval '24 hours'</literal> to the same initial
   <type>timestamp with time zone</type> produces
   <literal>timestamp with time zone '2005-04-03 13:00-06'</literal>, as there is
   a change in daylight saving time at <literal>2005-04-03 02:00</literal> in time zone
   <literal>CST7CDT</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 fractional parts (0 -
        59<footnote><simpara>60 if leap seconds are
        implemented by the operating system</simpara></footnote>)
       </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> converts time
    stamp <emphasis>without time zone</emphasis> to/from
    time stamp <emphasis>with time zone</emphasis>, and
    <emphasis>time</emphasis> 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="3">
      <thead>
       <row>
        <entry>Expression</entry>
        <entry>Return Type</entry>
        <entry>Description</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry>
         <literal><type>timestamp without time zone</type> AT TIME ZONE <replaceable>zone</replaceable></literal>
        </entry>
        <entry><type>timestamp with time zone</type></entry>
        <entry>Treat given time stamp <emphasis>without time zone</emphasis> as located in the specified time zone</entry>
       </row>

       <row>
        <entry>
         <literal><type>timestamp with time zone</type> AT TIME ZONE <replaceable>zone</replaceable></literal>
        </entry>
        <entry><type>timestamp without time zone</type></entry>
        <entry>Convert given time stamp <emphasis>with time zone</emphasis> to the new time
        zone, with no time zone designation</entry>
       </row>

       <row>
        <entry>
         <literal><type>time with time zone</type> AT TIME ZONE <replaceable>zone</replaceable></literal>
        </entry>
        <entry><type>time with time zone</type></entry>
        <entry>Convert given time <emphasis>with time zone</emphasis> to the new time zone</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    In these expressions, the desired time zone <replaceable>zone</replaceable> can be
    specified either as a text string (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"/>.
   </para>

   <para>
    Examples (assuming the local time zone 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 20:38:40-05' 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.   Converting <emphasis>time</emphasis>
    values to other time zones uses the currently active time zone rules
    since no date is supplied.
   </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>2001-12-23</computeroutput>

SELECT CURRENT_TIMESTAMP;
<lineannotation>Result: </lineannotation><computeroutput>2001-12-23 14:39:53.662522-05</computeroutput>

SELECT CURRENT_TIMESTAMP(2);
<lineannotation>Result: </lineannotation><computeroutput>2001-12-23 14:39:53.66-05</computeroutput>

SELECT LOCALTIMESTAMP;
<lineannotation>Result: </lineannotation><computeroutput>2001-12-23 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';  -- incorrect for use with DEFAULT
</programlisting>
   </para>

    <tip>
     <para>
      You do not want to use the third form when specifying a <literal>DEFAULT</literal>
      clause while creating a table.  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.
     </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(<replaceable>seconds</replaceable>)
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 <replaceable>seconds</replaceable> seconds have
    elapsed.  <replaceable>seconds</replaceable> is a value of type
    <type>double precision</type>, so fractional-second delays can be specified.
    <function>pg_sleep_for</function> is a convenience function for larger
    sleep times 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="4">
     <thead>
      <row>
       <entry>Function</entry>
       <entry>Description</entry>
       <entry>Example</entry>
       <entry>Example Result</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>
         <indexterm>
          <primary>enum_first</primary>
         </indexterm>
         <literal>enum_first(anyenum)</literal>
       </entry>
       <entry>Returns the first value of the input enum type</entry>
       <entry><literal>enum_first(null::rainbow)</literal></entry>
       <entry><literal>red</literal></entry>
      </row>
      <row>
       <entry>
         <indexterm>
          <primary>enum_last</primary>
         </indexterm>
         <literal>enum_last(anyenum)</literal>
       </entry>
       <entry>Returns the last value of the input enum type</entry>
       <entry><literal>enum_last(null::rainbow)</literal></entry>
       <entry><literal>purple</literal></entry>
      </row>
      <row>
       <entry>
         <indexterm>
          <primary>enum_range</primary>
         </indexterm>
         <literal>enum_range(anyenum)</literal>
       </entry>
       <entry>Returns all values of the input enum type in an ordered array</entry>
       <entry><literal>enum_range(null::rainbow)</literal></entry>
       <entry><literal>{red,orange,yellow,green,blue,purple}</literal></entry>
      </row>
      <row>
       <entry morerows="2"><literal>enum_range(anyenum, anyenum)</literal></entry>
       <entry morerows="2">
        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.
       </entry>
       <entry><literal>enum_range('orange'::rainbow, 'green'::rainbow)</literal></entry>
       <entry><literal>{orange,yellow,green}</literal></entry>
      </row>
      <row>
       <entry><literal>enum_range(NULL, 'green'::rainbow)</literal></entry>
       <entry><literal>{red,orange,yellow,green}</literal></entry>
      </row>
      <row>
       <entry><literal>enum_range('orange'::rainbow, NULL)</literal></entry>
       <entry><literal>{orange,yellow,green,blue,purple}</literal></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 suggested by 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>

   <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 these 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) likewise compare areas for these types.
    </para>
   </caution>

   <table id="functions-geometry-op-table">
     <title>Geometric Operators</title>
     <tgroup cols="3">
      <thead>
       <row>
        <entry>Operator</entry>
        <entry>Description</entry>
        <entry>Example</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry> <literal>+</literal> </entry>
        <entry>Translation</entry>
        <entry><literal>box '((0,0),(1,1))' + point '(2.0,0)'</literal></entry>
       </row>
       <row>
        <entry> <literal>-</literal> </entry>
        <entry>Translation</entry>
        <entry><literal>box '((0,0),(1,1))' - point '(2.0,0)'</literal></entry>
       </row>
       <row>
        <entry> <literal>*</literal> </entry>
        <entry>Scaling/rotation</entry>
        <entry><literal>box '((0,0),(1,1))' * point '(2.0,0)'</literal></entry>
       </row>
       <row>
        <entry> <literal>/</literal> </entry>
        <entry>Scaling/rotation</entry>
        <entry><literal>box '((0,0),(2,2))' / point '(2.0,0)'</literal></entry>
       </row>
       <row>
        <entry> <literal>#</literal> </entry>
        <entry>Point or box of intersection</entry>
        <entry><literal>box '((1,-1),(-1,1))' # box '((1,1),(-2,-2))'</literal></entry>
       </row>
       <row>
        <entry> <literal>#</literal> </entry>
        <entry>Number of points in path or polygon</entry>
        <entry><literal># path '((1,0),(0,1),(-1,0))'</literal></entry>
       </row>
       <row>
        <entry> <literal>@-@</literal> </entry>
        <entry>Length or circumference</entry>
        <entry><literal>@-@ path '((0,0),(1,0))'</literal></entry>
       </row>
       <row>
        <entry> <literal>@@</literal> </entry>
        <entry>Center</entry>
        <entry><literal>@@ circle '((0,0),10)'</literal></entry>
       </row>
       <row>
        <entry> <literal>##</literal> </entry>
        <entry>Closest point to first operand on second operand</entry>
        <entry><literal>point '(0,0)' ## lseg '((2,0),(0,2))'</literal></entry>
       </row>
       <row>
        <entry> <literal>&lt;-&gt;</literal> </entry>
        <entry>Distance between</entry>
        <entry><literal>circle '((0,0),1)' &lt;-&gt; circle '((5,0),1)'</literal></entry>
       </row>
       <row>
        <entry> <literal>&amp;&amp;</literal> </entry>
        <entry>Overlaps?  (One point in common makes this true.)</entry>
        <entry><literal>box '((0,0),(1,1))' &amp;&amp; box '((0,0),(2,2))'</literal></entry>
       </row>
       <row>
        <entry> <literal>&lt;&lt;</literal> </entry>
        <entry>Is strictly left of?</entry>
        <entry><literal>circle '((0,0),1)' &lt;&lt; circle '((5,0),1)'</literal></entry>
       </row>
       <row>
        <entry> <literal>&gt;&gt;</literal> </entry>
        <entry>Is strictly right of?</entry>
        <entry><literal>circle '((5,0),1)' &gt;&gt; circle '((0,0),1)'</literal></entry>
       </row>
       <row>
        <entry> <literal>&amp;&lt;</literal> </entry>
        <entry>Does not extend to the right of?</entry>
        <entry><literal>box '((0,0),(1,1))' &amp;&lt; box '((0,0),(2,2))'</literal></entry>
       </row>
       <row>
        <entry> <literal>&amp;&gt;</literal> </entry>
        <entry>Does not extend to the left of?</entry>
        <entry><literal>box '((0,0),(3,3))' &amp;&gt; box '((0,0),(2,2))'</literal></entry>
       </row>
       <row>
        <entry> <literal>&lt;&lt;|</literal> </entry>
        <entry>Is strictly below?</entry>
        <entry><literal>box '((0,0),(3,3))' &lt;&lt;| box '((3,4),(5,5))'</literal></entry>
       </row>
       <row>
        <entry> <literal>|&gt;&gt;</literal> </entry>
        <entry>Is strictly above?</entry>
        <entry><literal>box '((3,4),(5,5))' |&gt;&gt; box '((0,0),(3,3))'</literal></entry>
       </row>
       <row>
        <entry> <literal>&amp;&lt;|</literal> </entry>
        <entry>Does not extend above?</entry>
        <entry><literal>box '((0,0),(1,1))' &amp;&lt;| box '((0,0),(2,2))'</literal></entry>
       </row>
       <row>
        <entry> <literal>|&amp;&gt;</literal> </entry>
        <entry>Does not extend below?</entry>
        <entry><literal>box '((0,0),(3,3))' |&amp;&gt; box '((0,0),(2,2))'</literal></entry>
       </row>
       <row>
        <entry> <literal>&lt;^</literal> </entry>
        <entry>Is below (allows touching)?</entry>
        <entry><literal>circle '((0,0),1)' &lt;^ circle '((0,5),1)'</literal></entry>
       </row>
       <row>
        <entry> <literal>&gt;^</literal> </entry>
        <entry>Is above (allows touching)?</entry>
        <entry><literal>circle '((0,5),1)' &gt;^ circle '((0,0),1)'</literal></entry>
       </row>
       <row>
        <entry> <literal>?#</literal> </entry>
        <entry>Intersects?</entry>
        <entry><literal>lseg '((-1,0),(1,0))' ?# box '((-2,-2),(2,2))'</literal></entry>
       </row>
       <row>
        <entry> <literal>?-</literal> </entry>
        <entry>Is horizontal?</entry>
        <entry><literal>?- lseg '((-1,0),(1,0))'</literal></entry>
       </row>
       <row>
        <entry> <literal>?-</literal> </entry>
        <entry>Are horizontally aligned?</entry>
        <entry><literal>point '(1,0)' ?- point '(0,0)'</literal></entry>
       </row>
       <row>
        <entry> <literal>?|</literal> </entry>
        <entry>Is vertical?</entry>
        <entry><literal>?| lseg '((-1,0),(1,0))'</literal></entry>
       </row>
       <row>
        <entry> <literal>?|</literal> </entry>
        <entry>Are vertically aligned?</entry>
        <entry><literal>point '(0,1)' ?| point '(0,0)'</literal></entry>
       </row>
       <row>
        <entry> <literal>?-|</literal> </entry>
        <entry>Is perpendicular?</entry>
        <entry><literal>lseg '((0,0),(0,1))' ?-| lseg '((0,0),(1,0))'</literal></entry>
       </row>
       <row>
        <entry> <literal>?||</literal> </entry>
        <entry>Are parallel?</entry>
        <entry><literal>lseg '((-1,0),(1,0))' ?|| lseg '((-1,2),(1,2))'</literal></entry>
       </row>
       <row>
        <entry> <literal>@&gt;</literal> </entry>
        <entry>Contains?</entry>
        <entry><literal>circle '((0,0),2)' @&gt; point '(1,1)'</literal></entry>
       </row>
       <row>
        <entry> <literal>&lt;@</literal> </entry>
        <entry>Contained in or on?</entry>
        <entry><literal>point '(1,1)' &lt;@ circle '((0,0),2)'</literal></entry>
       </row>
       <row>
        <entry> <literal>~=</literal> </entry>
        <entry>Same as?</entry>
        <entry><literal>polygon '((0,0),(1,1))' ~= polygon '((1,1),(0,0))'</literal></entry>
       </row>
      </tbody>
     </tgroup>
   </table>

   <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>

   <indexterm>
    <primary>area</primary>
   </indexterm>
   <indexterm>
    <primary>center</primary>
   </indexterm>
   <indexterm>
    <primary>diameter</primary>
   </indexterm>
   <indexterm>
    <primary>height</primary>
   </indexterm>
   <indexterm>
    <primary>isclosed</primary>
   </indexterm>
   <indexterm>
    <primary>isopen</primary>
   </indexterm>
   <indexterm>
    <primary>length</primary>
   </indexterm>
   <indexterm>
    <primary>npoints</primary>
   </indexterm>
   <indexterm>
    <primary>pclose</primary>
   </indexterm>
   <indexterm>
    <primary>popen</primary>
   </indexterm>
   <indexterm>
    <primary>radius</primary>
   </indexterm>
   <indexterm>
    <primary>width</primary>
   </indexterm>

   <table id="functions-geometry-func-table">
     <title>Geometric Functions</title>
     <tgroup cols="4">
      <thead>
       <row>
        <entry>Function</entry>
        <entry>Return Type</entry>
        <entry>Description</entry>
        <entry>Example</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><literal><function>area(<replaceable>object</replaceable>)</function></literal></entry>
        <entry><type>double precision</type></entry>
        <entry>area</entry>
        <entry><literal>area(box '((0,0),(1,1))')</literal></entry>
       </row>
       <row>
        <entry><literal><function>center(<replaceable>object</replaceable>)</function></literal></entry>
        <entry><type>point</type></entry>
        <entry>center</entry>
        <entry><literal>center(box '((0,0),(1,2))')</literal></entry>
       </row>
       <row>
        <entry><literal><function>diameter(<type>circle</type>)</function></literal></entry>
        <entry><type>double precision</type></entry>
        <entry>diameter of circle</entry>
        <entry><literal>diameter(circle '((0,0),2.0)')</literal></entry>
       </row>
       <row>
        <entry><literal><function>height(<type>box</type>)</function></literal></entry>
        <entry><type>double precision</type></entry>
        <entry>vertical size of box</entry>
        <entry><literal>height(box '((0,0),(1,1))')</literal></entry>
       </row>
       <row>
        <entry><literal><function>isclosed(<type>path</type>)</function></literal></entry>
        <entry><type>boolean</type></entry>
        <entry>a closed path?</entry>
        <entry><literal>isclosed(path '((0,0),(1,1),(2,0))')</literal></entry>
       </row>
       <row>
        <entry><literal><function>isopen(<type>path</type>)</function></literal></entry>
        <entry><type>boolean</type></entry>
        <entry>an open path?</entry>
        <entry><literal>isopen(path '[(0,0),(1,1),(2,0)]')</literal></entry>
       </row>
       <row>
        <entry><literal><function>length(<replaceable>object</replaceable>)</function></literal></entry>
        <entry><type>double precision</type></entry>
        <entry>length</entry>
        <entry><literal>length(path '((-1,0),(1,0))')</literal></entry>
       </row>
       <row>
        <entry><literal><function>npoints(<type>path</type>)</function></literal></entry>
        <entry><type>int</type></entry>
        <entry>number of points</entry>
        <entry><literal>npoints(path '[(0,0),(1,1),(2,0)]')</literal></entry>
       </row>
       <row>
        <entry><literal><function>npoints(<type>polygon</type>)</function></literal></entry>
        <entry><type>int</type></entry>
        <entry>number of points</entry>
        <entry><literal>npoints(polygon '((1,1),(0,0))')</literal></entry>
       </row>
       <row>
        <entry><literal><function>pclose(<type>path</type>)</function></literal></entry>
        <entry><type>path</type></entry>
        <entry>convert path to closed</entry>
        <entry><literal>pclose(path '[(0,0),(1,1),(2,0)]')</literal></entry>
       </row>
       <row>
        <entry><literal><function>popen(<type>path</type>)</function></literal></entry>
        <entry><type>path</type></entry>
        <entry>convert path to open</entry>
        <entry><literal>popen(path '((0,0),(1,1),(2,0))')</literal></entry>
       </row>
       <row>
        <entry><literal><function>radius(<type>circle</type>)</function></literal></entry>
        <entry><type>double precision</type></entry>
        <entry>radius of circle</entry>
        <entry><literal>radius(circle '((0,0),2.0)')</literal></entry>
       </row>
       <row>
        <entry><literal><function>width(<type>box</type>)</function></literal></entry>
        <entry><type>double precision</type></entry>
        <entry>horizontal size of box</entry>
        <entry><literal>width(box '((0,0),(1,1))')</literal></entry>
       </row>
      </tbody>
     </tgroup>
   </table>

   <table id="functions-geometry-conv-table">
     <title>Geometric Type Conversion Functions</title>
     <tgroup cols="4">
      <thead>
       <row>
        <entry>Function</entry>
        <entry>Return Type</entry>
        <entry>Description</entry>
        <entry>Example</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry>
         <indexterm>
          <primary>box</primary>
         </indexterm>
         <literal><function>box(<type>circle</type>)</function></literal>
        </entry>
        <entry><type>box</type></entry>
        <entry>circle to box</entry>
        <entry><literal>box(circle '((0,0),2.0)')</literal></entry>
       </row>
       <row>
        <entry><literal><function>box(<type>point</type>)</function></literal></entry>
        <entry><type>box</type></entry>
        <entry>point to empty box</entry>
        <entry><literal>box(point '(0,0)')</literal></entry>
       </row>
       <row>
        <entry><literal><function>box(<type>point</type>, <type>point</type>)</function></literal></entry>
        <entry><type>box</type></entry>
        <entry>points to box</entry>
        <entry><literal>box(point '(0,0)', point '(1,1)')</literal></entry>
       </row>
       <row>
        <entry><literal><function>box(<type>polygon</type>)</function></literal></entry>
        <entry><type>box</type></entry>
        <entry>polygon to box</entry>
        <entry><literal>box(polygon '((0,0),(1,1),(2,0))')</literal></entry>
       </row>
       <row>
        <entry><literal><function>bound_box(<type>box</type>, <type>box</type>)</function></literal></entry>
        <entry><type>box</type></entry>
        <entry>boxes to bounding box</entry>
        <entry><literal>bound_box(box '((0,0),(1,1))', box '((3,3),(4,4))')</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>circle</primary>
         </indexterm>
         <literal><function>circle(<type>box</type>)</function></literal>
        </entry>
        <entry><type>circle</type></entry>
        <entry>box to circle</entry>
        <entry><literal>circle(box '((0,0),(1,1))')</literal></entry>
       </row>
       <row>
        <entry><literal><function>circle(<type>point</type>, <type>double precision</type>)</function></literal></entry>
        <entry><type>circle</type></entry>
        <entry>center and radius to circle</entry>
        <entry><literal>circle(point '(0,0)', 2.0)</literal></entry>
       </row>
       <row>
        <entry><literal><function>circle(<type>polygon</type>)</function></literal></entry>
        <entry><type>circle</type></entry>
        <entry>polygon to circle</entry>
        <entry><literal>circle(polygon '((0,0),(1,1),(2,0))')</literal></entry>
       </row>
       <row>
        <entry><literal><function>line(<type>point</type>, <type>point</type>)</function></literal></entry>
        <entry><type>line</type></entry>
        <entry>points to line</entry>
        <entry><literal>line(point '(-1,0)', point '(1,0)')</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>lseg</primary>
         </indexterm>
         <literal><function>lseg(<type>box</type>)</function></literal>
        </entry>
        <entry><type>lseg</type></entry>
        <entry>box diagonal to line segment</entry>
        <entry><literal>lseg(box '((-1,0),(1,0))')</literal></entry>
       </row>
       <row>
        <entry><literal><function>lseg(<type>point</type>, <type>point</type>)</function></literal></entry>
        <entry><type>lseg</type></entry>
        <entry>points to line segment</entry>
        <entry><literal>lseg(point '(-1,0)', point '(1,0)')</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>path</primary>
         </indexterm>
         <literal><function>path(<type>polygon</type>)</function></literal>
        </entry>
        <entry><type>path</type></entry>
        <entry>polygon to path</entry>
        <entry><literal>path(polygon '((0,0),(1,1),(2,0))')</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>point</primary>
         </indexterm>
         <literal><function>point</function>(<type>double
         precision</type>, <type>double precision</type>)</literal>
        </entry>
        <entry><type>point</type></entry>
        <entry>construct point</entry>
        <entry><literal>point(23.4, -44.5)</literal></entry>
       </row>
       <row>
        <entry><literal><function>point(<type>box</type>)</function></literal></entry>
        <entry><type>point</type></entry>
        <entry>center of box</entry>
        <entry><literal>point(box '((-1,0),(1,0))')</literal></entry>
       </row>
       <row>
        <entry><literal><function>point(<type>circle</type>)</function></literal></entry>
        <entry><type>point</type></entry>
        <entry>center of circle</entry>
        <entry><literal>point(circle '((0,0),2.0)')</literal></entry>
       </row>
       <row>
        <entry><literal><function>point(<type>lseg</type>)</function></literal></entry>
        <entry><type>point</type></entry>
        <entry>center of line segment</entry>
        <entry><literal>point(lseg '((-1,0),(1,0))')</literal></entry>
       </row>
       <row>
        <entry><literal><function>point(<type>polygon</type>)</function></literal></entry>
        <entry><type>point</type></entry>
        <entry>center of polygon</entry>
        <entry><literal>point(polygon '((0,0),(1,1),(2,0))')</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>polygon</primary>
         </indexterm>
         <literal><function>polygon(<type>box</type>)</function></literal>
        </entry>
        <entry><type>polygon</type></entry>
        <entry>box to 4-point polygon</entry>
        <entry><literal>polygon(box '((0,0),(1,1))')</literal></entry>
       </row>
       <row>
        <entry><literal><function>polygon(<type>circle</type>)</function></literal></entry>
        <entry><type>polygon</type></entry>
        <entry>circle to 12-point polygon</entry>
        <entry><literal>polygon(circle '((0,0),2.0)')</literal></entry>
       </row>
       <row>
        <entry><literal><function>polygon(<replaceable class="parameter">npts</replaceable>, <type>circle</type>)</function></literal></entry>
        <entry><type>polygon</type></entry>
        <entry>circle to <replaceable class="parameter">npts</replaceable>-point polygon</entry>
        <entry><literal>polygon(12, circle '((0,0),2.0)')</literal></entry>
       </row>
       <row>
        <entry><literal><function>polygon(<type>path</type>)</function></literal></entry>
        <entry><type>polygon</type></entry>
        <entry>path to polygon</entry>
        <entry><literal>polygon(path '((0,0),(1,1),(2,0))')</literal></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>

    <para>
     The <function>area</function> function works for the types
     <type>box</type>, <type>circle</type>, and <type>path</type>.
     The <function>area</function> function only works on the
     <type>path</type> data type if the points in the
     <type>path</type> are non-intersecting.  For example, the
     <type>path</type>
     <literal>'((0,0),(0,1),(2,1),(2,2),(1,2),(1,0),(0,0))'::PATH</literal>
     will not work;  however, the following visually identical
     <type>path</type>
     <literal>'((0,0),(0,1),(1,1),(1,2),(2,2),(2,1),(1,1),(1,0),(0,0))'::PATH</literal>
     will work.  If the concept of an intersecting versus
     non-intersecting <type>path</type> is confusing, draw both of the
     above <type>path</type>s side by side on a piece of graph paper.
    </para>

  </sect1>


 <sect1 id="functions-net">
  <title>Network Address Functions and Operators</title>

  <para>
   <xref linkend="cidr-inet-operators-table"/> shows the operators
   available for the <type>cidr</type> and <type>inet</type> types.
   The operators <literal>&lt;&lt;</literal>,
   <literal>&lt;&lt;=</literal>, <literal>&gt;&gt;</literal>,
   <literal>&gt;&gt;=</literal>, and <literal>&amp;&amp;</literal>
   test for subnet inclusion.  They
   consider only the network parts of the two addresses (ignoring any
   host part) and determine whether one network is identical to
   or a subnet of the other.
  </para>

    <table id="cidr-inet-operators-table">
     <title><type>cidr</type> and <type>inet</type> Operators</title>
     <tgroup cols="3">
      <thead>
       <row>
        <entry>Operator</entry>
        <entry>Description</entry>
        <entry>Example</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry> <literal>&lt;</literal> </entry>
        <entry>is less than</entry>
        <entry><literal>inet '192.168.1.5' &lt; inet '192.168.1.6'</literal></entry>
       </row>
       <row>
        <entry> <literal>&lt;=</literal> </entry>
        <entry>is less than or equal</entry>
        <entry><literal>inet '192.168.1.5' &lt;= inet '192.168.1.5'</literal></entry>
       </row>
       <row>
        <entry> <literal>=</literal> </entry>
        <entry>equals</entry>
        <entry><literal>inet '192.168.1.5' = inet '192.168.1.5'</literal></entry>
       </row>
       <row>
        <entry> <literal>&gt;=</literal> </entry>
        <entry>is greater or equal</entry>
        <entry><literal>inet '192.168.1.5' &gt;= inet '192.168.1.5'</literal></entry>
       </row>
       <row>
        <entry> <literal>&gt;</literal> </entry>
        <entry>is greater than</entry>
        <entry><literal>inet '192.168.1.5' &gt; inet '192.168.1.4'</literal></entry>
       </row>
       <row>
        <entry> <literal>&lt;&gt;</literal> </entry>
        <entry>is not equal</entry>
        <entry><literal>inet '192.168.1.5' &lt;&gt; inet '192.168.1.4'</literal></entry>
       </row>
       <row>
        <entry> <literal>&lt;&lt;</literal> </entry>
        <entry>is contained by</entry>
        <entry><literal>inet '192.168.1.5' &lt;&lt; inet '192.168.1/24'</literal></entry>
       </row>
       <row>
        <entry> <literal>&lt;&lt;=</literal> </entry>
        <entry>is contained by or equals</entry>
        <entry><literal>inet '192.168.1/24' &lt;&lt;= inet '192.168.1/24'</literal></entry>
       </row>
       <row>
        <entry> <literal>&gt;&gt;</literal> </entry>
        <entry>contains</entry>
        <entry><literal>inet '192.168.1/24' &gt;&gt; inet '192.168.1.5'</literal></entry>
       </row>
       <row>
        <entry> <literal>&gt;&gt;=</literal> </entry>
        <entry>contains or equals</entry>
        <entry><literal>inet '192.168.1/24' &gt;&gt;= inet '192.168.1/24'</literal></entry>
       </row>
       <row>
        <entry> <literal>&amp;&amp;</literal> </entry>
        <entry>contains or is contained by</entry>
        <entry><literal>inet '192.168.1/24' &amp;&amp; inet '192.168.1.80/28'</literal></entry>
       </row>
       <row>
        <entry> <literal>~</literal> </entry>
        <entry>bitwise NOT</entry>
        <entry><literal>~ inet '192.168.1.6'</literal></entry>
       </row>
       <row>
        <entry> <literal>&amp;</literal> </entry>
        <entry>bitwise AND</entry>
        <entry><literal>inet '192.168.1.6' &amp; inet '0.0.0.255'</literal></entry>
       </row>
       <row>
        <entry> <literal>|</literal> </entry>
        <entry>bitwise OR</entry>
        <entry><literal>inet '192.168.1.6' | inet '0.0.0.255'</literal></entry>
       </row>
       <row>
        <entry> <literal>+</literal> </entry>
        <entry>addition</entry>
        <entry><literal>inet '192.168.1.6' + 25</literal></entry>
       </row>
       <row>
        <entry> <literal>-</literal> </entry>
        <entry>subtraction</entry>
        <entry><literal>inet '192.168.1.43' - 36</literal></entry>
       </row>
       <row>
        <entry> <literal>-</literal> </entry>
        <entry>subtraction</entry>
        <entry><literal>inet '192.168.1.43' - inet '192.168.1.19'</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

  <para>
   <xref linkend="cidr-inet-functions-table"/> shows the functions
   available for use with the <type>cidr</type> and <type>inet</type>
   types.  The <function>abbrev</function>, <function>host</function>,
   and <function>text</function>
   functions are primarily intended to offer alternative display
   formats.
  </para>

    <table id="cidr-inet-functions-table">
     <title><type>cidr</type> and <type>inet</type> Functions</title>
     <tgroup cols="5">
      <thead>
       <row>
        <entry>Function</entry>
        <entry>Return Type</entry>
        <entry>Description</entry>
        <entry>Example</entry>
        <entry>Result</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry>
         <indexterm>
          <primary>abbrev</primary>
         </indexterm>
         <literal><function>abbrev(<type>inet</type>)</function></literal>
        </entry>
        <entry><type>text</type></entry>
        <entry>abbreviated display format as text</entry>
        <entry><literal>abbrev(inet '10.1.0.0/16')</literal></entry>
        <entry><literal>10.1.0.0/16</literal></entry>
       </row>
       <row>
        <entry><literal><function>abbrev(<type>cidr</type>)</function></literal></entry>
        <entry><type>text</type></entry>
        <entry>abbreviated display format as text</entry>
        <entry><literal>abbrev(cidr '10.1.0.0/16')</literal></entry>
        <entry><literal>10.1/16</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>broadcast</primary>
         </indexterm>
         <literal><function>broadcast(<type>inet</type>)</function></literal>
        </entry>
        <entry><type>inet</type></entry>
        <entry>broadcast address for network</entry>
        <entry><literal>broadcast('192.168.1.5/24')</literal></entry>
        <entry><literal>192.168.1.255/24</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>family</primary>
         </indexterm>
         <literal><function>family(<type>inet</type>)</function></literal>
        </entry>
        <entry><type>int</type></entry>
        <entry>extract family of address; <literal>4</literal> for IPv4,
         <literal>6</literal> for IPv6</entry>
        <entry><literal>family('::1')</literal></entry>
        <entry><literal>6</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>host</primary>
         </indexterm>
         <literal><function>host(<type>inet</type>)</function></literal>
        </entry>
        <entry><type>text</type></entry>
        <entry>extract IP address as text</entry>
        <entry><literal>host('192.168.1.5/24')</literal></entry>
        <entry><literal>192.168.1.5</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>hostmask</primary>
         </indexterm>
         <literal><function>hostmask(<type>inet</type>)</function></literal>
        </entry>
        <entry><type>inet</type></entry>
        <entry>construct host mask for network</entry>
        <entry><literal>hostmask('192.168.23.20/30')</literal></entry>
        <entry><literal>0.0.0.3</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>masklen</primary>
         </indexterm>
         <literal><function>masklen(<type>inet</type>)</function></literal>
        </entry>
        <entry><type>int</type></entry>
        <entry>extract netmask length</entry>
        <entry><literal>masklen('192.168.1.5/24')</literal></entry>
        <entry><literal>24</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>netmask</primary>
         </indexterm>
         <literal><function>netmask(<type>inet</type>)</function></literal>
        </entry>
        <entry><type>inet</type></entry>
        <entry>construct netmask for network</entry>
        <entry><literal>netmask('192.168.1.5/24')</literal></entry>
        <entry><literal>255.255.255.0</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>network</primary>
         </indexterm>
         <literal><function>network(<type>inet</type>)</function></literal>
        </entry>
        <entry><type>cidr</type></entry>
        <entry>extract network part of address</entry>
        <entry><literal>network('192.168.1.5/24')</literal></entry>
        <entry><literal>192.168.1.0/24</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>set_masklen</primary>
         </indexterm>
         <literal><function>set_masklen(<type>inet</type>, <type>int</type>)</function></literal>
        </entry>
        <entry><type>inet</type></entry>
        <entry>set netmask length for <type>inet</type> value</entry>
        <entry><literal>set_masklen('192.168.1.5/24', 16)</literal></entry>
        <entry><literal>192.168.1.5/16</literal></entry>
       </row>
       <row>
        <entry><literal><function>set_masklen(<type>cidr</type>, <type>int</type>)</function></literal></entry>
        <entry><type>cidr</type></entry>
        <entry>set netmask length for <type>cidr</type> value</entry>
        <entry><literal>set_masklen('192.168.1.0/24'::cidr, 16)</literal></entry>
        <entry><literal>192.168.0.0/16</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>text</primary>
         </indexterm>
         <literal><function>text(<type>inet</type>)</function></literal>
        </entry>
        <entry><type>text</type></entry>
        <entry>extract IP address and netmask length as text</entry>
        <entry><literal>text(inet '192.168.1.5')</literal></entry>
        <entry><literal>192.168.1.5/32</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>inet_same_family</primary>
         </indexterm>
         <literal><function>inet_same_family(<type>inet</type>, <type>inet</type>)</function></literal>
        </entry>
        <entry><type>boolean</type></entry>
        <entry>are the addresses from the same family?</entry>
        <entry><literal>inet_same_family('192.168.1.5/24', '::1')</literal></entry>
        <entry><literal>false</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>inet_merge</primary>
         </indexterm>
         <literal><function>inet_merge(<type>inet</type>, <type>inet</type>)</function></literal>
        </entry>
        <entry><type>cidr</type></entry>
        <entry>the smallest network which includes both of the given networks</entry>
        <entry><literal>inet_merge('192.168.1.5/24', '192.168.2.5/24')</literal></entry>
        <entry><literal>192.168.0.0/22</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

  <para>
   Any <type>cidr</type> value can be cast to <type>inet</type> implicitly
   or explicitly; therefore, the functions shown above 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.
   In addition,
   you can cast a text value to <type>inet</type> or <type>cidr</type>
   using normal casting syntax: for example,
   <literal>inet(<replaceable>expression</replaceable>)</literal> or
   <literal><replaceable>colname</replaceable>::cidr</literal>.
  </para>

  <para>
   <xref linkend="macaddr-functions-table"/> shows the functions
   available for use with the <type>macaddr</type> type.  The function
   <literal><function>trunc(<type>macaddr</type>)</function></literal> returns a MAC
   address with the last 3 bytes set to zero.  This can be used to
   associate the remaining prefix with a manufacturer.
  </para>

    <table id="macaddr-functions-table">
     <title><type>macaddr</type> Functions</title>
     <tgroup cols="5">
      <thead>
       <row>
        <entry>Function</entry>
        <entry>Return Type</entry>
        <entry>Description</entry>
        <entry>Example</entry>
        <entry>Result</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry>
         <indexterm>
          <primary>trunc</primary>
         </indexterm>
         <literal><function>trunc(<type>macaddr</type>)</function></literal>
        </entry>
        <entry><type>macaddr</type></entry>
        <entry>set last 3 bytes to zero</entry>
        <entry><literal>trunc(macaddr '12:34:56:78:90:ab')</literal></entry>
        <entry><literal>12:34:56:00:00:00</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    The <type>macaddr</type> type also supports the standard relational
    operators (<literal>&gt;</literal>, <literal>&lt;=</literal>, etc.) for
    lexicographical ordering, and the bitwise arithmetic operators
    (<literal>~</literal>, <literal>&amp;</literal> and <literal>|</literal>)
    for NOT, AND and OR.
   </para>

   <para>
   <xref linkend="macaddr8-functions-table"/> shows the functions
   available for use with the <type>macaddr8</type> type.  The function
   <literal><function>trunc(<type>macaddr8</type>)</function></literal> returns a MAC
   address with the last 5 bytes set to zero.  This can be used to
   associate the remaining prefix with a manufacturer.
  </para>

    <table id="macaddr8-functions-table">
     <title><type>macaddr8</type> Functions</title>
     <tgroup cols="5">
      <thead>
       <row>
        <entry>Function</entry>
        <entry>Return Type</entry>
        <entry>Description</entry>
        <entry>Example</entry>
        <entry>Result</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry>
         <indexterm>
          <primary>trunc</primary>
         </indexterm>
         <literal><function>trunc(<type>macaddr8</type>)</function></literal>
        </entry>
        <entry><type>macaddr8</type></entry>
        <entry>set last 5 bytes to zero</entry>
        <entry><literal>trunc(macaddr8 '12:34:56:78:90:ab:cd:ef')</literal></entry>
        <entry><literal>12:34:56:00:00:00:00:00</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>macaddr8_set7bit</primary>
         </indexterm>
         <literal><function>macaddr8_set7bit(<type>macaddr8</type>)</function></literal>
        </entry>
        <entry><type>macaddr8</type></entry>
        <entry>set 7th bit to one, also known as modified EUI-64, for inclusion in an IPv6 address</entry>
        <entry><literal>macaddr8_set7bit(macaddr8 '00:34:56:ab:cd:ef')</literal></entry>
        <entry><literal>02:34:56:ff:fe:ab:cd:ef</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    The <type>macaddr8</type> type also supports the standard relational
    operators (<literal>&gt;</literal>, <literal>&lt;=</literal>, etc.) for
    ordering, and the bitwise arithmetic operators (<literal>~</literal>,
    <literal>&amp;</literal> and <literal>|</literal>) for NOT, AND and OR.
   </para>

  </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="5">
      <thead>
       <row>
        <entry>Operator</entry>
        <entry>Return Type</entry>
        <entry>Description</entry>
        <entry>Example</entry>
        <entry>Result</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry> <literal>@@</literal> </entry>
        <entry><type>boolean</type></entry>
        <entry><type>tsvector</type> matches <type>tsquery</type> ?</entry>
        <entry><literal>to_tsvector('fat cats ate rats') @@ to_tsquery('cat &amp; rat')</literal></entry>
        <entry><literal>t</literal></entry>
       </row>
       <row>
        <entry> <literal>@@@</literal> </entry>
        <entry><type>boolean</type></entry>
        <entry>deprecated synonym for <literal>@@</literal></entry>
        <entry><literal>to_tsvector('fat cats ate rats') @@@ to_tsquery('cat &amp; rat')</literal></entry>
        <entry><literal>t</literal></entry>
       </row>
       <row>
        <entry> <literal>||</literal> </entry>
        <entry><type>tsvector</type></entry>
        <entry>concatenate <type>tsvector</type>s</entry>
        <entry><literal>'a:1 b:2'::tsvector || 'c:1 d:2 b:3'::tsvector</literal></entry>
        <entry><literal>'a':1 'b':2,5 'c':3 'd':4</literal></entry>
       </row>
       <row>
        <entry> <literal>&amp;&amp;</literal> </entry>
        <entry><type>tsquery</type></entry>
        <entry>AND <type>tsquery</type>s together</entry>
        <entry><literal>'fat | rat'::tsquery &amp;&amp; 'cat'::tsquery</literal></entry>
        <entry><literal>( 'fat' | 'rat' ) &amp; 'cat'</literal></entry>
       </row>
       <row>
        <entry> <literal>||</literal> </entry>
        <entry><type>tsquery</type></entry>
        <entry>OR <type>tsquery</type>s together</entry>
        <entry><literal>'fat | rat'::tsquery || 'cat'::tsquery</literal></entry>
        <entry><literal>( 'fat' | 'rat' ) | 'cat'</literal></entry>
       </row>
       <row>
        <entry> <literal>!!</literal> </entry>
        <entry><type>tsquery</type></entry>
        <entry>negate a <type>tsquery</type></entry>
        <entry><literal>!! 'cat'::tsquery</literal></entry>
        <entry><literal>!'cat'</literal></entry>
       </row>
       <row>
        <entry> <literal>&lt;-&gt;</literal> </entry>
        <entry><type>tsquery</type></entry>
        <entry><type>tsquery</type> followed by <type>tsquery</type></entry>
        <entry><literal>to_tsquery('fat') &lt;-&gt; to_tsquery('rat')</literal></entry>
        <entry><literal>'fat' &lt;-&gt; 'rat'</literal></entry>
       </row>
       <row>
        <entry> <literal>@&gt;</literal> </entry>
        <entry><type>boolean</type></entry>
        <entry><type>tsquery</type> contains another ?</entry>
        <entry><literal>'cat'::tsquery @&gt; 'cat &amp; rat'::tsquery</literal></entry>
        <entry><literal>f</literal></entry>
       </row>
       <row>
        <entry> <literal>&lt;@</literal> </entry>
        <entry><type>boolean</type></entry>
        <entry><type>tsquery</type> is contained in ?</entry>
        <entry><literal>'cat'::tsquery &lt;@ 'cat &amp; rat'::tsquery</literal></entry>
        <entry><literal>t</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <note>
     <para>
      The <type>tsquery</type> containment operators consider only the lexemes
      listed in the two queries, ignoring the combining operators.
     </para>
    </note>

    <para>
     In addition to the operators shown in the table, the ordinary B-tree
     comparison operators (<literal>=</literal>, <literal>&lt;</literal>, etc) are defined
     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="5">
      <thead>
       <row>
        <entry>Function</entry>
        <entry>Return Type</entry>
        <entry>Description</entry>
        <entry>Example</entry>
        <entry>Result</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry>
         <indexterm>
          <primary>array_to_tsvector</primary>
         </indexterm>
         <literal><function>array_to_tsvector(<type>text[]</type>)</function></literal>
        </entry>
        <entry><type>tsvector</type></entry>
        <entry>convert array of lexemes to <type>tsvector</type></entry>
        <entry><literal>array_to_tsvector('{fat,cat,rat}'::text[])</literal></entry>
        <entry><literal>'cat' 'fat' 'rat'</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>get_current_ts_config</primary>
         </indexterm>
         <literal><function>get_current_ts_config()</function></literal>
        </entry>
        <entry><type>regconfig</type></entry>
        <entry>get default text search configuration</entry>
        <entry><literal>get_current_ts_config()</literal></entry>
        <entry><literal>english</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>length</primary>
         </indexterm>
         <literal><function>length(<type>tsvector</type>)</function></literal>
        </entry>
        <entry><type>integer</type></entry>
        <entry>number of lexemes in <type>tsvector</type></entry>
        <entry><literal>length('fat:2,4 cat:3 rat:5A'::tsvector)</literal></entry>
        <entry><literal>3</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>numnode</primary>
         </indexterm>
         <literal><function>numnode(<type>tsquery</type>)</function></literal>
        </entry>
        <entry><type>integer</type></entry>
        <entry>number of lexemes plus operators in <type>tsquery</type></entry>
        <entry><literal> numnode('(fat &amp; rat) | cat'::tsquery)</literal></entry>
        <entry><literal>5</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>plainto_tsquery</primary>
         </indexterm>
         <literal><function>plainto_tsquery(<optional> <replaceable class="parameter">config</replaceable> <type>regconfig</type> , </optional> <replaceable class="parameter">query</replaceable> <type>text</type>)</function></literal>
        </entry>
        <entry><type>tsquery</type></entry>
        <entry>produce <type>tsquery</type> ignoring punctuation</entry>
        <entry><literal>plainto_tsquery('english', 'The Fat Rats')</literal></entry>
        <entry><literal>'fat' &amp; 'rat'</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>phraseto_tsquery</primary>
         </indexterm>
         <literal><function>phraseto_tsquery(<optional> <replaceable class="parameter">config</replaceable> <type>regconfig</type> , </optional> <replaceable class="parameter">query</replaceable> <type>text</type>)</function></literal>
        </entry>
        <entry><type>tsquery</type></entry>
        <entry>produce <type>tsquery</type> that searches for a phrase,
         ignoring punctuation</entry>
        <entry><literal>phraseto_tsquery('english', 'The Fat Rats')</literal></entry>
        <entry><literal>'fat' &lt;-&gt; 'rat'</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>websearch_to_tsquery</primary>
         </indexterm>
          <literal><function>websearch_to_tsquery(<optional> <replaceable class="parameter">config</replaceable> <type>regconfig</type> , </optional> <replaceable class="parameter">query</replaceable> <type>text</type>)</function></literal>
         </entry>
        <entry><type>tsquery</type></entry>
        <entry>produce <type>tsquery</type> from a web search style query</entry>
        <entry><literal>websearch_to_tsquery('english', '"fat rat" or rat')</literal></entry>
        <entry><literal>'fat' &lt;-&gt; 'rat' | 'rat'</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>querytree</primary>
         </indexterm>
         <literal><function>querytree(<replaceable class="parameter">query</replaceable> <type>tsquery</type>)</function></literal>
        </entry>
        <entry><type>text</type></entry>
        <entry>get indexable part of a <type>tsquery</type></entry>
        <entry><literal>querytree('foo &amp; ! bar'::tsquery)</literal></entry>
        <entry><literal>'foo'</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>setweight</primary>
         </indexterm>
         <literal><function>setweight(<replaceable class="parameter">vector</replaceable> <type>tsvector</type>, <replaceable class="parameter">weight</replaceable> <type>"char"</type>)</function></literal>
        </entry>
        <entry><type>tsvector</type></entry>
        <entry>assign <replaceable class="parameter">weight</replaceable> to each element of <replaceable class="parameter">vector</replaceable></entry>
        <entry><literal>setweight('fat:2,4 cat:3 rat:5B'::tsvector, 'A')</literal></entry>
        <entry><literal>'cat':3A 'fat':2A,4A 'rat':5A</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>setweight</primary>
          <secondary>setweight for specific lexeme(s)</secondary>
         </indexterm>
         <literal><function>setweight(<replaceable class="parameter">vector</replaceable> <type>tsvector</type>, <replaceable class="parameter">weight</replaceable> <type>"char"</type>, <replaceable class="parameter">lexemes</replaceable> <type>text[]</type>)</function></literal>
        </entry>
        <entry><type>tsvector</type></entry>
        <entry>assign <replaceable class="parameter">weight</replaceable> to elements of <replaceable class="parameter">vector</replaceable> that are listed in <replaceable class="parameter">lexemes</replaceable></entry>
        <entry><literal>setweight('fat:2,4 cat:3 rat:5B'::tsvector, 'A', '{cat,rat}')</literal></entry>
        <entry><literal>'cat':3A 'fat':2,4 'rat':5A</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>strip</primary>
         </indexterm>
         <literal><function>strip(<type>tsvector</type>)</function></literal>
        </entry>
        <entry><type>tsvector</type></entry>
        <entry>remove positions and weights from <type>tsvector</type></entry>
        <entry><literal>strip('fat:2,4 cat:3 rat:5A'::tsvector)</literal></entry>
        <entry><literal>'cat' 'fat' 'rat'</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>to_tsquery</primary>
         </indexterm>
         <literal><function>to_tsquery(<optional> <replaceable class="parameter">config</replaceable> <type>regconfig</type> , </optional> <replaceable class="parameter">query</replaceable> <type>text</type>)</function></literal>
        </entry>
        <entry><type>tsquery</type></entry>
        <entry>normalize words and convert to <type>tsquery</type></entry>
        <entry><literal>to_tsquery('english', 'The &amp; Fat &amp; Rats')</literal></entry>
        <entry><literal>'fat' &amp; 'rat'</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>to_tsvector</primary>
         </indexterm>
         <literal><function>to_tsvector(<optional> <replaceable class="parameter">config</replaceable> <type>regconfig</type> , </optional> <replaceable class="parameter">document</replaceable> <type>text</type>)</function></literal>
        </entry>
        <entry><type>tsvector</type></entry>
        <entry>reduce document text to <type>tsvector</type></entry>
        <entry><literal>to_tsvector('english', 'The Fat Rats')</literal></entry>
        <entry><literal>'fat':2 'rat':3</literal></entry>
       </row>
       <row>
        <entry>
         <literal><function>to_tsvector(<optional> <replaceable class="parameter">config</replaceable> <type>regconfig</type> , </optional> <replaceable class="parameter">document</replaceable> <type>json(b)</type>)</function></literal>
        </entry>
        <entry><type>tsvector</type></entry>
        <entry>
          reduce each string value in the document to a <type>tsvector</type>, and then
          concatenate those in document order to produce a single <type>tsvector</type>
        </entry>
        <entry><literal>to_tsvector('english', '{"a": "The Fat Rats"}'::json)</literal></entry>
        <entry><literal>'fat':2 'rat':3</literal></entry>
       </row>
       <row>
        <entry>
         <literal><function>json(b)_to_tsvector(<optional> <replaceable class="parameter">config</replaceable> <type>regconfig</type>,
         </optional> <replaceable class="parameter">document</replaceable> <type>json(b)</type>,
         <replaceable class="parameter">filter</replaceable> <type>json(b)</type>)</function></literal>
        </entry>
        <entry><type>tsvector</type></entry>
        <entry>
          reduce each value in the document, specified by <replaceable class="parameter">filter</replaceable> to a <type>tsvector</type>,
          and then concatenate those in document order to produce a single <type>tsvector</type>.
          <replaceable class="parameter">filter</replaceable> is a <type>jsonb</type> array, that enumerates what kind of elements need to be included
          into the resulting <type>tsvector</type>. Possible values for <replaceable class="parameter">filter</replaceable> are
          <literal>"string"</literal> (to include all string values), <literal>"numeric"</literal> (to include all numeric values in the string format),
          <literal>"boolean"</literal> (to include all Boolean values in the string format <literal>"true"</literal>/<literal>"false"</literal>),
          <literal>"key"</literal> (to include all keys) or <literal>"all"</literal> (to include all above). These values
          can be combined together to include, e.g. all string and numeric values.
        </entry>
        <entry><literal>json_to_tsvector('english', '{"a": "The Fat Rats", "b": 123}'::json, '["string", "numeric"]')</literal></entry>
        <entry><literal>'123':5 'fat':2 'rat':3</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>ts_delete</primary>
         </indexterm>
         <literal><function>ts_delete(<replaceable class="parameter">vector</replaceable> <type>tsvector</type>, <replaceable class="parameter">lexeme</replaceable> <type>text</type>)</function></literal>
        </entry>
        <entry><type>tsvector</type></entry>
        <entry>remove given <replaceable class="parameter">lexeme</replaceable> from <replaceable class="parameter">vector</replaceable></entry>
        <entry><literal>ts_delete('fat:2,4 cat:3 rat:5A'::tsvector, 'fat')</literal></entry>
        <entry><literal>'cat':3 'rat':5A</literal></entry>
       </row>
       <row>
        <entry>
         <!-- previous indexterm entry covers this too -->
         <literal><function>ts_delete(<replaceable class="parameter">vector</replaceable> <type>tsvector</type>, <replaceable class="parameter">lexemes</replaceable> <type>text[]</type>)</function></literal>
        </entry>
        <entry><type>tsvector</type></entry>
        <entry>remove any occurrence of lexemes in <replaceable class="parameter">lexemes</replaceable> from <replaceable class="parameter">vector</replaceable></entry>
        <entry><literal>ts_delete('fat:2,4 cat:3 rat:5A'::tsvector, ARRAY['fat','rat'])</literal></entry>
        <entry><literal>'cat':3</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>ts_filter</primary>
         </indexterm>
         <literal><function>ts_filter(<replaceable class="parameter">vector</replaceable> <type>tsvector</type>, <replaceable class="parameter">weights</replaceable> <type>"char"[]</type>)</function></literal>
        </entry>
        <entry><type>tsvector</type></entry>
        <entry>select only elements with given <replaceable class="parameter">weights</replaceable> from <replaceable class="parameter">vector</replaceable></entry>
        <entry><literal>ts_filter('fat:2,4 cat:3b rat:5A'::tsvector, '{a,b}')</literal></entry>
        <entry><literal>'cat':3B 'rat':5A</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>ts_headline</primary>
         </indexterm>
         <literal><function>ts_headline(<optional> <replaceable class="parameter">config</replaceable> <type>regconfig</type>, </optional> <replaceable class="parameter">document</replaceable> <type>text</type>, <replaceable class="parameter">query</replaceable> <type>tsquery</type> <optional>, <replaceable class="parameter">options</replaceable> <type>text</type> </optional>)</function></literal>
        </entry>
        <entry><type>text</type></entry>
        <entry>display a query match</entry>
        <entry><literal>ts_headline('x y z', 'z'::tsquery)</literal></entry>
        <entry><literal>x y &lt;b&gt;z&lt;/b&gt;</literal></entry>
       </row>
       <row>
        <entry>
         <literal><function>ts_headline(<optional> <replaceable class="parameter">config</replaceable> <type>regconfig</type>, </optional> <replaceable class="parameter">document</replaceable> <type>json(b)</type>, <replaceable class="parameter">query</replaceable> <type>tsquery</type> <optional>, <replaceable class="parameter">options</replaceable> <type>text</type> </optional>)</function></literal>
        </entry>
        <entry><type>text</type></entry>
        <entry>display a query match</entry>
        <entry><literal>ts_headline('{"a":"x y z"}'::json, 'z'::tsquery)</literal></entry>
        <entry><literal>{"a":"x y &lt;b&gt;z&lt;/b&gt;"}</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>ts_rank</primary>
         </indexterm>
         <literal><function>ts_rank(<optional> <replaceable class="parameter">weights</replaceable> <type>float4[]</type>, </optional> <replaceable class="parameter">vector</replaceable> <type>tsvector</type>, <replaceable class="parameter">query</replaceable> <type>tsquery</type> <optional>, <replaceable class="parameter">normalization</replaceable> <type>integer</type> </optional>)</function></literal>
        </entry>
        <entry><type>float4</type></entry>
        <entry>rank document for query</entry>
        <entry><literal>ts_rank(textsearch, query)</literal></entry>
        <entry><literal>0.818</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>ts_rank_cd</primary>
         </indexterm>
         <literal><function>ts_rank_cd(<optional> <replaceable class="parameter">weights</replaceable> <type>float4[]</type>, </optional> <replaceable class="parameter">vector</replaceable> <type>tsvector</type>, <replaceable class="parameter">query</replaceable> <type>tsquery</type> <optional>, <replaceable class="parameter">normalization</replaceable> <type>integer</type> </optional>)</function></literal>
        </entry>
        <entry><type>float4</type></entry>
        <entry>rank document for query using cover density</entry>
        <entry><literal>ts_rank_cd('{0.1, 0.2, 0.4, 1.0}', textsearch, query)</literal></entry>
        <entry><literal>2.01317</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>ts_rewrite</primary>
         </indexterm>
         <literal><function>ts_rewrite(<replaceable class="parameter">query</replaceable> <type>tsquery</type>, <replaceable class="parameter">target</replaceable> <type>tsquery</type>, <replaceable class="parameter">substitute</replaceable> <type>tsquery</type>)</function></literal>
        </entry>
        <entry><type>tsquery</type></entry>
        <entry>replace <replaceable>target</replaceable> with <replaceable>substitute</replaceable>
         within query</entry>
        <entry><literal>ts_rewrite('a &amp; b'::tsquery, 'a'::tsquery, 'foo|bar'::tsquery)</literal></entry>
        <entry><literal>'b' &amp; ( 'foo' | 'bar' )</literal></entry>
       </row>
       <row>
        <entry><literal><function>ts_rewrite(<replaceable class="parameter">query</replaceable> <type>tsquery</type>, <replaceable class="parameter">select</replaceable> <type>text</type>)</function></literal></entry>
        <entry><type>tsquery</type></entry>
        <entry>replace using targets and substitutes from a <command>SELECT</command> command</entry>
        <entry><literal>SELECT ts_rewrite('a &amp; b'::tsquery, 'SELECT t,s FROM aliases')</literal></entry>
        <entry><literal>'b' &amp; ( 'foo' | 'bar' )</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>tsquery_phrase</primary>
         </indexterm>
         <literal><function>tsquery_phrase(<replaceable class="parameter">query1</replaceable> <type>tsquery</type>, <replaceable class="parameter">query2</replaceable> <type>tsquery</type>)</function></literal>
        </entry>
        <entry><type>tsquery</type></entry>
        <entry>make query that searches for <replaceable>query1</replaceable> followed
         by <replaceable>query2</replaceable> (same as <literal>&lt;-&gt;</literal>
         operator)</entry>
        <entry><literal>tsquery_phrase(to_tsquery('fat'), to_tsquery('cat'))</literal></entry>
        <entry><literal>'fat' &lt;-&gt; 'cat'</literal></entry>
       </row>
       <row>
        <entry>
         <literal><function>tsquery_phrase(<replaceable class="parameter">query1</replaceable> <type>tsquery</type>, <replaceable class="parameter">query2</replaceable> <type>tsquery</type>, <replaceable class="parameter">distance</replaceable> <type>integer</type>)</function></literal>
        </entry>
        <entry><type>tsquery</type></entry>
        <entry>make query that searches for <replaceable>query1</replaceable> followed by
         <replaceable>query2</replaceable> at distance <replaceable>distance</replaceable></entry>
        <entry><literal>tsquery_phrase(to_tsquery('fat'), to_tsquery('cat'), 10)</literal></entry>
        <entry><literal>'fat' &lt;10&gt; 'cat'</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>tsvector_to_array</primary>
         </indexterm>
         <literal><function>tsvector_to_array(<type>tsvector</type>)</function></literal>
        </entry>
        <entry><type>text[]</type></entry>
        <entry>convert <type>tsvector</type> to array of lexemes</entry>
        <entry><literal>tsvector_to_array('fat:2,4 cat:3 rat:5A'::tsvector)</literal></entry>
        <entry><literal>{cat,fat,rat}</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>tsvector_update_trigger</primary>
         </indexterm>
         <literal><function>tsvector_update_trigger()</function></literal>
        </entry>
        <entry><type>trigger</type></entry>
        <entry>trigger function for automatic <type>tsvector</type> column update</entry>
        <entry><literal>CREATE TRIGGER ... tsvector_update_trigger(tsvcol, 'pg_catalog.swedish', title, body)</literal></entry>
        <entry><literal></literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>tsvector_update_trigger_column</primary>
         </indexterm>
         <literal><function>tsvector_update_trigger_column()</function></literal>
        </entry>
        <entry><type>trigger</type></entry>
        <entry>trigger function for automatic <type>tsvector</type> column update</entry>
        <entry><literal>CREATE TRIGGER ... tsvector_update_trigger_column(tsvcol, configcol, title, body)</literal></entry>
        <entry><literal></literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>unnest</primary>
          <secondary>for tsvector</secondary>
         </indexterm>
         <literal><function>unnest(<type>tsvector</type>, OUT <replaceable class="parameter">lexeme</replaceable> <type>text</type>, OUT <replaceable class="parameter">positions</replaceable> <type>smallint[]</type>, OUT <replaceable class="parameter">weights</replaceable> <type>text</type>)</function></literal>
        </entry>
        <entry><type>setof record</type></entry>
        <entry>expand a <type>tsvector</type> to a set of rows</entry>
        <entry><literal>unnest('fat:2,4 cat:3 rat:5A'::tsvector)</literal></entry>
        <entry><literal>(cat,{3},{D}) ...</literal></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 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="5">
      <thead>
       <row>
        <entry>Function</entry>
        <entry>Return Type</entry>
        <entry>Description</entry>
        <entry>Example</entry>
        <entry>Result</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry>
         <indexterm>
          <primary>ts_debug</primary>
         </indexterm>
         <literal><function>ts_debug(<optional> <replaceable class="parameter">config</replaceable> <type>regconfig</type>, </optional> <replaceable class="parameter">document</replaceable> <type>text</type>, OUT <replaceable class="parameter">alias</replaceable> <type>text</type>, OUT <replaceable class="parameter">description</replaceable> <type>text</type>, OUT <replaceable class="parameter">token</replaceable> <type>text</type>, OUT <replaceable class="parameter">dictionaries</replaceable> <type>regdictionary[]</type>, OUT <replaceable class="parameter">dictionary</replaceable> <type>regdictionary</type>, OUT <replaceable class="parameter">lexemes</replaceable> <type>text[]</type>)</function></literal>
        </entry>
        <entry><type>setof record</type></entry>
        <entry>test a configuration</entry>
        <entry><literal>ts_debug('english', 'The Brightest supernovaes')</literal></entry>
        <entry><literal>(asciiword,"Word, all ASCII",The,{english_stem},english_stem,{}) ...</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>ts_lexize</primary>
         </indexterm>
         <literal><function>ts_lexize(<replaceable class="parameter">dict</replaceable> <type>regdictionary</type>, <replaceable class="parameter">token</replaceable> <type>text</type>)</function></literal>
        </entry>
        <entry><type>text[]</type></entry>
        <entry>test a dictionary</entry>
        <entry><literal>ts_lexize('english_stem', 'stars')</literal></entry>
        <entry><literal>{star}</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>ts_parse</primary>
         </indexterm>
         <literal><function>ts_parse(<replaceable class="parameter">parser_name</replaceable> <type>text</type>, <replaceable class="parameter">document</replaceable> <type>text</type>, OUT <replaceable class="parameter">tokid</replaceable> <type>integer</type>, OUT <replaceable class="parameter">token</replaceable> <type>text</type>)</function></literal>
        </entry>
        <entry><type>setof record</type></entry>
        <entry>test a parser</entry>
        <entry><literal>ts_parse('default', 'foo - bar')</literal></entry>
        <entry><literal>(1,foo) ...</literal></entry>
       </row>
       <row>
        <entry><literal><function>ts_parse(<replaceable class="parameter">parser_oid</replaceable> <type>oid</type>, <replaceable class="parameter">document</replaceable> <type>text</type>, OUT <replaceable class="parameter">tokid</replaceable> <type>integer</type>, OUT <replaceable class="parameter">token</replaceable> <type>text</type>)</function></literal></entry>
        <entry><type>setof record</type></entry>
        <entry>test a parser</entry>
        <entry><literal>ts_parse(3722, 'foo - bar')</literal></entry>
        <entry><literal>(1,foo) ...</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>ts_token_type</primary>
         </indexterm>
         <literal><function>ts_token_type(<replaceable class="parameter">parser_name</replaceable> <type>text</type>, OUT <replaceable class="parameter">tokid</replaceable> <type>integer</type>, OUT <replaceable class="parameter">alias</replaceable> <type>text</type>, OUT <replaceable class="parameter">description</replaceable> <type>text</type>)</function></literal>
        </entry>
        <entry><type>setof record</type></entry>
        <entry>get token types defined by parser</entry>
        <entry><literal>ts_token_type('default')</literal></entry>
        <entry><literal>(1,asciiword,"Word, all ASCII") ...</literal></entry>
       </row>
       <row>
        <entry><literal><function>ts_token_type(<replaceable class="parameter">parser_oid</replaceable> <type>oid</type>, OUT <replaceable class="parameter">tokid</replaceable> <type>integer</type>, OUT <replaceable class="parameter">alias</replaceable> <type>text</type>, OUT <replaceable class="parameter">description</replaceable> <type>text</type>)</function></literal></entry>
        <entry><type>setof record</type></entry>
        <entry>get token types defined by parser</entry>
        <entry><literal>ts_token_type(3722)</literal></entry>
        <entry><literal>(1,asciiword,"Word, all ASCII") ...</literal></entry>
       </row>
       <row>
        <entry>
         <indexterm>
          <primary>ts_stat</primary>
         </indexterm>
         <literal><function>ts_stat(<replaceable class="parameter">sqlquery</replaceable> <type>text</type>, <optional> <replaceable class="parameter">weights</replaceable> <type>text</type>, </optional> OUT <replaceable class="parameter">word</replaceable> <type>text</type>, OUT <replaceable class="parameter">ndoc</replaceable> <type>integer</type>, OUT <replaceable class="parameter">nentry</replaceable> <type>integer</type>)</function></literal>
        </entry>
        <entry><type>setof record</type></entry>
        <entry>get statistics of a <type>tsvector</type> column</entry>
        <entry><literal>ts_stat('SELECT vector from apod')</literal></entry>
        <entry><literal>(foo,10,15) ...</literal></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>
gen_random_uuid() returns uuid
</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>
 </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 are 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>(<replaceable>text</replaceable>)
</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> so that the resulting construct is 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>(<replaceable>xml</replaceable><optional>, ...</optional>)
</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>(name <replaceable>name</replaceable> <optional>, xmlattributes(<replaceable>value</replaceable> <optional>AS <replaceable>attname</replaceable></optional> <optional>, ... </optional>)</optional> <optional><replaceable>, content, ...</replaceable></optional>)
</synopsis>

    <para>
     The <function>xmlelement</function> expression produces an XML
     element with the given name, attributes, and 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>AS <replaceable>name</replaceable></optional> <optional>, ...</optional>)
</synopsis>

    <para>
     The <function>xmlforest</function> expression produces an XML
     forest (sequence) of elements using the given names and content.
    </para>

    <para>
     Examples:
<screen><![CDATA[
SELECT xmlforest('abc' AS foo, 123 AS bar);

          xmlforest
------------------------------
 <foo>abc</foo><bar>123</bar>


SELECT xmlforest(table_name, column_name)
FROM information_schema.columns
WHERE table_schema = 'pg_catalog';

                                         xmlforest
-------------------------------------------------------------------------------------------
 <table_name>pg_authid</table_name><column_name>rolname</column_name>
 <table_name>pg_authid</table_name><column_name>rolsuper</column_name>
 ...
]]></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>(name <replaceable>target</replaceable> <optional>, <replaceable>content</replaceable></optional>)
</synopsis>

    <para>
     The <function>xmlpi</function> expression creates an XML
     processing instruction.  The content, 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>(<replaceable>xml</replaceable>, version <replaceable>text</replaceable> | no value <optional>, standalone yes|no|no value</optional>)
</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>(<replaceable>xml</replaceable>)
</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>
<replaceable>xml</replaceable> IS DOCUMENT
</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>
<replaceable>xml</replaceable> IS NOT DOCUMENT
</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>(<replaceable>text</replaceable> PASSING <optional>BY { REF | VALUE }</optional> <replaceable>xml</replaceable> <optional>BY { REF | VALUE }</optional>)
</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"/>.
     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>(<replaceable>text</replaceable>)
<function>xml_is_well_formed_document</function>(<replaceable>text</replaceable>)
<function>xml_is_well_formed_content</function>(<replaceable>text</replaceable>)
</synopsis>

    <para>
     These functions check whether a <type>text</type> string is 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>(<replaceable>xpath</replaceable>, <replaceable>xml</replaceable> <optional>, <replaceable>nsarray</replaceable></optional>)
</synopsis>

    <para>
     The function <function>xpath</function> evaluates the XPath 1.0
     expression <replaceable>xpath</replaceable> (a <type>text</type> value)
     against the XML value
     <replaceable>xml</replaceable>.  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>(<replaceable>xpath</replaceable>, <replaceable>xml</replaceable> <optional>, <replaceable>nsarray</replaceable></optional>)
</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>XMLNAMESPACES(<replaceable>namespace uri</replaceable> AS <replaceable>namespace name</replaceable><optional>, ...</optional>), </optional>
          <replaceable>row_expression</replaceable> PASSING <optional>BY { REF | VALUE }</optional> <replaceable>document_expression</replaceable> <optional>BY { REF | VALUE }</optional>
          COLUMNS <replaceable>name</replaceable> { <replaceable>type</replaceable> <optional>PATH <replaceable>column_expression</replaceable></optional> <optional>DEFAULT <replaceable>default_expression</replaceable></optional> <optional>NOT NULL | NULL</optional>
                        | FOR ORDINALITY }
                   <optional>, ...</optional>
)
</synopsis>

    <para>
     The <function>xmltable</function> function produces a table based
     on the given XML value, an XPath filter to extract rows, and a
     set of column definitions.
    </para>

    <para>
     The optional <literal>XMLNAMESPACES</literal> clause is a comma-separated
     list of namespaces.  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 that is evaluated, passing the
     <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"/>.
     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 mandatory <literal>COLUMNS</literal> clause specifies the list
     of columns in the output table.
     Each entry describes a single column.
     See the syntax summary above for the format.
     The column name and type are required; 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 (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>
     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>
     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>
     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>
table_to_xml(tbl regclass, nulls boolean, tableforest boolean, targetns text)
query_to_xml(query text, nulls boolean, tableforest boolean, targetns text)
cursor_to_xml(cursor refcursor, count int, nulls boolean,
              tableforest boolean, targetns text)
</synopsis>
    The return type of each function is <type>xml</type>.
   </para>

   <para>
    <function>table_to_xml</function> maps the content of the named
    table, passed as parameter <parameter>tbl</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>
table_to_xmlschema(tbl regclass, nulls boolean, tableforest boolean, targetns text)
query_to_xmlschema(query text, nulls boolean, tableforest boolean, targetns text)
cursor_to_xmlschema(cursor refcursor, nulls boolean, tableforest boolean, targetns text)
</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>
table_to_xml_and_xmlschema(tbl regclass, nulls boolean, tableforest boolean, targetns text)
query_to_xml_and_xmlschema(query text, nulls boolean, tableforest boolean, targetns text)
</synopsis>
   </para>

   <para>
    In addition, the following functions are available to produce
    analogous mappings of entire schemas or the entire current
    database:
<synopsis>
schema_to_xml(schema name, nulls boolean, tableforest boolean, targetns text)
schema_to_xmlschema(schema name, nulls boolean, tableforest boolean, targetns text)
schema_to_xml_and_xmlschema(schema name, nulls boolean, tableforest boolean, targetns text)

database_to_xml(nulls boolean, tableforest boolean, targetns text)
database_to_xmlschema(nulls boolean, tableforest boolean, targetns text)
database_to_xml_and_xmlschema(nulls boolean, tableforest boolean, targetns text)
</synopsis>

    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>

   <figure 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>
   </figure>
  </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"/>).
  </para>

  <table id="functions-json-op-table">
     <title><type>json</type> and <type>jsonb</type> Operators</title>
     <tgroup cols="6">
      <thead>
       <row>
        <entry>Operator</entry>
        <entry>Right Operand Type</entry>
        <entry>Return type</entry>
        <entry>Description</entry>
        <entry>Example</entry>
        <entry>Example Result</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><literal>-&gt;</literal></entry>
        <entry><type>int</type></entry>
        <entry><type>json</type> or <type>jsonb</type></entry>
        <entry>Get JSON array element (indexed from zero, negative
        integers count from the end)</entry>
        <entry><literal>'[{"a":"foo"},{"b":"bar"},{"c":"baz"}]'::json-&gt;2</literal></entry>
        <entry><literal>{"c":"baz"}</literal></entry>
       </row>
       <row>
        <entry><literal>-&gt;</literal></entry>
        <entry><type>text</type></entry>
        <entry><type>json</type> or <type>jsonb</type></entry>
        <entry>Get JSON object field by key</entry>
        <entry><literal>'{"a": {"b":"foo"}}'::json-&gt;'a'</literal></entry>
        <entry><literal>{"b":"foo"}</literal></entry>
       </row>
        <row>
        <entry><literal>-&gt;&gt;</literal></entry>
        <entry><type>int</type></entry>
        <entry><type>text</type></entry>
        <entry>Get JSON array element as <type>text</type></entry>
        <entry><literal>'[1,2,3]'::json-&gt;&gt;2</literal></entry>
        <entry><literal>3</literal></entry>
       </row>
       <row>
        <entry><literal>-&gt;&gt;</literal></entry>
        <entry><type>text</type></entry>
        <entry><type>text</type></entry>
        <entry>Get JSON object field as <type>text</type></entry>
        <entry><literal>'{"a":1,"b":2}'::json-&gt;&gt;'b'</literal></entry>
        <entry><literal>2</literal></entry>
       </row>
       <row>
        <entry><literal>#&gt;</literal></entry>
        <entry><type>text[]</type></entry>
        <entry><type>json</type> or <type>jsonb</type></entry>
        <entry>Get JSON object at the specified path</entry>
        <entry><literal>'{"a": {"b":{"c": "foo"}}}'::json#&gt;'{a,b}'</literal></entry>
        <entry><literal>{"c": "foo"}</literal></entry>
       </row>
       <row>
        <entry><literal>#&gt;&gt;</literal></entry>
        <entry><type>text[]</type></entry>
        <entry><type>text</type></entry>
        <entry>Get JSON object at the specified path as <type>text</type></entry>
        <entry><literal>'{"a":[1,2,3],"b":[4,5,6]}'::json#&gt;&gt;'{a,2}'</literal></entry>
        <entry><literal>3</literal></entry>
       </row>
      </tbody>
     </tgroup>
   </table>

  <note>
   <para>
    There are parallel variants of these operators for both the
    <type>json</type> and <type>jsonb</type> types.
    The field/element/path extraction operators
    return the same type as their left-hand input (either <type>json</type>
    or <type>jsonb</type>), except for those specified as
    returning <type>text</type>, which coerce the value to text.
    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 element exists.  The
    field/element/path extraction operators that accept integer JSON
    array subscripts all support negative subscripting from the end of
    arrays.
   </para>
  </note>
  <para>
   The standard comparison operators shown in  <xref
   linkend="functions-comparison-op-table"/> are available for
   <type>jsonb</type>, but not for <type>json</type>. They follow the
   ordering rules for B-tree operations outlined at <xref
   linkend="json-indexing"/>.
  </para>
  <para>
   Some further operators also exist only for <type>jsonb</type>, as shown
   in <xref linkend="functions-jsonb-op-table"/>.
   Many of these operators can be indexed by
   <type>jsonb</type> operator classes.  For a full description of
   <type>jsonb</type> containment and existence semantics, see <xref
   linkend="json-containment"/>.  <xref linkend="json-indexing"/>
   describes how these operators can be used to effectively index
   <type>jsonb</type> data.
  </para>
  <table id="functions-jsonb-op-table">
     <title>Additional <type>jsonb</type> Operators</title>
     <tgroup cols="4">
      <thead>
       <row>
        <entry>Operator</entry>
        <entry>Right Operand Type</entry>
        <entry>Description</entry>
        <entry>Example</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><literal>@&gt;</literal></entry>
        <entry><type>jsonb</type></entry>
        <entry>Does the left JSON value contain the right JSON
        path/value entries at the top level?</entry>
        <entry><literal>'{"a":1, "b":2}'::jsonb &#64;&gt; '{"b":2}'::jsonb</literal></entry>
       </row>
       <row>
        <entry><literal>&lt;@</literal></entry>
        <entry><type>jsonb</type></entry>
        <entry>Are the left JSON path/value entries contained at the top level within
        the right JSON value?</entry>
        <entry><literal>'{"b":2}'::jsonb &lt;@ '{"a":1, "b":2}'::jsonb</literal></entry>
       </row>
       <row>
        <entry><literal>?</literal></entry>
        <entry><type>text</type></entry>
        <entry>Does the <emphasis>string</emphasis> exist as a top-level
        key within the JSON value?</entry>
        <entry><literal>'{"a":1, "b":2}'::jsonb ? 'b'</literal></entry>
       </row>
       <row>
        <entry><literal>?|</literal></entry>
        <entry><type>text[]</type></entry>
        <entry>Do any of these array <emphasis>strings</emphasis>
        exist as top-level keys?</entry>
        <entry><literal>'{"a":1, "b":2, "c":3}'::jsonb ?| array['b', 'c']</literal></entry>
       </row>
       <row>
        <entry><literal>?&amp;</literal></entry>
        <entry><type>text[]</type></entry>
        <entry>Do all of these array <emphasis>strings</emphasis> exist
        as top-level keys?</entry>
        <entry><literal>'["a", "b"]'::jsonb ?&amp; array['a', 'b']</literal></entry>
       </row>
       <row>
        <entry><literal>||</literal></entry>
        <entry><type>jsonb</type></entry>
        <entry>Concatenate two <type>jsonb</type> values into a new <type>jsonb</type> value</entry>
        <entry><literal>'["a", "b"]'::jsonb || '["c", "d"]'::jsonb</literal></entry>
       </row>
       <row>
        <entry><literal>-</literal></entry>
        <entry><type>text</type></entry>
        <entry>Delete key/value pair or <emphasis>string</emphasis>
        element from left operand.  Key/value pairs are matched based
        on their key value.</entry>
        <entry><literal>'{"a": "b"}'::jsonb - 'a' </literal></entry>
       </row>
       <row>
        <entry><literal>-</literal></entry>
        <entry><type>text[]</type></entry>
        <entry>Delete multiple key/value pairs or <emphasis>string</emphasis>
        elements from left operand.  Key/value pairs are matched based
        on their key value.</entry>
        <entry><literal>'{"a": "b", "c": "d"}'::jsonb - '{a,c}'::text[] </literal></entry>
       </row>
       <row>
        <entry><literal>-</literal></entry>
        <entry><type>integer</type></entry>
        <entry>Delete the array element with specified index (Negative
        integers count from the end).  Throws an error if top level
        container is not an array.</entry>
        <entry><literal>'["a", "b"]'::jsonb - 1 </literal></entry>
       </row>
       <row>
        <entry><literal>#-</literal></entry>
        <entry><type>text[]</type></entry>
        <entry>Delete the field or element with specified path (for
        JSON arrays, negative integers count from the end)</entry>
        <entry><literal>'["a", {"b":1}]'::jsonb #- '{1,b}'</literal></entry>
       </row>
       <row>
        <entry><literal>@?</literal></entry>
        <entry><type>jsonpath</type></entry>
        <entry>Does JSON path return any item for the specified JSON value?</entry>
        <entry><literal>'{"a":[1,2,3,4,5]}'::jsonb @? '$.a[*] ? (@ > 2)'</literal></entry>
       </row>
       <row>
        <entry><literal>@@</literal></entry>
        <entry><type>jsonpath</type></entry>
        <entry>Returns the result of JSON path predicate check for the specified JSON value.
        Only the first item of the result is taken into account.  If the
        result is not Boolean, then <literal>null</literal> is returned.</entry>
        <entry><literal>'{"a":[1,2,3,4,5]}'::jsonb @@ '$.a[*] > 2'</literal></entry>
       </row>
      </tbody>
     </tgroup>
   </table>

  <note>
   <para>
    The <literal>||</literal> operator concatenates the elements at the top level of
    each of its operands. It does not operate recursively. For example, if
    both operands are objects with a common key field name, the value of the
    field in the result will just be the value from the right hand operand.
   </para>
  </note>

  <note>
   <para>
    The <literal>@?</literal> and <literal>@@</literal> operators suppress
    the following errors: lacking object field or array element, unexpected
    JSON item type, datetime and numeric errors.
    This behavior might be helpful while searching over JSON document
    collections of varying structure.
   </para>
  </note>

  <para>
   <xref linkend="functions-json-creation-table"/> shows the functions that are
   available for creating <type>json</type> and <type>jsonb</type> values.
   (There are no equivalent functions for <type>jsonb</type>, of the <literal>row_to_json</literal>
   and <literal>array_to_json</literal> functions. However, the <literal>to_jsonb</literal>
   function supplies much the same functionality as these functions would.)
  </para>

  <indexterm>
   <primary>to_json</primary>
  </indexterm>
  <indexterm>
   <primary>array_to_json</primary>
  </indexterm>
  <indexterm>
   <primary>row_to_json</primary>
  </indexterm>
  <indexterm>
   <primary>json_build_array</primary>
  </indexterm>
  <indexterm>
   <primary>json_build_object</primary>
  </indexterm>
  <indexterm>
   <primary>json_object</primary>
  </indexterm>
  <indexterm>
   <primary>to_jsonb</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_build_array</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_build_object</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_object</primary>
  </indexterm>

  <table id="functions-json-creation-table">
    <title>JSON Creation Functions</title>
    <tgroup cols="4">
     <thead>
      <row>
       <entry>Function</entry>
       <entry>Description</entry>
       <entry>Example</entry>
       <entry>Example Result</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry><para><literal>to_json(anyelement)</literal>
          </para><para><literal>to_jsonb(anyelement)</literal>
       </para></entry>
       <entry>
         Returns the value as <type>json</type> or <type>jsonb</type>.
         Arrays and composites are converted
         (recursively) to arrays and objects; otherwise, if there is a cast
         from the type to <type>json</type>, the cast function will be used to
         perform the conversion; otherwise, a scalar value is produced.
         For any scalar type other than a number, a Boolean, or a null value,
         the text representation will be used, in such a fashion that it is a
         valid <type>json</type> or <type>jsonb</type> value.
       </entry>
       <entry><literal>to_json('Fred said "Hi."'::text)</literal></entry>
       <entry><literal>"Fred said \"Hi.\""</literal></entry>
      </row>
      <row>
       <entry>
         <literal>array_to_json(anyarray [, pretty_bool])</literal>
       </entry>
       <entry>
         Returns the array as a JSON array. A PostgreSQL multidimensional array
         becomes a JSON array of arrays. Line feeds will be added between
         dimension-1 elements if <parameter>pretty_bool</parameter> is true.
       </entry>
       <entry><literal>array_to_json('{{1,5},{99,100}}'::int[])</literal></entry>
       <entry><literal>[[1,5],[99,100]]</literal></entry>
      </row>
      <row>
       <entry>
         <literal>row_to_json(record [, pretty_bool])</literal>
       </entry>
       <entry>
         Returns the row as a JSON object. Line feeds will be added between
         level-1 elements if <parameter>pretty_bool</parameter> is true.
       </entry>
       <entry><literal>row_to_json(row(1,'foo'))</literal></entry>
       <entry><literal>{"f1":1,"f2":"foo"}</literal></entry>
      </row>
      <row>
       <entry><para><literal>json_build_array(VARIADIC "any")</literal>
          </para><para><literal>jsonb_build_array(VARIADIC "any")</literal>
       </para></entry>
       <entry>
         Builds a possibly-heterogeneously-typed JSON array out of a variadic
         argument list.
       </entry>
       <entry><literal>json_build_array(1,2,'3',4,5)</literal></entry>
       <entry><literal>[1, 2, "3", 4, 5]</literal></entry>
      </row>
      <row>
       <entry><para><literal>json_build_object(VARIADIC "any")</literal>
          </para><para><literal>jsonb_build_object(VARIADIC "any")</literal>
       </para></entry>
       <entry>
         Builds a JSON object out of a variadic argument list.  By
         convention, the argument list consists of alternating
         keys and values.
       </entry>
       <entry><literal>json_build_object('foo',1,'bar',2)</literal></entry>
       <entry><literal>{"foo": 1, "bar": 2}</literal></entry>
      </row>
      <row>
       <entry><para><literal>json_object(text[])</literal>
          </para><para><literal>jsonb_object(text[])</literal>
       </para></entry>
       <entry>
         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.
       </entry>
       <entry><para><literal>json_object('{a, 1, b, "def", c, 3.5}')</literal></para>
        <para><literal>json_object('{{a, 1},{b, "def"},{c, 3.5}}')</literal></para></entry>
       <entry><literal>{"a": "1", "b": "def", "c": "3.5"}</literal></entry>
      </row>
      <row>
       <entry><para><literal>json_object(keys text[], values text[])</literal>
          </para><para><literal>jsonb_object(keys text[], values text[])</literal>
       </para></entry>
       <entry>
         This form of <function>json_object</function> takes keys and values pairwise from two separate
         arrays. In all other respects it is identical to the one-argument form.
       </entry>
       <entry><literal>json_object('{a, b}', '{1,2}')</literal></entry>
       <entry><literal>{"a": "1", "b": "2"}</literal></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <note>
    <para>
     <function>array_to_json</function> and <function>row_to_json</function> have the same
     behavior as <function>to_json</function> except for offering a pretty-printing
     option.  The behavior described for <function>to_json</function> likewise applies
     to each individual value converted by the other JSON creation functions.
    </para>
  </note>

  <note>
    <para>
     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>
  </note>

  <para>
   <xref linkend="functions-json-processing-table"/> shows the functions that
   are available for processing <type>json</type> and <type>jsonb</type> values.
  </para>

  <indexterm>
   <primary>json_array_length</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_array_length</primary>
  </indexterm>
  <indexterm>
   <primary>json_each</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_each</primary>
  </indexterm>
  <indexterm>
   <primary>json_each_text</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_each_text</primary>
  </indexterm>
  <indexterm>
   <primary>json_extract_path</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_extract_path</primary>
  </indexterm>
  <indexterm>
   <primary>json_extract_path_text</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_extract_path_text</primary>
  </indexterm>
  <indexterm>
   <primary>json_object_keys</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_object_keys</primary>
  </indexterm>
  <indexterm>
   <primary>json_populate_record</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_populate_record</primary>
  </indexterm>
  <indexterm>
   <primary>json_populate_recordset</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_populate_recordset</primary>
  </indexterm>
  <indexterm>
   <primary>json_array_elements</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_array_elements</primary>
  </indexterm>
  <indexterm>
   <primary>json_array_elements_text</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_array_elements_text</primary>
  </indexterm>
  <indexterm>
   <primary>json_typeof</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_typeof</primary>
  </indexterm>
  <indexterm>
   <primary>json_to_record</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_to_record</primary>
  </indexterm>
  <indexterm>
   <primary>json_to_recordset</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_to_recordset</primary>
  </indexterm>
  <indexterm>
   <primary>json_strip_nulls</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_strip_nulls</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_set</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_insert</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_pretty</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_path_exists</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_path_exists_tz</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_path_match</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_path_match_tz</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_path_query</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_path_query_tz</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_path_query_array</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_path_query_array_tz</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_path_query_first</primary>
  </indexterm>
  <indexterm>
   <primary>jsonb_path_query_first_tz</primary>
  </indexterm>

  <table id="functions-json-processing-table">
    <title>JSON Processing Functions</title>
    <tgroup cols="5">
     <thead>
      <row>
       <entry>Function</entry>
       <entry>Return Type</entry>
       <entry>Description</entry>
       <entry>Example</entry>
       <entry>Example Result</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry><para><literal>json_array_length(json)</literal>
         </para><para><literal>jsonb_array_length(jsonb)</literal>
       </para></entry>
       <entry><type>int</type></entry>
       <entry>
         Returns the number of elements in the outermost JSON array.
       </entry>
       <entry><literal>json_array_length('[1,2,3,{"f1":1,"f2":[5,6]},4]')</literal></entry>
       <entry><literal>5</literal></entry>
      </row>
      <row>
       <entry><para><literal>json_each(json)</literal>
         </para><para><literal>jsonb_each(jsonb)</literal>
       </para></entry>
       <entry><para><literal>setof key text, value json</literal>
         </para><para><literal>setof key text, value jsonb</literal>
       </para></entry>
       <entry>
         Expands the outermost JSON object into a set of key/value pairs.
       </entry>
       <entry><literal>select * from json_each('{"a":"foo", "b":"bar"}')</literal></entry>
       <entry>
<programlisting>
 key | value
-----+-------
 a   | "foo"
 b   | "bar"
</programlisting>
       </entry>
      </row>
      <row>
       <entry><para><literal>json_each_text(json)</literal>
         </para><para><literal>jsonb_each_text(jsonb)</literal>
       </para></entry>
       <entry><type>setof key text, value text</type></entry>
       <entry>
         Expands the outermost JSON object into a set of key/value pairs. The
         returned values will be of type <type>text</type>.
       </entry>
       <entry><literal>select * from json_each_text('{"a":"foo", "b":"bar"}')</literal></entry>
       <entry>
<programlisting>
 key | value
-----+-------
 a   | foo
 b   | bar
</programlisting>
       </entry>
      </row>
      <row>
       <entry><para><literal>json_extract_path(from_json json, VARIADIC path_elems text[])</literal>
        </para><para><literal>jsonb_extract_path(from_json jsonb, VARIADIC path_elems text[])</literal>
       </para></entry>
       <entry><para><type>json</type></para><para><type>jsonb</type>
       </para></entry>
       <entry>
         Returns JSON value pointed to by <replaceable>path_elems</replaceable>
         (equivalent to <literal>#&gt;</literal> operator).
       </entry>
       <entry><literal>json_extract_path('{"f2":{"f3":1},"f4":{"f5":99,"f6":"foo"}}','f4')</literal></entry>
       <entry><literal>{"f5":99,"f6":"foo"}</literal></entry>
      </row>
      <row>
       <entry><para><literal>json_extract_path_text(from_json json, VARIADIC path_elems text[])</literal>
         </para><para><literal>jsonb_extract_path_text(from_json jsonb, VARIADIC path_elems text[])</literal>
       </para></entry>
       <entry><type>text</type></entry>
       <entry>
         Returns JSON value pointed to by <replaceable>path_elems</replaceable>
         as <type>text</type>
         (equivalent to <literal>#&gt;&gt;</literal> operator).
       </entry>
       <entry><literal>json_extract_path_text('{"f2":{"f3":1},"f4":{"f5":99,"f6":"foo"}}','f4', 'f6')</literal></entry>
       <entry><literal>foo</literal></entry>
      </row>
      <row>
       <entry><para><literal>json_object_keys(json)</literal>
         </para><para><literal>jsonb_object_keys(jsonb)</literal>
       </para></entry>
       <entry><type>setof text</type></entry>
       <entry>
          Returns set of keys in the outermost JSON object.
       </entry>
       <entry><literal>json_object_keys('{"f1":"abc","f2":{"f3":"a", "f4":"b"}}')</literal></entry>
       <entry>
<programlisting>
 json_object_keys
------------------
 f1
 f2
</programlisting>
       </entry>
      </row>
      <row>
       <entry><para><literal>json_populate_record(base anyelement, from_json json)</literal>
         </para><para><literal>jsonb_populate_record(base anyelement, from_json jsonb)</literal>
       </para></entry>
       <entry><type>anyelement</type></entry>
       <entry>
         Expands the object in <replaceable>from_json</replaceable> to a row
         whose columns match the record type defined by <replaceable>base</replaceable>
         (see note below).
       </entry>
       <entry><literal>select * from json_populate_record(null::myrowtype, '{"a": 1, "b": ["2", "a b"], "c": {"d": 4, "e": "a  b c"}}')</literal></entry>
       <entry>
<programlisting>
 a |   b       |      c
---+-----------+-------------
 1 | {2,"a b"} | (4,"a b c")
</programlisting>
       </entry>
      </row>
      <row>
       <entry><para><literal>json_populate_recordset(base anyelement, from_json json)</literal>
         </para><para><literal>jsonb_populate_recordset(base anyelement, from_json jsonb)</literal>
       </para></entry>
       <entry><type>setof anyelement</type></entry>
       <entry>
         Expands the outermost array of objects
         in <replaceable>from_json</replaceable> to a set of rows whose
         columns match the record type defined by <replaceable>base</replaceable> (see
         note below).
       </entry>
       <entry><literal>select * from json_populate_recordset(null::myrowtype, '[{"a":1,"b":2},{"a":3,"b":4}]')</literal></entry>
       <entry>
<programlisting>
 a | b
---+---
 1 | 2
 3 | 4
</programlisting>
       </entry>
      </row>
      <row>
       <entry><para><literal>json_array_elements(json)</literal>
         </para><para><literal>jsonb_array_elements(jsonb)</literal>
       </para></entry>
       <entry><para><type>setof json</type>
         </para><para><type>setof jsonb</type>
       </para></entry>
       <entry>
         Expands a JSON array to a set of JSON values.
       </entry>
       <entry><literal>select * from json_array_elements('[1,true, [2,false]]')</literal></entry>
       <entry>
<programlisting>
   value
-----------
 1
 true
 [2,false]
</programlisting>
       </entry>
      </row>
      <row>
       <entry><para><literal>json_array_elements_text(json)</literal>
         </para><para><literal>jsonb_array_elements_text(jsonb)</literal>
       </para></entry>
       <entry><type>setof text</type></entry>
       <entry>
         Expands a JSON array to a set of <type>text</type> values.
       </entry>
       <entry><literal>select * from json_array_elements_text('["foo", "bar"]')</literal></entry>
       <entry>
<programlisting>
   value
-----------
 foo
 bar
</programlisting>
       </entry>
      </row>
      <row>
       <entry><para><literal>json_typeof(json)</literal>
         </para><para><literal>jsonb_typeof(jsonb)</literal>
       </para></entry>
       <entry><type>text</type></entry>
       <entry>
         Returns the type of the outermost 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>.
       </entry>
       <entry><literal>json_typeof('-123.4')</literal></entry>
       <entry><literal>number</literal></entry>
      </row>
      <row>
       <entry><para><literal>json_to_record(json)</literal>
          </para><para><literal>jsonb_to_record(jsonb)</literal>
       </para></entry>
       <entry><type>record</type></entry>
       <entry>
         Builds an arbitrary record from a JSON object (see note below).  As
         with all functions returning <type>record</type>, the caller must
         explicitly define the structure of the record with an <literal>AS</literal>
         clause.
       </entry>
       <entry><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></entry>
       <entry>
<programlisting>
 a |    b    |    c    | d |       r
---+---------+---------+---+---------------
 1 | [1,2,3] | {1,2,3} |   | (123,"a b c")
</programlisting>
       </entry>
      </row>
      <row>
       <entry><para><literal>json_to_recordset(json)</literal>
         </para><para><literal>jsonb_to_recordset(jsonb)</literal>
       </para></entry>
       <entry><type>setof record</type></entry>
       <entry>
         Builds an arbitrary set of records from a JSON array of objects (see
         note below).  As with all functions returning <type>record</type>, the
         caller must explicitly define the structure of the record with
         an <literal>AS</literal> clause.
       </entry>
       <entry><literal>select * from json_to_recordset('[{"a":1,"b":"foo"},{"a":"2","c":"bar"}]') as x(a int, b text);</literal></entry>
       <entry>
<programlisting>
 a |  b
---+-----
 1 | foo
 2 |
</programlisting>
       </entry>
      </row>
      <row>
       <entry><para><literal>json_strip_nulls(from_json json)</literal>
         </para><para><literal>jsonb_strip_nulls(from_json jsonb)</literal>
       </para></entry>
       <entry><para><type>json</type></para><para><type>jsonb</type></para></entry>
       <entry>
         Returns <replaceable>from_json</replaceable>
         with all object fields that have null values omitted. Other null values
         are untouched.
       </entry>
       <entry><literal>json_strip_nulls('[{"f1":1,"f2":null},2,null,3]')</literal></entry>
       <entry><literal>[{"f1":1},2,null,3]</literal></entry>
       </row>
      <row>
       <entry><para><literal>jsonb_set(target jsonb, path text[], new_value jsonb <optional>, create_missing boolean</optional>)</literal>
         </para></entry>
       <entry><para><type>jsonb</type></para></entry>
       <entry>
         Returns <replaceable>target</replaceable>
         with the section designated by <replaceable>path</replaceable>
         replaced by <replaceable>new_value</replaceable>, or with
         <replaceable>new_value</replaceable> added if
         <replaceable>create_missing</replaceable> is true (default is
         <literal>true</literal>) and the item
         designated by <replaceable>path</replaceable> does not exist.
         As with the path oriented operators, negative integers that
         appear in <replaceable>path</replaceable> count from the end
         of JSON arrays.
       </entry>
       <entry><para><literal>jsonb_set('[{"f1":1,"f2":null},2,null,3]', '{0,f1}','[2,3,4]', false)</literal>
         </para><para><literal>jsonb_set('[{"f1":1,"f2":null},2]', '{0,f3}','[2,3,4]')</literal>
         </para></entry>
       <entry><para><literal>[{"f1":[2,3,4],"f2":null},2,null,3]</literal>
         </para><para><literal>[{"f1": 1, "f2": null, "f3": [2, 3, 4]}, 2]</literal>
        </para></entry>
       </row>
      <row>
       <entry>
           <para><literal>
           jsonb_insert(target jsonb, path text[], new_value jsonb <optional>, insert_after boolean</optional>)
           </literal></para>
       </entry>
       <entry><para><type>jsonb</type></para></entry>
       <entry>
         Returns <replaceable>target</replaceable> with
         <replaceable>new_value</replaceable> inserted. If
         <replaceable>target</replaceable> section designated by
         <replaceable>path</replaceable> is in a JSONB array,
         <replaceable>new_value</replaceable> will be inserted before target or
         after if <replaceable>insert_after</replaceable> is true (default is
         <literal>false</literal>). If <replaceable>target</replaceable> section
         designated by <replaceable>path</replaceable> is in JSONB object,
         <replaceable>new_value</replaceable> will be inserted only if
         <replaceable>target</replaceable> does not exist. As with the path
         oriented operators, negative integers that appear in
         <replaceable>path</replaceable> count from the end of JSON arrays.
       </entry>
       <entry>
           <para><literal>
               jsonb_insert('{"a": [0,1,2]}', '{a, 1}', '"new_value"')
           </literal></para>
           <para><literal>
               jsonb_insert('{"a": [0,1,2]}', '{a, 1}', '"new_value"', true)
           </literal></para>
       </entry>
       <entry><para><literal>{"a": [0, "new_value", 1, 2]}</literal>
         </para><para><literal>{"a": [0, 1, "new_value", 2]}</literal>
        </para></entry>
       </row>
      <row>
       <entry><para><literal>jsonb_pretty(from_json jsonb)</literal>
         </para></entry>
       <entry><para><type>text</type></para></entry>
       <entry>
         Returns <replaceable>from_json</replaceable>
         as indented JSON text.
       </entry>
       <entry><literal>jsonb_pretty('[{"f1":1,"f2":null},2,null,3]')</literal></entry>
       <entry>
<programlisting>
[
    {
        "f1": 1,
        "f2": null
    },
    2,
    null,
    3
]
</programlisting>
        </entry>
       </row>
       <row>
        <entry>
         <para><literal>
           jsonb_path_exists(target jsonb, path jsonpath <optional>, vars jsonb <optional>, silent bool</optional></optional>)
         </literal></para>
         <para><literal>
           jsonb_path_exists_tz(target jsonb, path jsonpath <optional>, vars jsonb <optional>, silent bool</optional></optional>)
         </literal></para>
        </entry>
        <entry><type>boolean</type></entry>
        <entry>
          Checks whether JSON path returns any item for the specified JSON
          value.
        </entry>
        <entry>
         <para><literal>
           jsonb_path_exists('{"a":[1,2,3,4,5]}', '$.a[*] ? (@ >= $min &amp;&amp; @ &lt;= $max)', '{"min":2,"max":4}')
         </literal></para>
        </entry>
        <entry>
          <para><literal>true</literal></para>
        </entry>
       </row>
       <row>
        <entry>
         <para><literal>
           jsonb_path_match(target jsonb, path jsonpath <optional>, vars jsonb <optional>, silent bool</optional></optional>)
         </literal></para>
         <para><literal>
           jsonb_path_match_tz(target jsonb, path jsonpath <optional>, vars jsonb <optional>, silent bool</optional></optional>)
         </literal></para>
        </entry>
        <entry><type>boolean</type></entry>
        <entry>
          Returns the result of JSON path predicate check for the specified JSON value.
          Only the first item of the result is taken into account.  If the
          result is not Boolean, then <literal>null</literal> is returned.
        </entry>
        <entry>
         <para><literal>
           jsonb_path_match('{"a":[1,2,3,4,5]}', 'exists($.a[*] ? (@ >= $min &amp;&amp; @ &lt;= $max))', '{"min":2,"max":4}')
        </literal></para>
        </entry>
        <entry>
          <para><literal>true</literal></para>
        </entry>
       </row>
       <row>
        <entry>
         <para><literal>
           jsonb_path_query(target jsonb, path jsonpath <optional>, vars jsonb <optional>, silent bool</optional></optional>)
         </literal></para>
         <para><literal>
           jsonb_path_query_tz(target jsonb, path jsonpath <optional>, vars jsonb <optional>, silent bool</optional></optional>)
         </literal></para>
        </entry>
        <entry><type>setof jsonb</type></entry>
        <entry>
          Gets all JSON items returned by JSON path for the specified JSON
          value.
        </entry>
        <entry>
         <para><literal>
           select * from jsonb_path_query('{"a":[1,2,3,4,5]}', '$.a[*] ? (@ >= $min &amp;&amp; @ &lt;= $max)', '{"min":2,"max":4}');
         </literal></para>
        </entry>
        <entry>
         <para>
<programlisting>
 jsonb_path_query
------------------
 2
 3
 4
</programlisting>
         </para>
        </entry>
       </row>
       <row>
        <entry>
         <para><literal>
           jsonb_path_query_array(target jsonb, path jsonpath <optional>, vars jsonb <optional>, silent bool</optional></optional>)
         </literal></para>
         <para><literal>
           jsonb_path_query_array_tz(target jsonb, path jsonpath <optional>, vars jsonb <optional>, silent bool</optional></optional>)
         </literal></para>
        </entry>
        <entry><type>jsonb</type></entry>
        <entry>
          Gets all JSON items returned by JSON path for the specified JSON
          value and wraps result into an array.
        </entry>
        <entry>
         <para><literal>
           jsonb_path_query_array('{"a":[1,2,3,4,5]}', '$.a[*] ? (@ >= $min &amp;&amp; @ &lt;= $max)', '{"min":2,"max":4}')
         </literal></para>
        </entry>
        <entry>
          <para><literal>[2, 3, 4]</literal></para>
        </entry>
       </row>
       <row>
        <entry>
         <para><literal>
           jsonb_path_query_first(target jsonb, path jsonpath <optional>, vars jsonb <optional>, silent bool</optional></optional>)
         </literal></para>
         <para><literal>
           jsonb_path_query_first_tz(target jsonb, path jsonpath <optional>, vars jsonb <optional>, silent bool</optional></optional>)
         </literal></para>
        </entry>
        <entry><type>jsonb</type></entry>
        <entry>
          Gets the first JSON item returned by JSON path for the specified JSON
          value.  Returns <literal>NULL</literal> on no results.
        </entry>
        <entry>
         <para><literal>
           jsonb_path_query_first('{"a":[1,2,3,4,5]}', '$.a[*] ? (@ >= $min &amp;&amp; @ &lt;= $max)', '{"min":2,"max":4}')
         </literal></para>
        </entry>
        <entry>
          <para><literal>2</literal></para>
        </entry>
       </row>
     </tbody>
    </tgroup>
   </table>

  <note>
    <para>
      Many of these functions and operators will convert Unicode escapes in
      JSON strings to the appropriate single character.  This is a non-issue
      if the input is type <type>jsonb</type>, because the conversion was already
      done; but for <type>json</type> input, this may result in throwing an error,
      as noted in <xref linkend="datatype-json"/>.
    </para>
  </note>

  <note>
   <para>
    The functions
    <function>json[b]_populate_record</function>,
    <function>json[b]_populate_recordset</function>,
    <function>json[b]_to_record</function> and
    <function>json[b]_to_recordset</function>
    operate on a JSON object, or array of objects, and extract the values
    associated with keys whose names match column names of the output row
    type.
    Object fields that do not correspond to any output column name are
    ignored, and output columns that do not match any object field will be
    filled with nulls.
    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 literal, 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 examples for these functions use constants, the typical use
    would be to reference a table in the <literal>FROM</literal> clause
    and use one of its <type>json</type> or <type>jsonb</type> columns
    as an argument to the function.  Extracted key values can then be
    referenced in other parts of the query, like <literal>WHERE</literal>
    clauses and target lists.  Extracting multiple values in this
    way can improve performance over extracting them separately with
    per-key operators.
   </para>
  </note>

  <note>
    <para>
      All the items of the <literal>path</literal> parameter of <literal>jsonb_set</literal>
      as well as <literal>jsonb_insert</literal> except the last item must be present
      in the <literal>target</literal>. If <literal>create_missing</literal> is false, all
      items of the <literal>path</literal> parameter of <literal>jsonb_set</literal> must be
      present. If these conditions are not met the <literal>target</literal> is
      returned unchanged.
    </para>
    <para>
      If the last path item is an object key, it will be created if it
      is absent and given the new value. If the last path item is an array
      index, if it is positive the item to set is found by counting from
      the left, and if negative by counting from the right - <literal>-1</literal>
      designates the rightmost element, and so on.
      If the item is out of the range -array_length .. array_length -1,
      and create_missing is true, the new value is added at the beginning
      of the array if the item is negative, and at the end of the array if
      it is positive.
    </para>
  </note>

  <note>
    <para>
      The <literal>json_typeof</literal> function's <literal>null</literal> return value
      should not be confused with a SQL NULL.  While
      calling <literal>json_typeof('null'::json)</literal> will
      return <literal>null</literal>, calling <literal>json_typeof(NULL::json)</literal>
      will return a SQL NULL.
    </para>
  </note>

  <note>
    <para>
      If the argument to <literal>json_strip_nulls</literal> contains duplicate
      field names in any object, the result could be semantically somewhat
      different, depending on the order in which they occur. This is not an
      issue for <literal>jsonb_strip_nulls</literal> since <type>jsonb</type> values never have
      duplicate object field names.
    </para>
  </note>

  <note>
   <para>
    The <literal>jsonb_path_*</literal> functions have optional
    <literal>vars</literal> and <literal>silent</literal> arguments.
   </para>
   <para>
    If the <parameter>vars</parameter> argument is specified, it provides an
    object containing named variables to be substituted into a
    <literal>jsonpath</literal> expression.
   </para>
   <para>
    If the <parameter>silent</parameter> argument is specified and has the
    <literal>true</literal> value, these functions suppress the same errors
    as the <literal>@?</literal> and <literal>@@</literal> operators.
   </para>
  </note>

  <note>
   <para>
    Some of the <literal>jsonb_path_*</literal> functions have the
    <literal>_tz</literal> suffix. These functions have been implemented to
    support comparison of date/time values that involves implicit
    timezone-aware casts. Since operations with time zones are not immutable,
    these functions are qualified as stable. Their counterparts without the
    suffix do not support such casts, so they are immutable and can be used for
    such use-cases as expression indexes
    (see <xref linkend="indexes-expressional"/>). There is no difference
    between these functions for other <type>jsonpath</type> operations.
   </para>
  </note>

  <para>
    See also <xref linkend="functions-aggregate"/> for the aggregate
    function <function>json_agg</function> which aggregates record
    values as JSON, and 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 SQL/JSON item is returned.
   Path expressions are written in the SQL/JSON path language
   and can also include arithmetic expressions and functions.
   Query functions treat the provided expression as a
   text string, so it must be enclosed in single quotes.
  </para>

  <para>
   A path expression consists of a sequence of elements allowed
   by the <type>jsonpath</type> data type.
   The path expression is evaluated from left to right, but
   you can use parentheses to change the order of operations.
   If the evaluation is successful, a sequence of SQL/JSON items
   (<firstterm>SQL/JSON sequence</firstterm>) 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 data to be queried (the
   <firstterm>context item</firstterm>), use the <literal>$</literal> sign
   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 the
   content of 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 for all the preceding JSON objects:
<programlisting>
'$.track.segments'
</programlisting>
  </para>

  <para>
   If the item to retrieve is an element of an array, you have
   to unnest this array using the <literal>[*]</literal> operator. For example,
   the following path will return 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. Note that the SQL/JSON arrays 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 an array size:
<programlisting>
'$.track.segments.size()'
</programlisting>
   For more examples of using <type>jsonpath</type> operators
   and methods within path expressions, see
   <xref linkend="functions-sqljson-path-operators"/>.
  </para>

  <para>
   When defining the path, you can also use one or more
   <firstterm>filter expressions</firstterm> that work similar 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 specified right after the path evaluation step
   to which they are applied. The result of this 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 filter expressions
   return <literal>true</literal>.
  </para>

  <para>
   Functions and operators that can be used in filter expressions are listed
   in <xref linkend="functions-sqljson-filter-ex-table"/>. The path
   evaluation result to be filtered is denoted by the <literal>@</literal>
   variable. To refer to a JSON element stored at a lower nesting level,
   add one or more accessor operators after <literal>@</literal>.
  </para>

  <para>
   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 time of segments with such values instead, you have to
   filter out irrelevant segments before returning the start time, 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 on the same nesting level, if
   required. For example, the following expression selects 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 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="jsonpath-regular-expressions">
    <title>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>

   <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>.  <xref
    linkend="functions-sqljson-filter-ex-table"/> shows the available filter
    expression elements.
   </para>

   <table id="functions-sqljson-op-table">
    <title><type>jsonpath</type> Operators and Methods</title>
     <tgroup cols="5">
      <thead>
       <row>
        <entry>Operator/Method</entry>
        <entry>Description</entry>
        <entry>Example JSON</entry>
        <entry>Example Query</entry>
        <entry>Result</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><literal>+</literal> (unary)</entry>
        <entry>Plus operator that iterates over the SQL/JSON sequence</entry>
        <entry><literal>{"x": [2.85, -14.7, -9.4]}</literal></entry>
        <entry><literal>+ $.x.floor()</literal></entry>
        <entry><literal>2, -15, -10</literal></entry>
       </row>
       <row>
        <entry><literal>-</literal> (unary)</entry>
        <entry>Minus operator that iterates over the SQL/JSON sequence</entry>
        <entry><literal>{"x": [2.85, -14.7, -9.4]}</literal></entry>
        <entry><literal>- $.x.floor()</literal></entry>
        <entry><literal>-2, 15, 10</literal></entry>
       </row>
       <row>
        <entry><literal>+</literal> (binary)</entry>
        <entry>Addition</entry>
        <entry><literal>[2]</literal></entry>
        <entry><literal>2 + $[0]</literal></entry>
        <entry><literal>4</literal></entry>
       </row>
       <row>
        <entry><literal>-</literal> (binary)</entry>
        <entry>Subtraction</entry>
        <entry><literal>[2]</literal></entry>
        <entry><literal>4 - $[0]</literal></entry>
        <entry><literal>2</literal></entry>
       </row>
       <row>
        <entry><literal>*</literal></entry>
        <entry>Multiplication</entry>
        <entry><literal>[4]</literal></entry>
        <entry><literal>2 * $[0]</literal></entry>
        <entry><literal>8</literal></entry>
       </row>
       <row>
        <entry><literal>/</literal></entry>
        <entry>Division</entry>
        <entry><literal>[8]</literal></entry>
        <entry><literal>$[0] / 2</literal></entry>
        <entry><literal>4</literal></entry>
       </row>
       <row>
        <entry><literal>%</literal></entry>
        <entry>Modulus</entry>
        <entry><literal>[32]</literal></entry>
        <entry><literal>$[0] % 10</literal></entry>
        <entry><literal>2</literal></entry>
       </row>
       <row>
        <entry><literal>type()</literal></entry>
        <entry>Type of the SQL/JSON item</entry>
        <entry><literal>[1, "2", {}]</literal></entry>
        <entry><literal>$[*].type()</literal></entry>
        <entry><literal>"number", "string", "object"</literal></entry>
       </row>
       <row>
        <entry><literal>size()</literal></entry>
        <entry>Size of the SQL/JSON item</entry>
        <entry><literal>{"m": [11, 15]}</literal></entry>
        <entry><literal>$.m.size()</literal></entry>
        <entry><literal>2</literal></entry>
       </row>
       <row>
        <entry><literal>double()</literal></entry>
        <entry>Approximate floating-point number converted from an SQL/JSON number or a string</entry>
        <entry><literal>{"len": "1.9"}</literal></entry>
        <entry><literal>$.len.double() * 2</literal></entry>
        <entry><literal>3.8</literal></entry>
       </row>
       <row>
        <entry><literal>ceiling()</literal></entry>
        <entry>Nearest integer greater than or equal to the SQL/JSON number</entry>
        <entry><literal>{"h": 1.3}</literal></entry>
        <entry><literal>$.h.ceiling()</literal></entry>
        <entry><literal>2</literal></entry>
       </row>
       <row>
        <entry><literal>floor()</literal></entry>
        <entry>Nearest integer less than or equal to the SQL/JSON number</entry>
        <entry><literal>{"h": 1.3}</literal></entry>
        <entry><literal>$.h.floor()</literal></entry>
        <entry><literal>1</literal></entry>
       </row>
       <row>
        <entry><literal>abs()</literal></entry>
        <entry>Absolute value of the SQL/JSON number</entry>
        <entry><literal>{"z": -0.3}</literal></entry>
        <entry><literal>$.z.abs()</literal></entry>
        <entry><literal>0.3</literal></entry>
       </row>
       <row>
        <entry><literal>datetime()</literal></entry>
        <entry>Date/time value converted from a string</entry>
        <entry><literal>["2015-8-1", "2015-08-12"]</literal></entry>
        <entry><literal>$[*] ? (@.datetime() &lt; "2015-08-2". datetime())</literal></entry>
        <entry><literal>2015-8-1</literal></entry>
       </row>
       <row>
        <entry><literal>datetime(<replaceable>template</replaceable>)</literal></entry>
        <entry>Date/time value converted from a string using the specified template</entry>
        <entry><literal>["12:30", "18:40"]</literal></entry>
        <entry><literal>$[*].datetime("HH24:MI")</literal></entry>
        <entry><literal>"12:30:00", "18:40:00"</literal></entry>
       </row>
       <row>
        <entry><literal>keyvalue()</literal></entry>
        <entry>
          Sequence of object's key-value pairs represented as array of items
          containing three fields (<literal>"key"</literal>,
          <literal>"value"</literal>, and <literal>"id"</literal>).
          <literal>"id"</literal> is a unique identifier of the object
          key-value pair belongs to.
        </entry>
        <entry><literal>{"x": "20", "y": 32}</literal></entry>
        <entry><literal>$.keyvalue()</literal></entry>
        <entry><literal>{"key": "x", "value": "20", "id": 0}, {"key": "y", "value": 32, "id": 0}</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <note>
     <para>
      The result type of <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 the result type dynamically.
     </para>
     <para>
      The <literal>datetime()</literal> method sequentially tries 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 the corresponding data type.
     </para>
     <para>
      The <literal>datetime(<replaceable>template</replaceable>)</literal>
      method determines the result type by the provided template string.
     </para>
     <para>
      The <literal>datetime()</literal> and
      <literal>datetime(<replaceable>template</replaceable>)</literal> methods
      use the same parsing rules as <literal>to_timestamp</literal> SQL
      function does (see <xref linkend="functions-formatting"/>) with three
      exceptions.  At first, these methods doesn't allow unmatched template
      patterns.  At second, only following separators are allowed in the
      template string: minus sign, period, solidus, comma, apostrophe,
      semicolon, colon and space.  At third, separators in the template string
      must exactly match the input string.
     </para>
    </note>

    <table id="functions-sqljson-filter-ex-table">
     <title><type>jsonpath</type> Filter Expression Elements</title>
     <tgroup cols="5">
      <thead>
       <row>
        <entry>Value/Predicate</entry>
        <entry>Description</entry>
        <entry>Example JSON</entry>
        <entry>Example Query</entry>
        <entry>Result</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><literal>==</literal></entry>
        <entry>Equality operator</entry>
        <entry><literal>[1, 2, 1, 3]</literal></entry>
        <entry><literal>$[*] ? (@ == 1)</literal></entry>
        <entry><literal>1, 1</literal></entry>
       </row>
       <row>
        <entry><literal>!=</literal></entry>
        <entry>Non-equality operator</entry>
        <entry><literal>[1, 2, 1, 3]</literal></entry>
        <entry><literal>$[*] ? (@ != 1)</literal></entry>
        <entry><literal>2, 3</literal></entry>
       </row>
       <row>
        <entry><literal>&lt;&gt;</literal></entry>
        <entry>Non-equality operator (same as <literal>!=</literal>)</entry>
        <entry><literal>[1, 2, 1, 3]</literal></entry>
        <entry><literal>$[*] ? (@ &lt;&gt; 1)</literal></entry>
        <entry><literal>2, 3</literal></entry>
       </row>
       <row>
        <entry><literal>&lt;</literal></entry>
        <entry>Less-than operator</entry>
        <entry><literal>[1, 2, 3]</literal></entry>
        <entry><literal>$[*] ? (@ &lt; 2)</literal></entry>
        <entry><literal>1</literal></entry>
       </row>
       <row>
        <entry><literal>&lt;=</literal></entry>
        <entry>Less-than-or-equal-to operator</entry>
        <entry><literal>[1, 2, 3]</literal></entry>
        <entry><literal>$[*] ? (@ &lt;= 2)</literal></entry>
        <entry><literal>1, 2</literal></entry>
       </row>
       <row>
        <entry><literal>&gt;</literal></entry>
        <entry>Greater-than operator</entry>
        <entry><literal>[1, 2, 3]</literal></entry>
        <entry><literal>$[*] ? (@ &gt; 2)</literal></entry>
        <entry><literal>3</literal></entry>
       </row>
       <row>
        <entry><literal>&gt;=</literal></entry>
        <entry>Greater-than-or-equal-to operator</entry>
        <entry><literal>[1, 2, 3]</literal></entry>
        <entry><literal>$[*] ? (@ &gt;= 2)</literal></entry>
        <entry><literal>2, 3</literal></entry>
       </row>
       <row>
        <entry><literal>true</literal></entry>
        <entry>Value used to perform comparison with JSON <literal>true</literal> literal</entry>
        <entry><literal>[{"name": "John", "parent": false},
                           {"name": "Chris", "parent": true}]</literal></entry>
        <entry><literal>$[*] ? (@.parent == true)</literal></entry>
        <entry><literal>{"name": "Chris", "parent": true}</literal></entry>
       </row>
       <row>
        <entry><literal>false</literal></entry>
        <entry>Value used to perform comparison with JSON <literal>false</literal> literal</entry>
        <entry><literal>[{"name": "John", "parent": false},
                           {"name": "Chris", "parent": true}]</literal></entry>
        <entry><literal>$[*] ? (@.parent == false)</literal></entry>
        <entry><literal>{"name": "John", "parent": false}</literal></entry>
       </row>
       <row>
        <entry><literal>null</literal></entry>
        <entry>Value used to perform comparison with JSON <literal>null</literal> value</entry>
        <entry><literal>[{"name": "Mary", "job": null},
                         {"name": "Michael", "job": "driver"}]</literal></entry>
        <entry><literal>$[*] ? (@.job == null) .name</literal></entry>
        <entry><literal>"Mary"</literal></entry>
       </row>
       <row>
        <entry><literal>&amp;&amp;</literal></entry>
        <entry>Boolean AND</entry>
        <entry><literal>[1, 3, 7]</literal></entry>
        <entry><literal>$[*] ? (@ &gt; 1 &amp;&amp; @ &lt; 5)</literal></entry>
        <entry><literal>3</literal></entry>
       </row>
       <row>
        <entry><literal>||</literal></entry>
        <entry>Boolean OR</entry>
        <entry><literal>[1, 3, 7]</literal></entry>
        <entry><literal>$[*] ? (@ &lt; 1 || @ &gt; 5)</literal></entry>
        <entry><literal>7</literal></entry>
       </row>
       <row>
        <entry><literal>!</literal></entry>
        <entry>Boolean NOT</entry>
        <entry><literal>[1, 3, 7]</literal></entry>
        <entry><literal>$[*] ? (!(@ &lt; 5))</literal></entry>
        <entry><literal>7</literal></entry>
       </row>
       <row>
        <entry><literal>like_regex</literal></entry>
        <entry>
          Tests whether the first operand matches the regular expression
          given by the second operand, optionally with modifications
          described by a string of <literal>flag</literal> characters (see
          <xref linkend="jsonpath-regular-expressions"/>)
        </entry>
        <entry><literal>["abc", "abd", "aBdC", "abdacb", "babc"]</literal></entry>
        <entry><literal>$[*] ? (@ like_regex "^ab.*c" flag "i")</literal></entry>
        <entry><literal>"abc", "aBdC", "abdacb"</literal></entry>
       </row>
       <row>
        <entry><literal>starts with</literal></entry>
        <entry>Tests whether the second operand is an initial substring of the first operand</entry>
        <entry><literal>["John Smith", "Mary Stone", "Bob Johnson"]</literal></entry>
        <entry><literal>$[*] ? (@ starts with "John")</literal></entry>
        <entry><literal>"John Smith"</literal></entry>
       </row>
       <row>
        <entry><literal>exists</literal></entry>
        <entry>Tests whether a path expression matches at least one SQL/JSON item</entry>
        <entry><literal>{"x": [1, 2], "y": [2, 4]}</literal></entry>
        <entry><literal>strict $.* ? (exists (@ ? (@[*] > 2)))</literal></entry>
        <entry><literal>2, 4</literal></entry>
       </row>
       <row>
        <entry><literal>is unknown</literal></entry>
        <entry>Tests whether a Boolean condition is <literal>unknown</literal></entry>
        <entry><literal>[-1, 2, 7, "infinity"]</literal></entry>
        <entry><literal>$[*] ? ((@ > 0) is unknown)</literal></entry>
        <entry><literal>"infinity"</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <note>
     <para>
      When different date/time values are 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> &mdash; to <type>timetz</type>.
     </para>
    </note>
   </sect3>
  </sect2>
 </sect1>

 <sect1 id="functions-sequence">
  <title>Sequence Manipulation Functions</title>

  <indexterm>
   <primary>sequence</primary>
  </indexterm>
  <indexterm>
   <primary>nextval</primary>
  </indexterm>
  <indexterm>
   <primary>currval</primary>
  </indexterm>
  <indexterm>
   <primary>lastval</primary>
  </indexterm>
  <indexterm>
   <primary>setval</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="3">
     <thead>
      <row><entry>Function</entry> <entry>Return Type</entry> <entry>Description</entry></row>
     </thead>

     <tbody>
      <row>
        <entry><literal><function>currval(<type>regclass</type>)</function></literal></entry>
        <entry><type>bigint</type></entry>
        <entry>Return value most recently obtained with
        <function>nextval</function> for specified sequence</entry>
      </row>
      <row>
        <entry><literal><function>lastval()</function></literal></entry>
        <entry><type>bigint</type></entry>
        <entry>Return value most recently obtained with
        <function>nextval</function> for any sequence</entry>
      </row>
      <row>
        <entry><literal><function>nextval(<type>regclass</type>)</function></literal></entry>
        <entry><type>bigint</type></entry>
        <entry>Advance sequence and return new value</entry>
      </row>
      <row>
        <entry><literal><function>setval(<type>regclass</type>, <type>bigint</type>)</function></literal></entry>
        <entry><type>bigint</type></entry>
        <entry>Set sequence's current value</entry>
      </row>
      <row>
        <entry><literal><function>setval(<type>regclass</type>, <type>bigint</type>, <type>boolean</type>)</function></literal></entry>
        <entry><type>bigint</type></entry>
        <entry>Set sequence's current value and <literal>is_called</literal> flag</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <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>

  <para>
   The available sequence functions are:

    <variablelist>
     <varlistentry>
      <term><function>nextval</function></term>
      <listitem>
       <para>
        Advance the sequence object to its next value and return that
        value.  This is done atomically: even if multiple sessions
        execute <function>nextval</function> concurrently, each will safely receive
        a distinct sequence value.
       </para>

       <para>
        If a 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
        special parameters in the <xref linkend="sql-createsequence"/> command;
        see its command reference page for more information.
       </para>

       <important>
        <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>
       </important>

       <para>
        This function requires <literal>USAGE</literal>
        or <literal>UPDATE</literal> privilege on the sequence.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><function>currval</function></term>
      <listitem>
       <para>
        Return 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>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><function>lastval</function></term>
      <listitem>
       <para>
        Return 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>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><function>setval</function></term>
      <listitem>
       <para>
        Reset the sequence object's counter value.  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 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,

<screen>
SELECT setval('foo', 42);           <lineannotation>Next <function>nextval</function> will return 43</lineannotation>
SELECT setval('foo', 42, true);     <lineannotation>Same as above</lineannotation>
SELECT setval('foo', 42, false);    <lineannotation>Next <function>nextval</function> will return 42</lineannotation>
</screen>

        The result returned by <function>setval</function> is just the value of its
        second argument.
       </para>
       <important>
        <para>
         Because sequences are non-transactional, changes made by
         <function>setval</function> are not undone if the transaction rolls
         back.
        </para>
       </important>

       <para>
        This function requires <literal>UPDATE</literal> privilege on the
        sequence.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
  </para>

 </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 operators
   available for array types.
  </para>

    <table id="array-operators-table">
     <title>Array Operators</title>
     <tgroup cols="4">
      <thead>
       <row>
        <entry>Operator</entry>
        <entry>Description</entry>
        <entry>Example</entry>
        <entry>Result</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry> <literal>=</literal> </entry>
        <entry>equal</entry>
        <entry><literal>ARRAY[1.1,2.1,3.1]::int[] = ARRAY[1,2,3]</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&lt;&gt;</literal> </entry>
        <entry>not equal</entry>
        <entry><literal>ARRAY[1,2,3] &lt;&gt; ARRAY[1,2,4]</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&lt;</literal> </entry>
        <entry>less than</entry>
        <entry><literal>ARRAY[1,2,3] &lt; ARRAY[1,2,4]</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&gt;</literal> </entry>
        <entry>greater than</entry>
        <entry><literal>ARRAY[1,4,3] &gt; ARRAY[1,2,4]</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&lt;=</literal> </entry>
        <entry>less than or equal</entry>
        <entry><literal>ARRAY[1,2,3] &lt;= ARRAY[1,2,3]</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&gt;=</literal> </entry>
        <entry>greater than or equal</entry>
        <entry><literal>ARRAY[1,4,3] &gt;= ARRAY[1,4,3]</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>@&gt;</literal> </entry>
        <entry>contains</entry>
        <entry><literal>ARRAY[1,4,3] @&gt; ARRAY[3,1,3]</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&lt;@</literal> </entry>
        <entry>is contained by</entry>
        <entry><literal>ARRAY[2,2,7] &lt;@ ARRAY[1,7,4,2,6]</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&amp;&amp;</literal> </entry>
        <entry>overlap (have elements in common)</entry>
        <entry><literal>ARRAY[1,4,3] &amp;&amp; ARRAY[2,1]</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>||</literal> </entry>
        <entry>array-to-array concatenation</entry>
        <entry><literal>ARRAY[1,2,3] || ARRAY[4,5,6]</literal></entry>
        <entry><literal>{1,2,3,4,5,6}</literal></entry>
       </row>

       <row>
        <entry> <literal>||</literal> </entry>
        <entry>array-to-array concatenation</entry>
        <entry><literal>ARRAY[1,2,3] || ARRAY[[4,5,6],[7,8,9]]</literal></entry>
        <entry><literal>{{1,2,3},{4,5,6},{7,8,9}}</literal></entry>
       </row>

       <row>
        <entry> <literal>||</literal> </entry>
        <entry>element-to-array concatenation</entry>
        <entry><literal>3 || ARRAY[4,5,6]</literal></entry>
        <entry><literal>{3,4,5,6}</literal></entry>
       </row>

       <row>
        <entry> <literal>||</literal> </entry>
        <entry>array-to-element concatenation</entry>
        <entry><literal>ARRAY[4,5,6] || 7</literal></entry>
        <entry><literal>{4,5,6,7}</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

  <para>
   The array ordering operators (<literal>&lt;</literal>,
   <literal>&gt;=</literal>, etc) 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>

  <para>
   The array containment operators (<literal>&lt;@</literal>
   and <literal>@&gt;</literal>) consider one array to be contained in
   another one if each of its elements appears in the other one.
   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>
   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>

  <indexterm>
    <primary>array_append</primary>
  </indexterm>
  <indexterm>
    <primary>array_cat</primary>
  </indexterm>
  <indexterm>
    <primary>array_ndims</primary>
  </indexterm>
  <indexterm>
    <primary>array_dims</primary>
  </indexterm>
  <indexterm>
    <primary>array_fill</primary>
  </indexterm>
  <indexterm>
    <primary>array_length</primary>
  </indexterm>
  <indexterm>
    <primary>array_lower</primary>
  </indexterm>
  <indexterm>
    <primary>array_position</primary>
  </indexterm>
  <indexterm>
    <primary>array_positions</primary>
  </indexterm>
  <indexterm>
    <primary>array_prepend</primary>
  </indexterm>
  <indexterm>
    <primary>array_remove</primary>
  </indexterm>
  <indexterm>
    <primary>array_replace</primary>
  </indexterm>
  <indexterm>
    <primary>array_to_string</primary>
  </indexterm>
 <indexterm>
    <primary>array_upper</primary>
  </indexterm>
  <indexterm>
    <primary>cardinality</primary>
  </indexterm>
  <indexterm>
    <primary>string_to_array</primary>
  </indexterm>
  <indexterm>
    <primary>unnest</primary>
  </indexterm>

    <table id="array-functions-table">
     <title>Array Functions</title>
     <tgroup cols="5">
      <thead>
       <row>
        <entry>Function</entry>
        <entry>Return Type</entry>
        <entry>Description</entry>
        <entry>Example</entry>
        <entry>Result</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry>
         <literal>
          <function>array_append</function>(<type>anyarray</type>, <type>anyelement</type>)
         </literal>
        </entry>
        <entry><type>anyarray</type></entry>
        <entry>append an element to the end of an array</entry>
        <entry><literal>array_append(ARRAY[1,2], 3)</literal></entry>
        <entry><literal>{1,2,3}</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>array_cat</function>(<type>anyarray</type>, <type>anyarray</type>)
         </literal>
        </entry>
        <entry><type>anyarray</type></entry>
        <entry>concatenate two arrays</entry>
        <entry><literal>array_cat(ARRAY[1,2,3], ARRAY[4,5])</literal></entry>
        <entry><literal>{1,2,3,4,5}</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>array_ndims</function>(<type>anyarray</type>)
         </literal>
        </entry>
        <entry><type>int</type></entry>
        <entry>returns the number of dimensions of the array</entry>
        <entry><literal>array_ndims(ARRAY[[1,2,3], [4,5,6]])</literal></entry>
        <entry><literal>2</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>array_dims</function>(<type>anyarray</type>)
         </literal>
        </entry>
        <entry><type>text</type></entry>
        <entry>returns a text representation of array's dimensions</entry>
        <entry><literal>array_dims(ARRAY[[1,2,3], [4,5,6]])</literal></entry>
        <entry><literal>[1:2][1:3]</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>array_fill</function>(<type>anyelement</type>, <type>int[]</type>
          <optional>, <type>int[]</type></optional>)
         </literal>
        </entry>
        <entry><type>anyarray</type></entry>
        <entry>returns an array initialized with supplied value and
         dimensions, optionally with lower bounds other than 1</entry>
        <entry><literal>array_fill(7, ARRAY[3], ARRAY[2])</literal></entry>
        <entry><literal>[2:4]={7,7,7}</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>array_length</function>(<type>anyarray</type>, <type>int</type>)
         </literal>
        </entry>
        <entry><type>int</type></entry>
        <entry>returns the length of the requested array dimension</entry>
        <entry><literal>array_length(array[1,2,3], 1)</literal></entry>
        <entry><literal>3</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>array_lower</function>(<type>anyarray</type>, <type>int</type>)
         </literal>
        </entry>
        <entry><type>int</type></entry>
        <entry>returns lower bound of the requested array dimension</entry>
        <entry><literal>array_lower('[0:2]={1,2,3}'::int[], 1)</literal></entry>
        <entry><literal>0</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>array_position</function>(<type>anyarray</type>, <type>anyelement</type> <optional>, <type>int</type></optional>)
         </literal>
        </entry>
        <entry><type>int</type></entry>
        <entry>returns the subscript of the first occurrence of the second
        argument in the array, starting at the element indicated by the third
        argument or at the first element (array must be one-dimensional)</entry>
        <entry><literal>array_position(ARRAY['sun','mon','tue','wed','thu','fri','sat'], 'mon')</literal></entry>
        <entry><literal>2</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>array_positions</function>(<type>anyarray</type>, <type>anyelement</type>)
         </literal>
        </entry>
        <entry><type>int[]</type></entry>
        <entry>returns an array of subscripts of all occurrences of the second
        argument in the array given as first argument (array must be
        one-dimensional)</entry>
        <entry><literal>array_positions(ARRAY['A','A','B','A'], 'A')</literal></entry>
        <entry><literal>{1,2,4}</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>array_prepend</function>(<type>anyelement</type>, <type>anyarray</type>)
         </literal>
        </entry>
        <entry><type>anyarray</type></entry>
        <entry>append an element to the beginning of an array</entry>
        <entry><literal>array_prepend(1, ARRAY[2,3])</literal></entry>
        <entry><literal>{1,2,3}</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>array_remove</function>(<type>anyarray</type>, <type>anyelement</type>)
         </literal>
        </entry>
        <entry><type>anyarray</type></entry>
        <entry>remove all elements equal to the given value from the array
         (array must be one-dimensional)</entry>
        <entry><literal>array_remove(ARRAY[1,2,3,2], 2)</literal></entry>
        <entry><literal>{1,3}</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>array_replace</function>(<type>anyarray</type>, <type>anyelement</type>, <type>anyelement</type>)
         </literal>
        </entry>
        <entry><type>anyarray</type></entry>
        <entry>replace each array element equal to the given value with a new value</entry>
        <entry><literal>array_replace(ARRAY[1,2,5,4], 5, 3)</literal></entry>
        <entry><literal>{1,2,3,4}</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>array_to_string</function>(<type>anyarray</type>, <type>text</type> <optional>, <type>text</type></optional>)
         </literal>
        </entry>
        <entry><type>text</type></entry>
        <entry>concatenates array elements using supplied delimiter and
         optional null string</entry>
        <entry><literal>array_to_string(ARRAY[1, 2, 3, NULL, 5], ',', '*')</literal></entry>
        <entry><literal>1,2,3,*,5</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>array_upper</function>(<type>anyarray</type>, <type>int</type>)
         </literal>
        </entry>
        <entry><type>int</type></entry>
        <entry>returns upper bound of the requested array dimension</entry>
        <entry><literal>array_upper(ARRAY[1,8,3,7], 1)</literal></entry>
        <entry><literal>4</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>cardinality</function>(<type>anyarray</type>)
         </literal>
        </entry>
        <entry><type>int</type></entry>
        <entry>returns the total number of elements in the array, or 0 if the array is empty</entry>
        <entry><literal>cardinality(ARRAY[[1,2],[3,4]])</literal></entry>
        <entry><literal>4</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>string_to_array</function>(<type>text</type>, <type>text</type> <optional>, <type>text</type></optional>)
         </literal>
        </entry>
        <entry><type>text[]</type></entry>
        <entry>splits string into array elements using supplied delimiter and
         optional null string</entry>
        <entry><literal>string_to_array('xx~^~yy~^~zz', '~^~', 'yy')</literal></entry>
        <entry><literal>{xx,NULL,zz}</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>unnest</function>(<type>anyarray</type>)
         </literal>
        </entry>
        <entry><type>setof anyelement</type></entry>
        <entry>expand an array to a set of rows</entry>
        <entry><literal>unnest(ARRAY[1,2])</literal></entry>
        <entry><literallayout class="monospaced">1
2</literallayout>(2 rows)</entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>unnest</function>(<type>anyarray</type>, <type>anyarray</type> [, ...])
         </literal>
        </entry>
        <entry><type>setof anyelement, anyelement [, ...]</type></entry>
        <entry>expand multiple arrays (possibly of different types) to a set
         of rows.  This is only allowed in the FROM clause; see
         <xref linkend="queries-tablefunctions"/></entry>
        <entry><literal>unnest(ARRAY[1,2],ARRAY['foo','bar','baz'])</literal></entry>
        <entry><literallayout class="monospaced">1    foo
2    bar
NULL baz</literallayout>(3 rows)</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    In <function>array_position</function> and <function>array_positions</function>,
    each array element is compared to the searched value using
    <literal>IS NOT DISTINCT FROM</literal> semantics.
   </para>

   <para>
    In <function>array_position</function>, <literal>NULL</literal> is returned
    if the value is not found.
   </para>

   <para>
    In <function>array_positions</function>, <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 instead.
   </para>

   <para>
    In <function>string_to_array</function>, if the delimiter parameter is
    NULL, each character in the input string will become a separate element in
    the resulting array.  If the delimiter is an empty string, then the entire
    input string is returned as a one-element array.  Otherwise the input
    string is split at each occurrence of the delimiter string.
   </para>

   <para>
    In <function>string_to_array</function>, if the null-string parameter
    is omitted or NULL, none of the substrings of the input will be replaced
    by NULL.
    In <function>array_to_string</function>, if the null-string parameter
    is omitted or NULL, any null elements in the array are simply skipped
    and not represented in the output string.
   </para>

   <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 NULL when
     the input string is of zero length.  Second, if the delimiter string is
     NULL, the function splits the input into individual characters, rather
     than returning NULL 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 operators
   available for range types.
  </para>

    <table id="range-operators-table">
     <title>Range Operators</title>
     <tgroup cols="4">
      <thead>
       <row>
        <entry>Operator</entry>
        <entry>Description</entry>
        <entry>Example</entry>
        <entry>Result</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry> <literal>=</literal> </entry>
        <entry>equal</entry>
        <entry><literal>int4range(1,5) = '[1,4]'::int4range</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&lt;&gt;</literal> </entry>
        <entry>not equal</entry>
        <entry><literal>numrange(1.1,2.2) &lt;&gt; numrange(1.1,2.3)</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&lt;</literal> </entry>
        <entry>less than</entry>
        <entry><literal>int4range(1,10) &lt; int4range(2,3)</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&gt;</literal> </entry>
        <entry>greater than</entry>
        <entry><literal>int4range(1,10) &gt; int4range(1,5)</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&lt;=</literal> </entry>
        <entry>less than or equal</entry>
        <entry><literal>numrange(1.1,2.2) &lt;= numrange(1.1,2.2)</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&gt;=</literal> </entry>
        <entry>greater than or equal</entry>
        <entry><literal>numrange(1.1,2.2) &gt;= numrange(1.1,2.0)</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>@&gt;</literal> </entry>
        <entry>contains range</entry>
        <entry><literal>int4range(2,4) @&gt; int4range(2,3)</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>@&gt;</literal> </entry>
        <entry>contains element</entry>
        <entry><literal>'[2011-01-01,2011-03-01)'::tsrange @&gt; '2011-01-10'::timestamp</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&lt;@</literal> </entry>
        <entry>range is contained by</entry>
        <entry><literal>int4range(2,4) &lt;@ int4range(1,7)</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&lt;@</literal> </entry>
        <entry>element is contained by</entry>
        <entry><literal>42 &lt;@ int4range(1,7)</literal></entry>
        <entry><literal>f</literal></entry>
       </row>

       <row>
        <entry> <literal>&amp;&amp;</literal> </entry>
        <entry>overlap (have points in common)</entry>
        <entry><literal>int8range(3,7) &amp;&amp; int8range(4,12)</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&lt;&lt;</literal> </entry>
        <entry>strictly left of</entry>
        <entry><literal>int8range(1,10) &lt;&lt; int8range(100,110)</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&gt;&gt;</literal> </entry>
        <entry>strictly right of</entry>
        <entry><literal>int8range(50,60) &gt;&gt; int8range(20,30)</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&amp;&lt;</literal> </entry>
        <entry>does not extend to the right of</entry>
        <entry><literal>int8range(1,20) &amp;&lt; int8range(18,20)</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&amp;&gt;</literal> </entry>
        <entry>does not extend to the left of</entry>
        <entry><literal>int8range(7,20) &amp;&gt; int8range(5,10)</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>-|-</literal> </entry>
        <entry>is adjacent to</entry>
        <entry><literal>numrange(1.1,2.2) -|- numrange(2.2,3.3)</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>+</literal> </entry>
        <entry>union</entry>
        <entry><literal>numrange(5,15) + numrange(10,20)</literal></entry>
        <entry><literal>[5,20)</literal></entry>
       </row>

       <row>
        <entry> <literal>*</literal> </entry>
        <entry>intersection</entry>
        <entry><literal>int8range(5,15) * int8range(10,20)</literal></entry>
        <entry><literal>[10,15)</literal></entry>
       </row>

       <row>
        <entry> <literal>-</literal> </entry>
        <entry>difference</entry>
        <entry><literal>int8range(5,15) - int8range(10,20)</literal></entry>
        <entry><literal>[5,10)</literal></entry>
       </row>

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

  <para>
   The simple comparison operators <literal>&lt;</literal>,
   <literal>&gt;</literal>, <literal>&lt;=</literal>, and
   <literal>&gt;=</literal> compare the lower bounds first, and only if those
   are equal, compare the upper bounds.  These comparisons are not usually
   very useful for ranges, but are provided to allow B-tree indexes to be
   constructed on ranges.
  </para>

  <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>
   The union and difference operators will fail if the resulting range would
   need to contain two disjoint sub-ranges, as such a range cannot be
   represented.
  </para>

  <para>
   <xref linkend="range-functions-table"/> shows the functions
   available for use with range types.
  </para>

  <indexterm>
    <primary>lower</primary>
  </indexterm>
  <indexterm>
    <primary>upper</primary>
  </indexterm>
  <indexterm>
    <primary>isempty</primary>
  </indexterm>
  <indexterm>
    <primary>lower_inc</primary>
  </indexterm>
  <indexterm>
    <primary>upper_inc</primary>
  </indexterm>
  <indexterm>
    <primary>lower_inf</primary>
  </indexterm>
  <indexterm>
    <primary>upper_inf</primary>
  </indexterm>

    <table id="range-functions-table">
     <title>Range Functions</title>
     <tgroup cols="5">
      <thead>
       <row>
        <entry>Function</entry>
        <entry>Return Type</entry>
        <entry>Description</entry>
        <entry>Example</entry>
        <entry>Result</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry>
         <literal>
          <function>lower</function>(<type>anyrange</type>)
         </literal>
        </entry>
        <entry>range's element type</entry>
        <entry>lower bound of range</entry>
        <entry><literal>lower(numrange(1.1,2.2))</literal></entry>
        <entry><literal>1.1</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>upper</function>(<type>anyrange</type>)
         </literal>
        </entry>
        <entry>range's element type</entry>
        <entry>upper bound of range</entry>
        <entry><literal>upper(numrange(1.1,2.2))</literal></entry>
        <entry><literal>2.2</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>isempty</function>(<type>anyrange</type>)
         </literal>
        </entry>
        <entry><type>boolean</type></entry>
        <entry>is the range empty?</entry>
        <entry><literal>isempty(numrange(1.1,2.2))</literal></entry>
        <entry><literal>false</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>lower_inc</function>(<type>anyrange</type>)
         </literal>
        </entry>
        <entry><type>boolean</type></entry>
        <entry>is the lower bound inclusive?</entry>
        <entry><literal>lower_inc(numrange(1.1,2.2))</literal></entry>
        <entry><literal>true</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>upper_inc</function>(<type>anyrange</type>)
         </literal>
        </entry>
        <entry><type>boolean</type></entry>
        <entry>is the upper bound inclusive?</entry>
        <entry><literal>upper_inc(numrange(1.1,2.2))</literal></entry>
        <entry><literal>false</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>lower_inf</function>(<type>anyrange</type>)
         </literal>
        </entry>
        <entry><type>boolean</type></entry>
        <entry>is the lower bound infinite?</entry>
        <entry><literal>lower_inf('(,)'::daterange)</literal></entry>
        <entry><literal>true</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>upper_inf</function>(<type>anyrange</type>)
         </literal>
        </entry>
        <entry><type>boolean</type></entry>
        <entry>is the upper bound infinite?</entry>
        <entry><literal>upper_inf('(,)'::daterange)</literal></entry>
        <entry><literal>true</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>range_merge</function>(<type>anyrange</type>, <type>anyrange</type>)
         </literal>
        </entry>
        <entry><type>anyrange</type></entry>
        <entry>the smallest range which includes both of the given ranges</entry>
        <entry><literal>range_merge('[1,2)'::int4range, '[3,4)'::int4range)</literal></entry>
        <entry><literal>[1,4)</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

  <para>
   The <function>lower</function> and  <function>upper</function> functions return null
   if the range is empty or the requested bound is infinite.
   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"/>
   and statistical aggregates 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>

  <table id="functions-aggregate-table">
   <title>General-Purpose Aggregate Functions</title>

   <tgroup cols="5">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Argument Type(s)</entry>
      <entry>Return Type</entry>
      <entry>Partial Mode</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry>
       <indexterm>
        <primary>array_agg</primary>
       </indexterm>
       <function>array_agg(<replaceable class="parameter">expression</replaceable>)</function>
      </entry>
      <entry>
       any non-array type
      </entry>
      <entry>
       array of the argument type
      </entry>
      <entry>No</entry>
      <entry>input values, including nulls, concatenated into an array</entry>
     </row>

     <row>
      <entry>
       <function>array_agg(<replaceable class="parameter">expression</replaceable>)</function>
      </entry>
      <entry>
       any array type
      </entry>
      <entry>
       same as argument data type
      </entry>
      <entry>No</entry>
      <entry>input arrays concatenated into array of one higher dimension
       (inputs must all have same dimensionality,
        and cannot be empty or null)</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>average</primary>
       </indexterm>
       <indexterm>
        <primary>avg</primary>
       </indexterm>
       <function>avg(<replaceable class="parameter">expression</replaceable>)</function>
      </entry>
      <entry>
       <type>smallint</type>, <type>int</type>,
       <type>bigint</type>, <type>real</type>, <type>double
       precision</type>, <type>numeric</type>, or <type>interval</type>
      </entry>
      <entry>
       <type>numeric</type> for any integer-type argument,
       <type>double precision</type> for a floating-point argument,
       otherwise the same as the argument data type
      </entry>
      <entry>Yes</entry>
      <entry>the average (arithmetic mean) of all non-null input values</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>bit_and</primary>
       </indexterm>
       <function>bit_and(<replaceable class="parameter">expression</replaceable>)</function>
      </entry>
      <entry>
       <type>smallint</type>, <type>int</type>, <type>bigint</type>, or
       <type>bit</type>
      </entry>
      <entry>
        same as argument data type
      </entry>
      <entry>Yes</entry>
      <entry>the bitwise AND of all non-null input values, or null if none</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>bit_or</primary>
       </indexterm>
       <function>bit_or(<replaceable class="parameter">expression</replaceable>)</function>
      </entry>
      <entry>
       <type>smallint</type>, <type>int</type>, <type>bigint</type>, or
       <type>bit</type>
      </entry>
      <entry>
        same as argument data type
      </entry>
      <entry>Yes</entry>
      <entry>the bitwise OR of all non-null input values, or null if none</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>bool_and</primary>
       </indexterm>
       <function>bool_and(<replaceable class="parameter">expression</replaceable>)</function>
      </entry>
      <entry>
       <type>bool</type>
      </entry>
      <entry>
       <type>bool</type>
      </entry>
      <entry>Yes</entry>
      <entry>true if all input values are true, otherwise false</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>bool_or</primary>
       </indexterm>
       <function>bool_or(<replaceable class="parameter">expression</replaceable>)</function>
      </entry>
      <entry>
       <type>bool</type>
      </entry>
      <entry>
       <type>bool</type>
      </entry>
      <entry>Yes</entry>
      <entry>true if at least one input value is true, otherwise false</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>count</primary>
       </indexterm>
       <function>count(*)</function>
      </entry>
      <entry></entry>
      <entry><type>bigint</type></entry>
      <entry>Yes</entry>
      <entry>number of input rows</entry>
     </row>

     <row>
      <entry><function>count(<replaceable class="parameter">expression</replaceable>)</function></entry>
      <entry>any</entry>
      <entry><type>bigint</type></entry>
      <entry>Yes</entry>
      <entry>
       number of input rows for which the value of <replaceable
       class="parameter">expression</replaceable> is not null
      </entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>every</primary>
       </indexterm>
       <function>every(<replaceable class="parameter">expression</replaceable>)</function>
      </entry>
      <entry>
       <type>bool</type>
      </entry>
      <entry>
       <type>bool</type>
      </entry>
      <entry>Yes</entry>
      <entry>equivalent to <function>bool_and</function></entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>json_agg</primary>
       </indexterm>
       <function>json_agg(<replaceable class="parameter">expression</replaceable>)</function>
      </entry>
      <entry>
       <type>any</type>
      </entry>
      <entry>
       <type>json</type>
      </entry>
      <entry>No</entry>
      <entry>aggregates values, including nulls, as a JSON array</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>jsonb_agg</primary>
       </indexterm>
       <function>jsonb_agg(<replaceable class="parameter">expression</replaceable>)</function>
      </entry>
      <entry>
       <type>any</type>
      </entry>
      <entry>
       <type>jsonb</type>
      </entry>
      <entry>No</entry>
      <entry>aggregates values, including nulls, as a JSON array</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>json_object_agg</primary>
       </indexterm>
       <function>json_object_agg(<replaceable class="parameter">name</replaceable>, <replaceable class="parameter">value</replaceable>)</function>
      </entry>
      <entry>
       <type>(any, any)</type>
      </entry>
      <entry>
       <type>json</type>
      </entry>
      <entry>No</entry>
      <entry>aggregates name/value pairs as a JSON object; values can be
       null, but not names</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>jsonb_object_agg</primary>
       </indexterm>
       <function>jsonb_object_agg(<replaceable class="parameter">name</replaceable>, <replaceable class="parameter">value</replaceable>)</function>
      </entry>
      <entry>
       <type>(any, any)</type>
      </entry>
      <entry>
       <type>jsonb</type>
      </entry>
      <entry>No</entry>
      <entry>aggregates name/value pairs as a JSON object; values can be
       null, but not names</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>max</primary>
       </indexterm>
       <function>max(<replaceable class="parameter">expression</replaceable>)</function>
      </entry>
      <entry>any numeric, string, date/time, network, pg_lsn, or enum type,
             or arrays of these types</entry>
      <entry>same as argument type</entry>
      <entry>Yes</entry>
      <entry>
       maximum value of <replaceable
       class="parameter">expression</replaceable> across all non-null input
       values
      </entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>min</primary>
       </indexterm>
       <function>min(<replaceable class="parameter">expression</replaceable>)</function>
      </entry>
      <entry>any numeric, string, date/time, network, pg_lsn, or enum type,
             or arrays of these types</entry>
      <entry>same as argument type</entry>
      <entry>Yes</entry>
      <entry>
       minimum value of <replaceable
       class="parameter">expression</replaceable> across all non-null input
       values
      </entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>string_agg</primary>
       </indexterm>
       <function>
         string_agg(<replaceable class="parameter">expression</replaceable>,
                    <replaceable class="parameter">delimiter</replaceable>)
       </function>
      </entry>
      <entry>
       (<type>text</type>, <type>text</type>) or (<type>bytea</type>, <type>bytea</type>)
      </entry>
      <entry>
       same as argument types
      </entry>
      <entry>No</entry>
      <entry>non-null input values concatenated into a string, separated by delimiter</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>sum</primary>
       </indexterm>
       <function>sum(<replaceable class="parameter">expression</replaceable>)</function>
      </entry>
      <entry>
       <type>smallint</type>, <type>int</type>,
       <type>bigint</type>, <type>real</type>, <type>double
       precision</type>, <type>numeric</type>,
       <type>interval</type>, or <type>money</type>
      </entry>
      <entry>
       <type>bigint</type> for <type>smallint</type> or
       <type>int</type> arguments, <type>numeric</type> for
       <type>bigint</type> arguments, otherwise the same as the
       argument data type
      </entry>
      <entry>Yes</entry>
      <entry>sum of <replaceable class="parameter">expression</replaceable>
       across all non-null input values</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>xmlagg</primary>
       </indexterm>
       <function>xmlagg(<replaceable class="parameter">expression</replaceable>)</function>
      </entry>
      <entry>
       <type>xml</type>
      </entry>
      <entry>
       <type>xml</type>
      </entry>
      <entry>No</entry>
      <entry>concatenation of non-null XML values
       (see also <xref linkend="functions-xml-xmlagg"/>)</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>
   Aggregate functions which support <firstterm>Partial Mode</firstterm>
   are eligible to participate in various optimizations, such as parallel
   aggregation.
  </para>

  <note>
    <indexterm>
      <primary>ANY</primary>
    </indexterm>
    <indexterm>
      <primary>SOME</primary>
    </indexterm>
    <para>
      Boolean aggregates <function>bool_and</function> and
      <function>bool_or</function> correspond to standard SQL aggregates
      <function>every</function> and <function>any</function> or
      <function>some</function>.
      As for <function>any</function> and <function>some</function>,
      it seems that 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 which includes all rows in
    the table.
   </para>
  </note>

  <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>

  <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.)  Where the description mentions
   <replaceable class="parameter">N</replaceable>, 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 <replaceable class="parameter">N</replaceable> 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="5">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Argument Type</entry>
      <entry>Return Type</entry>
      <entry>Partial Mode</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>

     <row>
      <entry>
       <indexterm>
        <primary>correlation</primary>
       </indexterm>
       <indexterm>
        <primary>corr</primary>
       </indexterm>
       <function>corr(<replaceable class="parameter">Y</replaceable>, <replaceable class="parameter">X</replaceable>)</function>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>Yes</entry>
      <entry>correlation coefficient</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>covariance</primary>
        <secondary>population</secondary>
       </indexterm>
       <indexterm>
        <primary>covar_pop</primary>
       </indexterm>
       <function>covar_pop(<replaceable class="parameter">Y</replaceable>, <replaceable class="parameter">X</replaceable>)</function>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>Yes</entry>
      <entry>population covariance</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>covariance</primary>
        <secondary>sample</secondary>
       </indexterm>
       <indexterm>
        <primary>covar_samp</primary>
       </indexterm>
       <function>covar_samp(<replaceable class="parameter">Y</replaceable>, <replaceable class="parameter">X</replaceable>)</function>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>Yes</entry>
      <entry>sample covariance</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>regr_avgx</primary>
       </indexterm>
       <function>regr_avgx(<replaceable class="parameter">Y</replaceable>, <replaceable class="parameter">X</replaceable>)</function>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>Yes</entry>
      <entry>average of the independent variable
      (<literal>sum(<replaceable class="parameter">X</replaceable>)/<replaceable class="parameter">N</replaceable></literal>)</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>regr_avgy</primary>
       </indexterm>
       <function>regr_avgy(<replaceable class="parameter">Y</replaceable>, <replaceable class="parameter">X</replaceable>)</function>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>Yes</entry>
      <entry>average of the dependent variable
      (<literal>sum(<replaceable class="parameter">Y</replaceable>)/<replaceable class="parameter">N</replaceable></literal>)</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>regr_count</primary>
       </indexterm>
       <function>regr_count(<replaceable class="parameter">Y</replaceable>, <replaceable class="parameter">X</replaceable>)</function>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>
       <type>bigint</type>
      </entry>
      <entry>Yes</entry>
      <entry>number of input rows in which both expressions are nonnull</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>regression intercept</primary>
       </indexterm>
       <indexterm>
        <primary>regr_intercept</primary>
       </indexterm>
       <function>regr_intercept(<replaceable class="parameter">Y</replaceable>, <replaceable class="parameter">X</replaceable>)</function>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>Yes</entry>
      <entry>y-intercept of the least-squares-fit linear equation
      determined by the (<replaceable
      class="parameter">X</replaceable>, <replaceable
      class="parameter">Y</replaceable>) pairs</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>regr_r2</primary>
       </indexterm>
       <function>regr_r2(<replaceable class="parameter">Y</replaceable>, <replaceable class="parameter">X</replaceable>)</function>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>Yes</entry>
      <entry>square of the correlation coefficient</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>regression slope</primary>
       </indexterm>
       <indexterm>
        <primary>regr_slope</primary>
       </indexterm>
       <function>regr_slope(<replaceable class="parameter">Y</replaceable>, <replaceable class="parameter">X</replaceable>)</function>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>Yes</entry>
      <entry>slope of the least-squares-fit linear equation determined
      by the (<replaceable class="parameter">X</replaceable>,
      <replaceable class="parameter">Y</replaceable>) pairs</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>regr_sxx</primary>
       </indexterm>
       <function>regr_sxx(<replaceable class="parameter">Y</replaceable>, <replaceable class="parameter">X</replaceable>)</function>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>Yes</entry>
      <entry><literal>sum(<replaceable
      class="parameter">X</replaceable>^2) - sum(<replaceable
      class="parameter">X</replaceable>)^2/<replaceable
      class="parameter">N</replaceable></literal> (<quote>sum of
      squares</quote> of the independent variable)</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>regr_sxy</primary>
       </indexterm>
       <function>regr_sxy(<replaceable class="parameter">Y</replaceable>, <replaceable class="parameter">X</replaceable>)</function>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>Yes</entry>
      <entry><literal>sum(<replaceable
      class="parameter">X</replaceable>*<replaceable
      class="parameter">Y</replaceable>) - sum(<replaceable
      class="parameter">X</replaceable>) * sum(<replaceable
      class="parameter">Y</replaceable>)/<replaceable
      class="parameter">N</replaceable></literal> (<quote>sum of
      products</quote> of independent times dependent
      variable)</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>regr_syy</primary>
       </indexterm>
       <function>regr_syy(<replaceable class="parameter">Y</replaceable>, <replaceable class="parameter">X</replaceable>)</function>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>Yes</entry>
      <entry><literal>sum(<replaceable
      class="parameter">Y</replaceable>^2) - sum(<replaceable
      class="parameter">Y</replaceable>)^2/<replaceable
      class="parameter">N</replaceable></literal> (<quote>sum of
      squares</quote> of the dependent variable)</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>standard deviation</primary>
       </indexterm>
       <indexterm>
        <primary>stddev</primary>
       </indexterm>
       <function>stddev(<replaceable class="parameter">expression</replaceable>)</function>
      </entry>
      <entry>
       <type>smallint</type>, <type>int</type>,
       <type>bigint</type>, <type>real</type>, <type>double
       precision</type>, or <type>numeric</type>
      </entry>
      <entry>
       <type>double precision</type> for floating-point arguments,
       otherwise <type>numeric</type>
      </entry>
      <entry>Yes</entry>
      <entry>historical alias for <function>stddev_samp</function></entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>standard deviation</primary>
        <secondary>population</secondary>
       </indexterm>
       <indexterm>
        <primary>stddev_pop</primary>
       </indexterm>
       <function>stddev_pop(<replaceable class="parameter">expression</replaceable>)</function>
      </entry>
      <entry>
       <type>smallint</type>, <type>int</type>,
       <type>bigint</type>, <type>real</type>, <type>double
       precision</type>, or <type>numeric</type>
      </entry>
      <entry>
       <type>double precision</type> for floating-point arguments,
       otherwise <type>numeric</type>
      </entry>
      <entry>Yes</entry>
      <entry>population standard deviation of the input values</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>standard deviation</primary>
        <secondary>sample</secondary>
       </indexterm>
       <indexterm>
        <primary>stddev_samp</primary>
       </indexterm>
       <function>stddev_samp(<replaceable class="parameter">expression</replaceable>)</function>
      </entry>
      <entry>
       <type>smallint</type>, <type>int</type>,
       <type>bigint</type>, <type>real</type>, <type>double
       precision</type>, or <type>numeric</type>
      </entry>
      <entry>
       <type>double precision</type> for floating-point arguments,
       otherwise <type>numeric</type>
      </entry>
      <entry>Yes</entry>
      <entry>sample standard deviation of the input values</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>variance</primary>
       </indexterm>
       <function>variance</function>(<replaceable class="parameter">expression</replaceable>)
      </entry>
      <entry>
       <type>smallint</type>, <type>int</type>,
       <type>bigint</type>, <type>real</type>, <type>double
       precision</type>, or <type>numeric</type>
      </entry>
      <entry>
       <type>double precision</type> for floating-point arguments,
       otherwise <type>numeric</type>
      </entry>
      <entry>Yes</entry>
      <entry>historical alias for <function>var_samp</function></entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>variance</primary>
        <secondary>population</secondary>
       </indexterm>
       <indexterm>
        <primary>var_pop</primary>
       </indexterm>
       <function>var_pop</function>(<replaceable class="parameter">expression</replaceable>)
      </entry>
      <entry>
       <type>smallint</type>, <type>int</type>,
       <type>bigint</type>, <type>real</type>, <type>double
       precision</type>, or <type>numeric</type>
      </entry>
      <entry>
       <type>double precision</type> for floating-point arguments,
       otherwise <type>numeric</type>
      </entry>
      <entry>Yes</entry>
      <entry>population variance of the input values (square of the population standard deviation)</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>variance</primary>
        <secondary>sample</secondary>
       </indexterm>
       <indexterm>
        <primary>var_samp</primary>
       </indexterm>
       <function>var_samp</function>(<replaceable class="parameter">expression</replaceable>)
      </entry>
      <entry>
       <type>smallint</type>, <type>int</type>,
       <type>bigint</type>, <type>real</type>, <type>double
       precision</type>, or <type>numeric</type>
      </entry>
      <entry>
       <type>double precision</type> for floating-point arguments,
       otherwise <type>numeric</type>
      </entry>
      <entry>Yes</entry>
      <entry>sample variance of the input values (square of the sample standard deviation)</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.
  </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="6">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Direct Argument Type(s)</entry>
      <entry>Aggregated Argument Type(s)</entry>
      <entry>Return Type</entry>
      <entry>Partial Mode</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>

     <row>
      <entry>
       <indexterm>
        <primary>mode</primary>
        <secondary>statistical</secondary>
       </indexterm>
       <function>mode() WITHIN GROUP (ORDER BY <replaceable class="parameter">sort_expression</replaceable>)</function>
      </entry>
      <entry>
      </entry>
      <entry>
       any sortable type
      </entry>
      <entry>
       same as sort expression
      </entry>
      <entry>No</entry>
      <entry>
       returns the most frequent input value (arbitrarily choosing the first
       one if there are multiple equally-frequent results)
      </entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>percentile</primary>
        <secondary>continuous</secondary>
       </indexterm>
       <function>percentile_cont(<replaceable class="parameter">fraction</replaceable>) WITHIN GROUP (ORDER BY <replaceable class="parameter">sort_expression</replaceable>)</function>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>
       <type>double precision</type> or <type>interval</type>
      </entry>
      <entry>
       same as sort expression
      </entry>
      <entry>No</entry>
      <entry>
       continuous percentile: returns a value corresponding to the specified
       fraction in the ordering, interpolating between adjacent input items if
       needed
      </entry>
     </row>

     <row>
      <entry>
       <function>percentile_cont(<replaceable class="parameter">fractions</replaceable>) WITHIN GROUP (ORDER BY <replaceable class="parameter">sort_expression</replaceable>)</function>
      </entry>
      <entry>
       <type>double precision[]</type>
      </entry>
      <entry>
       <type>double precision</type> or <type>interval</type>
      </entry>
      <entry>
       array of sort expression's type
      </entry>
      <entry>No</entry>
      <entry>
       multiple continuous percentile: returns an array of results matching
       the shape of the <replaceable>fractions</replaceable> parameter, with each
       non-null element replaced by the value corresponding to that percentile
      </entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>percentile</primary>
        <secondary>discrete</secondary>
       </indexterm>
       <function>percentile_disc(<replaceable class="parameter">fraction</replaceable>) WITHIN GROUP (ORDER BY <replaceable class="parameter">sort_expression</replaceable>)</function>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>
       any sortable type
      </entry>
      <entry>
       same as sort expression
      </entry>
      <entry>No</entry>
      <entry>
       discrete percentile: returns the first input value whose position in
       the ordering equals or exceeds the specified fraction
      </entry>
     </row>

     <row>
      <entry>
       <function>percentile_disc(<replaceable class="parameter">fractions</replaceable>) WITHIN GROUP (ORDER BY <replaceable class="parameter">sort_expression</replaceable>)</function>
      </entry>
      <entry>
       <type>double precision[]</type>
      </entry>
      <entry>
       any sortable type
      </entry>
      <entry>
       array of sort expression's type
      </entry>
      <entry>No</entry>
      <entry>
       multiple discrete percentile: returns an array of results matching the
       shape of the <replaceable>fractions</replaceable> parameter, with each non-null
       element replaced by the input value corresponding to that percentile
      </entry>
     </row>

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

  <para>
   All the aggregates listed in <xref linkend="functions-orderedset-table"/>
   ignore null values in their sorted input.  For those that take
   a <replaceable>fraction</replaceable> parameter, the fraction value must be
   between 0 and 1; an error is thrown if not.  However, a null fraction value
   simply produces a null result.
  </para>

  <indexterm>
   <primary>hypothetical-set aggregate</primary>
   <secondary>built-in</secondary>
  </indexterm>

  <para>
   Each of the 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 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 computed from the <replaceable>sorted_args</replaceable>.
  </para>

  <table id="functions-hypothetical-table">
   <title>Hypothetical-Set Aggregate Functions</title>

   <tgroup cols="6">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Direct Argument Type(s)</entry>
      <entry>Aggregated Argument Type(s)</entry>
      <entry>Return Type</entry>
      <entry>Partial Mode</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>

     <row>
      <entry>
       <indexterm>
        <primary>rank</primary>
        <secondary>hypothetical</secondary>
       </indexterm>
       <function>rank(<replaceable class="parameter">args</replaceable>) WITHIN GROUP (ORDER BY <replaceable class="parameter">sorted_args</replaceable>)</function>
      </entry>
      <entry>
       <literal>VARIADIC</literal> <type>"any"</type>
      </entry>
      <entry>
       <literal>VARIADIC</literal> <type>"any"</type>
      </entry>
      <entry>
       <type>bigint</type>
      </entry>
      <entry>No</entry>
      <entry>
       rank of the hypothetical row, with gaps for duplicate rows
      </entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>dense_rank</primary>
        <secondary>hypothetical</secondary>
       </indexterm>
       <function>dense_rank(<replaceable class="parameter">args</replaceable>) WITHIN GROUP (ORDER BY <replaceable class="parameter">sorted_args</replaceable>)</function>
      </entry>
      <entry>
       <literal>VARIADIC</literal> <type>"any"</type>
      </entry>
      <entry>
       <literal>VARIADIC</literal> <type>"any"</type>
      </entry>
      <entry>
       <type>bigint</type>
      </entry>
      <entry>No</entry>
      <entry>
       rank of the hypothetical row, without gaps
      </entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>percent_rank</primary>
        <secondary>hypothetical</secondary>
       </indexterm>
       <function>percent_rank(<replaceable class="parameter">args</replaceable>) WITHIN GROUP (ORDER BY <replaceable class="parameter">sorted_args</replaceable>)</function>
      </entry>
      <entry>
       <literal>VARIADIC</literal> <type>"any"</type>
      </entry>
      <entry>
       <literal>VARIADIC</literal> <type>"any"</type>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>No</entry>
      <entry>
       relative rank of the hypothetical row, ranging from 0 to 1
      </entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>cume_dist</primary>
        <secondary>hypothetical</secondary>
       </indexterm>
       <function>cume_dist(<replaceable class="parameter">args</replaceable>) WITHIN GROUP (ORDER BY <replaceable class="parameter">sorted_args</replaceable>)</function>
      </entry>
      <entry>
       <literal>VARIADIC</literal> <type>"any"</type>
      </entry>
      <entry>
       <literal>VARIADIC</literal> <type>"any"</type>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>No</entry>
      <entry>
       relative rank of the hypothetical row, ranging from
       1/<replaceable>N</replaceable> to 1
      </entry>
     </row>

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

  <para>
   For each of these hypothetical-set aggregates, 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-grouping-table">
   <title>Grouping Operations</title>

   <tgroup cols="3">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Return Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>

     <row>
      <entry>
       <indexterm>
        <primary>GROUPING</primary>
       </indexterm>
       <function>GROUPING(<replaceable class="parameter">args...</replaceable>)</function>
      </entry>
      <entry>
       <type>integer</type>
      </entry>
      <entry>
       Integer bit mask indicating which arguments are not being included in the current
       grouping set
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

   <para>
    Grouping operations are used in conjunction with grouping sets (see
    <xref linkend="queries-grouping-sets"/>) to distinguish result rows.  The
    arguments to the <literal>GROUPING</literal> operation are not actually evaluated,
    but they must match exactly expressions given in the <literal>GROUP BY</literal>
    clause of the associated query level.  Bits are assigned with the rightmost
    argument being the least-significant bit; each bit is 0 if the corresponding
    expression is included in the grouping criteria of the grouping set generating
    the result row, and 1 if it is not.  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>
   </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
   general-purpose or statistical
   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 non-window aggregates
   and return a single row for the entire set.
  </para>

  <table id="functions-window-table">
   <title>General-Purpose Window Functions</title>

   <tgroup cols="3">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Return Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry>
       <indexterm>
        <primary>row_number</primary>
       </indexterm>
       <function>row_number()</function>
      </entry>
      <entry>
       <type>bigint</type>
      </entry>
      <entry>number of the current row within its partition, counting from 1</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>rank</primary>
       </indexterm>
       <function>rank()</function>
      </entry>
      <entry>
       <type>bigint</type>
      </entry>
      <entry>rank of the current row with gaps; same as <function>row_number</function> of its first peer</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>dense_rank</primary>
       </indexterm>
       <function>dense_rank()</function>
      </entry>
      <entry>
       <type>bigint</type>
      </entry>
      <entry>rank of the current row without gaps; this function counts peer groups</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>percent_rank</primary>
       </indexterm>
       <function>percent_rank()</function>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>relative rank of the current row: (<function>rank</function> - 1) / (total partition rows - 1)</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>cume_dist</primary>
       </indexterm>
       <function>cume_dist()</function>
      </entry>
      <entry>
       <type>double precision</type>
      </entry>
      <entry>cumulative distribution: (number of partition rows preceding or peer with current row) / total partition rows</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>ntile</primary>
       </indexterm>
       <function>ntile(<replaceable class="parameter">num_buckets</replaceable> <type>integer</type>)</function>
      </entry>
      <entry>
       <type>integer</type>
      </entry>
      <entry>integer ranging from 1 to the argument value, dividing the
       partition as equally as possible</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>lag</primary>
       </indexterm>
       <function>
         lag(<replaceable class="parameter">value</replaceable> <type>anyelement</type>
             [, <replaceable class="parameter">offset</replaceable> <type>integer</type>
             [, <replaceable class="parameter">default</replaceable> <type>anyelement</type> ]])
       </function>
      </entry>
      <entry>
       <type>same type as <replaceable class="parameter">value</replaceable></type>
      </entry>
      <entry>
       returns <replaceable class="parameter">value</replaceable> evaluated at
       the row that is <replaceable class="parameter">offset</replaceable>
       rows before the current row within the partition; if there is no such
       row, instead return <replaceable class="parameter">default</replaceable>
       (which must be of the same type as
       <replaceable class="parameter">value</replaceable>).
       Both <replaceable class="parameter">offset</replaceable> and
       <replaceable class="parameter">default</replaceable> are evaluated
       with respect to the current row.  If omitted,
       <replaceable class="parameter">offset</replaceable> defaults to 1 and
       <replaceable class="parameter">default</replaceable> to null
      </entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>lead</primary>
       </indexterm>
       <function>
         lead(<replaceable class="parameter">value</replaceable> <type>anyelement</type>
              [, <replaceable class="parameter">offset</replaceable> <type>integer</type>
              [, <replaceable class="parameter">default</replaceable> <type>anyelement</type> ]])
       </function>
      </entry>
      <entry>
       <type>same type as <replaceable class="parameter">value</replaceable></type>
      </entry>
      <entry>
       returns <replaceable class="parameter">value</replaceable> evaluated at
       the row that is <replaceable class="parameter">offset</replaceable>
       rows after the current row within the partition; if there is no such
       row, instead return <replaceable class="parameter">default</replaceable>
       (which must be of the same type as
       <replaceable class="parameter">value</replaceable>).
       Both <replaceable class="parameter">offset</replaceable> and
       <replaceable class="parameter">default</replaceable> are evaluated
       with respect to the current row.  If omitted,
       <replaceable class="parameter">offset</replaceable> defaults to 1 and
       <replaceable class="parameter">default</replaceable> to null
      </entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>first_value</primary>
       </indexterm>
       <function>first_value(<replaceable class="parameter">value</replaceable> <type>any</type>)</function>
      </entry>
      <entry>
       <type>same type as <replaceable class="parameter">value</replaceable></type>
      </entry>
      <entry>
       returns <replaceable class="parameter">value</replaceable> evaluated
       at the row that is the first row of the window frame
      </entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>last_value</primary>
       </indexterm>
       <function>last_value(<replaceable class="parameter">value</replaceable> <type>any</type>)</function>
      </entry>
      <entry>
       <type>same type as <replaceable class="parameter">value</replaceable></type>
      </entry>
      <entry>
       returns <replaceable class="parameter">value</replaceable> evaluated
       at the row that is the last row of the window frame
      </entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>nth_value</primary>
       </indexterm>
       <function>
         nth_value(<replaceable class="parameter">value</replaceable> <type>any</type>, <replaceable class="parameter">nth</replaceable> <type>integer</type>)
       </function>
      </entry>
      <entry>
       <type>same type as <replaceable class="parameter">value</replaceable></type>
      </entry>
      <entry>
       returns <replaceable class="parameter">value</replaceable> evaluated
       at the row that is the <replaceable class="parameter">nth</replaceable>
       row of the window frame (counting from 1); null if no such row
      </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 peer rows.
  </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>

  <para>
   <function>cume_dist</function> computes the fraction of partition rows that
   are less than or equal to the current row and its peers, while
   <function>percent_rank</function> computes the fraction of partition rows that
   are less than the current row, assuming the current row does not exist
   in the partition.
  </para>

 </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 but are not intended to be generally useful for
   writing queries.
  </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>

  <indexterm>
   <primary>generate_series</primary>
  </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="4">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Argument Type</entry>
      <entry>Return Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry><literal><function>generate_series(<parameter>start</parameter>, <parameter>stop</parameter>)</function></literal></entry>
      <entry><type>int</type>, <type>bigint</type> or <type>numeric</type></entry>
      <entry><type>setof int</type>, <type>setof bigint</type>, or <type>setof numeric</type> (same as argument type)</entry>
      <entry>
       Generate a series of values, from <parameter>start</parameter> to <parameter>stop</parameter>
       with a step size of one
      </entry>
     </row>

     <row>
      <entry><literal><function>generate_series(<parameter>start</parameter>, <parameter>stop</parameter>, <parameter>step</parameter>)</function></literal></entry>
      <entry><type>int</type>, <type>bigint</type> or <type>numeric</type></entry>
      <entry><type>setof int</type>, <type>setof bigint</type> or <type>setof numeric</type> (same as argument type)</entry>
      <entry>
       Generate a series of values, from <parameter>start</parameter> to <parameter>stop</parameter>
       with a step size of <parameter>step</parameter>
      </entry>
     </row>

     <row>
      <entry><literal><function>generate_series(<parameter>start</parameter>, <parameter>stop</parameter>, <parameter>step</parameter> <type>interval</type>)</function></literal></entry>
      <entry><type>timestamp</type> or <type>timestamp with time zone</type></entry>
      <entry><type>setof timestamp</type> or <type>setof timestamp with time zone</type> (same as argument type)</entry>
      <entry>
       Generate a series of values, from <parameter>start</parameter> to <parameter>stop</parameter>
       with a step size of <parameter>step</parameter>
      </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 for <literal>NULL</literal> inputs. 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="3">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Return Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry><literal><function>generate_subscripts(<parameter>array anyarray</parameter>, <parameter>dim int</parameter>)</function></literal></entry>
      <entry><type>setof int</type></entry>
      <entry>
       Generate a series comprising the given array's subscripts.
      </entry>
     </row>

     <row>
      <entry><literal><function>generate_subscripts(<parameter>array anyarray</parameter>, <parameter>dim int</parameter>, <parameter>reverse boolean</parameter>)</function></literal></entry>
      <entry><type>setof int</type></entry>
      <entry>
       Generate a series comprising the given array's subscripts. When
       <parameter>reverse</parameter> is true, the series is returned in
       reverse order.
      </entry>
     </row>

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

  <indexterm>
   <primary>generate_subscripts</primary>
  </indexterm>

  <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 for NULL arrays (but valid subscripts are returned for NULL array
   elements).  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 output 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="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry></row>
     </thead>

     <tbody>
      <row>
       <entry><literal><function>current_catalog</function></literal></entry>
       <entry><type>name</type></entry>
       <entry>name of current database (called <quote>catalog</quote> in the SQL standard)</entry>
      </row>

      <row>
       <entry><literal><function>current_database()</function></literal></entry>
       <entry><type>name</type></entry>
       <entry>name of current database</entry>
      </row>

      <row>
       <entry><literal><function>current_query()</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>text of the currently executing query, as submitted
       by the client (might contain more than one statement)</entry>
      </row>

      <row>
       <entry><literal><function>current_role</function></literal></entry>
       <entry><type>name</type></entry>
       <entry>equivalent to <function>current_user</function></entry>
      </row>

      <row>
       <entry><literal><function>current_schema</function>[()]</literal></entry>
       <entry><type>name</type></entry>
       <entry>name of current schema</entry>
      </row>

      <row>
       <entry><literal><function>current_schemas(<type>boolean</type>)</function></literal></entry>
       <entry><type>name[]</type></entry>
       <entry>names of schemas in search path, optionally including implicit schemas</entry>
      </row>

      <row>
       <entry><literal><function>current_user</function></literal></entry>
       <entry><type>name</type></entry>
       <entry>user name of current execution context</entry>
      </row>

      <row>
       <entry><literal><function>inet_client_addr()</function></literal></entry>
       <entry><type>inet</type></entry>
       <entry>address of the remote connection</entry>
      </row>

      <row>
       <entry><literal><function>inet_client_port()</function></literal></entry>
       <entry><type>int</type></entry>
       <entry>port of the remote connection</entry>
      </row>

      <row>
       <entry><literal><function>inet_server_addr()</function></literal></entry>
       <entry><type>inet</type></entry>
       <entry>address of the local connection</entry>
      </row>

      <row>
       <entry><literal><function>inet_server_port()</function></literal></entry>
       <entry><type>int</type></entry>
       <entry>port of the local connection</entry>
      </row>

      <row>
       <!-- See also the entry for this in monitoring.sgml -->
       <entry><literal><function>pg_backend_pid()</function></literal></entry>
       <entry><type>int</type></entry>
       <entry>
        Process ID of the server process attached to the current session
       </entry>
      </row>

      <row>
       <entry><literal><function>pg_blocking_pids(<type>int</type>)</function></literal></entry>
       <entry><type>int[]</type></entry>
       <entry>Process ID(s) that are blocking specified server process ID from acquiring a lock</entry>
      </row>

      <row>
       <entry><literal><function>pg_conf_load_time()</function></literal></entry>
       <entry><type>timestamp with time zone</type></entry>
       <entry>configuration load time</entry>
      </row>

      <row>
       <entry><literal><function>pg_current_logfile(<optional><type>text</type></optional>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>Primary log file name, or log in the requested format,
       currently in use by the logging collector</entry>
      </row>

      <row>
       <entry><literal><function>pg_my_temp_schema()</function></literal></entry>
       <entry><type>oid</type></entry>
       <entry>OID of session's temporary schema, or 0 if none</entry>
      </row>

      <row>
       <entry><literal><function>pg_is_other_temp_schema(<type>oid</type>)</function></literal></entry>
       <entry><type>boolean</type></entry>
       <entry>is schema another session's temporary schema?</entry>
      </row>

      <row>
       <entry><literal><function>pg_jit_available()</function></literal></entry>
       <entry><type>boolean</type></entry>
       <entry>is <acronym>JIT</acronym> compilation available in this session
       (see <xref linkend="jit"/>)? Returns <literal>false</literal> if <xref
       linkend="guc-jit"/> is set to false.</entry>
      </row>

      <row>
       <entry><literal><function>pg_listening_channels()</function></literal></entry>
       <entry><type>setof text</type></entry>
       <entry>channel names that the session is currently listening on</entry>
      </row>

      <row>
       <entry><literal><function>pg_notification_queue_usage()</function></literal></entry>
       <entry><type>double</type></entry>
       <entry>fraction of the asynchronous notification queue currently occupied (0&ndash;1)</entry>
      </row>

      <row>
       <entry><literal><function>pg_postmaster_start_time()</function></literal></entry>
       <entry><type>timestamp with time zone</type></entry>
       <entry>server start time</entry>
      </row>

      <row>
       <entry><literal><function>pg_safe_snapshot_blocking_pids(<type>int</type>)</function></literal></entry>
       <entry><type>int[]</type></entry>
       <entry>Process ID(s) that are blocking specified server process ID from acquiring a safe snapshot</entry>
      </row>

      <row>
       <entry><literal><function>pg_trigger_depth()</function></literal></entry>
       <entry><type>int</type></entry>
       <entry>current nesting level of <productname>PostgreSQL</productname> triggers
       (0 if not called, directly or indirectly, from inside a trigger)</entry>
      </row>

      <row>
       <entry><literal><function>session_user</function></literal></entry>
       <entry><type>name</type></entry>
       <entry>session user name</entry>
      </row>

      <row>
       <entry><literal><function>user</function></literal></entry>
       <entry><type>name</type></entry>
       <entry>equivalent to <function>current_user</function></entry>
      </row>

      <row>
       <entry><literal><function>version()</function></literal></entry>
       <entry><type>text</type></entry>
       <entry><productname>PostgreSQL</productname> version information. See also <xref linkend="guc-server-version-num"/> for a machine-readable version.</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>

   <indexterm>
    <primary>current_catalog</primary>
   </indexterm>

   <indexterm>
    <primary>current_database</primary>
   </indexterm>

   <indexterm>
    <primary>current_query</primary>
   </indexterm>

   <indexterm>
    <primary>current_role</primary>
   </indexterm>

   <indexterm>
    <primary>current_schema</primary>
   </indexterm>

   <indexterm>
    <primary>current_schemas</primary>
   </indexterm>

   <indexterm>
    <primary>current_user</primary>
   </indexterm>

   <indexterm>
    <primary>pg_backend_pid</primary>
   </indexterm>

   <indexterm>
    <primary>schema</primary>
    <secondary>current</secondary>
   </indexterm>

   <indexterm>
    <primary>search path</primary>
    <secondary>current</secondary>
   </indexterm>

   <indexterm>
    <primary>session_user</primary>
   </indexterm>

   <indexterm>
    <primary>user</primary>
    <secondary>current</secondary>
   </indexterm>

   <indexterm>
    <primary>user</primary>
   </indexterm>

   <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>

   <para>
    <function>current_schema</function> 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.
    <function>current_schemas(boolean)</function> returns an array of the names of all
    schemas presently in the search path.  The Boolean option determines whether or not
    implicitly included system schemas such as <literal>pg_catalog</literal> are included in the
    returned search path.
   </para>

   <note>
    <para>
     The search path can be altered at run time.  The command is:
<programlisting>
SET search_path TO <replaceable>schema</replaceable> <optional>, <replaceable>schema</replaceable>, ...</optional>
</programlisting>
    </para>
   </note>

   <indexterm>
    <primary>inet_client_addr</primary>
   </indexterm>

   <indexterm>
    <primary>inet_client_port</primary>
   </indexterm>

   <indexterm>
    <primary>inet_server_addr</primary>
   </indexterm>

   <indexterm>
    <primary>inet_server_port</primary>
   </indexterm>

   <para>
     <function>inet_client_addr</function> returns the IP address of the
     current client, and <function>inet_client_port</function> returns the
     port number.
     <function>inet_server_addr</function> returns the IP address on which
     the server accepted the current connection, and
     <function>inet_server_port</function> returns the port number.
     All these functions return NULL if the current connection is via a
     Unix-domain socket.
   </para>

   <indexterm>
    <primary>pg_blocking_pids</primary>
   </indexterm>

   <para>
    <function>pg_blocking_pids</function> returns an array of the process IDs
    of the sessions that are blocking the server process with the specified
    process ID, or an empty array if there is no such server process or it is
    not blocked.  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 in
    the result of this function.
    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>

   <indexterm>
    <primary>pg_conf_load_time</primary>
   </indexterm>

   <para>
    <function>pg_conf_load_time</function> returns the
    <type>timestamp with time zone</type> 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>

   <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>

   <para>
    <function>pg_current_logfile</function> returns, as <type>text</type>,
    the path of the log file(s) currently in use by the logging collector.
    The path includes the <xref linkend="guc-log-directory"/> directory
    and the log file name.  Log collection must be enabled or the return value
    is <literal>NULL</literal>.  When multiple log files exist, each in a
    different format, <function>pg_current_logfile</function> called
    without arguments returns the path of the file having the first format
    found in the ordered list: <systemitem>stderr</systemitem>, <systemitem>csvlog</systemitem>.
    <literal>NULL</literal> is returned when no log file has any of these
    formats.  To request a specific file format supply, as <type>text</type>,
    either <systemitem>csvlog</systemitem> or <systemitem>stderr</systemitem> as the value of the
    optional parameter. The return value is <literal>NULL</literal> when the
    log format requested is not a configured
    <xref linkend="guc-log-destination"/>.  The
    <function>pg_current_logfile</function> reflects the contents of the
    <filename>current_logfiles</filename> file.
   </para>

   <indexterm>
    <primary>pg_my_temp_schema</primary>
   </indexterm>

   <indexterm>
    <primary>pg_is_other_temp_schema</primary>
   </indexterm>

   <para>
    <function>pg_my_temp_schema</function> returns the OID of the current
    session's temporary schema, or zero if it has none (because it has not
    created any temporary tables).
    <function>pg_is_other_temp_schema</function> 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>

   <indexterm>
    <primary>pg_listening_channels</primary>
   </indexterm>

   <indexterm>
    <primary>pg_notification_queue_usage</primary>
   </indexterm>

   <para>
    <function>pg_listening_channels</function> returns a set of names of
    asynchronous notification channels that the current session is listening
    to.  <function>pg_notification_queue_usage</function> returns the
    fraction of the total available space for notifications currently
    occupied by notifications that are waiting to be processed, as a
    <type>double</type> in the range 0&ndash;1.
    See <xref linkend="sql-listen"/> and <xref linkend="sql-notify"/>
    for more information.
   </para>

   <indexterm>
    <primary>pg_postmaster_start_time</primary>
   </indexterm>

   <para>
    <function>pg_postmaster_start_time</function> returns the
    <type>timestamp with time zone</type> when the
    server started.
   </para>

   <indexterm>
    <primary>pg_safe_snapshot_blocking_pids</primary>
   </indexterm>

   <para>
    <function>pg_safe_snapshot_blocking_pids</function> returns an array of
    the process IDs 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.  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.  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>

   <indexterm>
    <primary>version</primary>
   </indexterm>

   <para>
    <function>version</function> 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, <xref linkend="guc-server-version-num"/>.
    Software developers should use <literal>server_version_num</literal>
    (available since 8.2) or <xref linkend="libpq-PQserverVersion"/> instead
    of parsing the text version.
   </para>

  <indexterm>
   <primary>privilege</primary>
   <secondary>querying</secondary>
  </indexterm>

  <para>
   <xref linkend="functions-info-access-table"/> lists functions that
   allow the user to query object access privileges programmatically.
   See <xref linkend="ddl-priv"/> for more information about
   privileges.
  </para>

   <table id="functions-info-access-table">
    <title>Access Privilege Inquiry Functions</title>
    <tgroup cols="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry></row>
     </thead>

     <tbody>
      <row>
       <entry><literal><function>has_any_column_privilege</function>(<parameter>user</parameter>,
                                  <parameter>table</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does user have privilege for any column of table</entry>
      </row>
      <row>
       <entry><literal><function>has_any_column_privilege</function>(<parameter>table</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does current user have privilege for any column of table</entry>
      </row>
      <row>
       <entry><literal><function>has_column_privilege</function>(<parameter>user</parameter>,
                                  <parameter>table</parameter>,
                                  <parameter>column</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does user have privilege for column</entry>
      </row>
      <row>
       <entry><literal><function>has_column_privilege</function>(<parameter>table</parameter>,
                                  <parameter>column</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does current user have privilege for column</entry>
      </row>
      <row>
       <entry><literal><function>has_database_privilege</function>(<parameter>user</parameter>,
                                  <parameter>database</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does user have privilege for database</entry>
      </row>
      <row>
       <entry><literal><function>has_database_privilege</function>(<parameter>database</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does current user have privilege for database</entry>
      </row>
      <row>
       <entry><literal><function>has_foreign_data_wrapper_privilege</function>(<parameter>user</parameter>,
                                  <parameter>fdw</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does user have privilege for foreign-data wrapper</entry>
      </row>
      <row>
       <entry><literal><function>has_foreign_data_wrapper_privilege</function>(<parameter>fdw</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does current user have privilege for foreign-data wrapper</entry>
      </row>
      <row>
       <entry><literal><function>has_function_privilege</function>(<parameter>user</parameter>,
                                  <parameter>function</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does user have privilege for function</entry>
      </row>
      <row>
       <entry><literal><function>has_function_privilege</function>(<parameter>function</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does current user have privilege for function</entry>
      </row>
      <row>
       <entry><literal><function>has_language_privilege</function>(<parameter>user</parameter>,
                                  <parameter>language</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does user have privilege for language</entry>
      </row>
      <row>
       <entry><literal><function>has_language_privilege</function>(<parameter>language</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does current user have privilege for language</entry>
      </row>
      <row>
       <entry><literal><function>has_schema_privilege</function>(<parameter>user</parameter>,
                                  <parameter>schema</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does user have privilege for schema</entry>
      </row>
      <row>
       <entry><literal><function>has_schema_privilege</function>(<parameter>schema</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does current user have privilege for schema</entry>
      </row>
      <row>
       <entry><literal><function>has_sequence_privilege</function>(<parameter>user</parameter>,
                                  <parameter>sequence</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does user have privilege for sequence</entry>
      </row>
      <row>
       <entry><literal><function>has_sequence_privilege</function>(<parameter>sequence</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does current user have privilege for sequence</entry>
      </row>
      <row>
       <entry><literal><function>has_server_privilege</function>(<parameter>user</parameter>,
                                  <parameter>server</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does user have privilege for foreign server</entry>
      </row>
      <row>
       <entry><literal><function>has_server_privilege</function>(<parameter>server</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does current user have privilege for foreign server</entry>
      </row>
      <row>
       <entry><literal><function>has_table_privilege</function>(<parameter>user</parameter>,
                                  <parameter>table</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does user have privilege for table</entry>
      </row>
      <row>
       <entry><literal><function>has_table_privilege</function>(<parameter>table</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does current user have privilege for table</entry>
      </row>
      <row>
       <entry><literal><function>has_tablespace_privilege</function>(<parameter>user</parameter>,
                                  <parameter>tablespace</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does user have privilege for tablespace</entry>
      </row>
      <row>
       <entry><literal><function>has_tablespace_privilege</function>(<parameter>tablespace</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does current user have privilege for tablespace</entry>
      </row>
      <row>
       <entry><literal><function>has_type_privilege</function>(<parameter>user</parameter>,
                                  <parameter>type</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does user have privilege for type</entry>
      </row>
      <row>
       <entry><literal><function>has_type_privilege</function>(<parameter>type</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does current user have privilege for type</entry>
      </row>
      <row>
       <entry><literal><function>pg_has_role</function>(<parameter>user</parameter>,
                                  <parameter>role</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does user have privilege for role</entry>
      </row>
      <row>
       <entry><literal><function>pg_has_role</function>(<parameter>role</parameter>,
                                  <parameter>privilege</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does current user have privilege for role</entry>
      </row>
      <row>
       <entry><literal><function>row_security_active</function>(<parameter>table</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>does current user have row level security active for table</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <indexterm>
    <primary>has_any_column_privilege</primary>
   </indexterm>
   <indexterm>
    <primary>has_column_privilege</primary>
   </indexterm>
   <indexterm>
    <primary>has_database_privilege</primary>
   </indexterm>
   <indexterm>
    <primary>has_function_privilege</primary>
   </indexterm>
   <indexterm>
    <primary>has_foreign_data_wrapper_privilege</primary>
   </indexterm>
   <indexterm>
    <primary>has_language_privilege</primary>
   </indexterm>
   <indexterm>
    <primary>has_schema_privilege</primary>
   </indexterm>
   <indexterm>
    <primary>has_server_privilege</primary>
   </indexterm>
   <indexterm>
    <primary>has_sequence_privilege</primary>
   </indexterm>
   <indexterm>
    <primary>has_table_privilege</primary>
   </indexterm>
   <indexterm>
    <primary>has_tablespace_privilege</primary>
   </indexterm>
   <indexterm>
    <primary>has_type_privilege</primary>
   </indexterm>
   <indexterm>
    <primary>pg_has_role</primary>
   </indexterm>
   <indexterm>
    <primary>row_security_active</primary>
   </indexterm>

   <para>
    <function>has_table_privilege</function> checks whether a user
    can access a table in a particular way.  The user can be
    specified by name, by OID (<literal>pg_authid.oid</literal>),
    <literal>public</literal> to indicate the PUBLIC pseudo-role, or if the argument is
    omitted
    <function>current_user</function> is assumed.  The table can be specified
    by name or by OID.  (Thus, there are actually six variants of
    <function>has_table_privilege</function>, which can be distinguished by
    the number and types of their arguments.)  When specifying by name,
    the name can be schema-qualified if necessary.
    The desired access privilege type
    is specified by a text string, which must evaluate to one of the
    values <literal>SELECT</literal>, <literal>INSERT</literal>,
    <literal>UPDATE</literal>, <literal>DELETE</literal>, <literal>TRUNCATE</literal>,
    <literal>REFERENCES</literal>, or <literal>TRIGGER</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 <literal>true</literal> 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>

   <para>
    <function>has_sequence_privilege</function> checks whether a user
    can access a sequence in a particular way.  The possibilities for its
    arguments are analogous to <function>has_table_privilege</function>.
    The desired access privilege type must evaluate to one of
    <literal>USAGE</literal>,
    <literal>SELECT</literal>, or
    <literal>UPDATE</literal>.
   </para>

   <para>
    <function>has_any_column_privilege</function> checks whether a user can
    access any column of a table in a particular way.
    Its argument possibilities
    are analogous to <function>has_table_privilege</function>,
    except that the desired access privilege type must evaluate to some
    combination of
    <literal>SELECT</literal>,
    <literal>INSERT</literal>,
    <literal>UPDATE</literal>, or
    <literal>REFERENCES</literal>.  Note that having any of these privileges
    at the table level implicitly grants it for each column of the table,
    so <function>has_any_column_privilege</function> will always return
    <literal>true</literal> if <function>has_table_privilege</function> does for the same
    arguments.  But <function>has_any_column_privilege</function> also succeeds if
    there is a column-level grant of the privilege for at least one column.
   </para>

   <para>
    <function>has_column_privilege</function> checks whether a user
    can access a column in a particular way.
    Its argument possibilities
    are analogous to <function>has_table_privilege</function>,
    with the addition that the column can be specified either by name
    or attribute number.
    The desired access privilege type must evaluate to some combination of
    <literal>SELECT</literal>,
    <literal>INSERT</literal>,
    <literal>UPDATE</literal>, or
    <literal>REFERENCES</literal>.  Note that having any of these privileges
    at the table level implicitly grants it for each column of the table.
   </para>

   <para>
    <function>has_database_privilege</function> checks whether a user
    can access a database in a particular way.
    Its argument possibilities
    are analogous to <function>has_table_privilege</function>.
    The desired access privilege type must evaluate to some combination of
    <literal>CREATE</literal>,
    <literal>CONNECT</literal>,
    <literal>TEMPORARY</literal>, or
    <literal>TEMP</literal> (which is equivalent to
    <literal>TEMPORARY</literal>).
   </para>

   <para>
    <function>has_function_privilege</function> checks whether a user
    can access a function in a particular way.
    Its argument possibilities
    are analogous to <function>has_table_privilege</function>.
    When specifying a function by a text string rather than by OID,
    the allowed input is the same as for the <type>regprocedure</type> data type
    (see <xref linkend="datatype-oid"/>).
    The desired access privilege type must evaluate to
    <literal>EXECUTE</literal>.
    An example is:
<programlisting>
SELECT has_function_privilege('joeuser', 'myfunc(int, text)', 'execute');
</programlisting>
   </para>

   <para>
    <function>has_foreign_data_wrapper_privilege</function> checks whether a user
    can access a foreign-data wrapper in a particular way.
    Its argument possibilities
    are analogous to <function>has_table_privilege</function>.
    The desired access privilege type must evaluate to
    <literal>USAGE</literal>.
   </para>

   <para>
    <function>has_language_privilege</function> checks whether a user
    can access a procedural language in a particular way.
    Its argument possibilities
    are analogous to <function>has_table_privilege</function>.
    The desired access privilege type must evaluate to
    <literal>USAGE</literal>.
   </para>

   <para>
    <function>has_schema_privilege</function> checks whether a user
    can access a schema in a particular way.
    Its argument possibilities
    are analogous to <function>has_table_privilege</function>.
    The desired access privilege type must evaluate to some combination of
    <literal>CREATE</literal> or
    <literal>USAGE</literal>.
   </para>

   <para>
    <function>has_server_privilege</function> checks whether a user
    can access a foreign server in a particular way.
    Its argument possibilities
    are analogous to <function>has_table_privilege</function>.
    The desired access privilege type must evaluate to
    <literal>USAGE</literal>.
   </para>

   <para>
    <function>has_tablespace_privilege</function> checks whether a user
    can access a tablespace in a particular way.
    Its argument possibilities
    are analogous to <function>has_table_privilege</function>.
    The desired access privilege type must evaluate to
    <literal>CREATE</literal>.
   </para>

   <para>
    <function>has_type_privilege</function> checks whether a user
    can access a type in a particular way.
    Its argument possibilities
    are analogous to <function>has_table_privilege</function>.
    When specifying a type by a text string rather than by OID,
    the allowed input is the same as for the <type>regtype</type> data type
    (see <xref linkend="datatype-oid"/>).
    The desired access privilege type must evaluate to
    <literal>USAGE</literal>.
   </para>

   <para>
    <function>pg_has_role</function> checks whether a user
    can access a role in a particular way.
    Its argument possibilities
    are analogous to <function>has_table_privilege</function>,
    except that <literal>public</literal> is not allowed as a user name.
    The desired access privilege type must evaluate to some combination of
    <literal>MEMBER</literal> or
    <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>.
   </para>

   <para>
    <function>row_security_active</function> checks whether row level
    security is active for the specified table in the context of the
    <function>current_user</function> and environment. The table can
    be specified by name or by OID.
   </para>

  <para>
   <xref linkend="functions-aclitem-fn-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>

   <indexterm>
    <primary>acldefault</primary>
   </indexterm>
   <indexterm>
    <primary>aclitemeq</primary>
   </indexterm>
   <indexterm>
    <primary>aclcontains</primary>
   </indexterm>
   <indexterm>
    <primary>aclexplode</primary>
   </indexterm>
   <indexterm>
    <primary>makeaclitem</primary>
   </indexterm>

    <table id="functions-aclitem-op-table">
     <title><type>aclitem</type> Operators</title>
     <tgroup cols="4">
      <thead>
       <row>
        <entry>Operator</entry>
        <entry>Description</entry>
        <entry>Example</entry>
        <entry>Result</entry>
       </row>
      </thead>
      <tbody>

       <row>
        <entry> <literal>=</literal> </entry>
        <entry>equal</entry>
        <entry><literal>'calvin=r*w/hobbes'::aclitem = 'calvin=r*w*/hobbes'::aclitem</literal></entry>
        <entry><literal>f</literal></entry>
       </row>

       <row>
        <entry> <literal>@&gt;</literal> </entry>
        <entry>contains element</entry>
        <entry><literal>'{calvin=r*w/hobbes,hobbes=r*w*/postgres}'::aclitem[] @&gt; 'calvin=r*w/hobbes'::aclitem</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>~</literal> </entry>
        <entry>contains element</entry>
        <entry><literal>'{calvin=r*w/hobbes,hobbes=r*w*/postgres}'::aclitem[] ~ 'calvin=r*w/hobbes'::aclitem</literal></entry>
        <entry><literal>t</literal></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="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry></row>
     </thead>
     <tbody>
      <row>
       <entry><literal><function>acldefault</function>(<parameter>type</parameter>,
        <parameter>ownerId</parameter>)</literal></entry>
       <entry><type>aclitem[]</type></entry>
       <entry>get the default access privileges for an object belonging to <parameter>ownerId</parameter></entry>
      </row>
      <row>
       <entry><literal><function>aclexplode</function>(<parameter>aclitem[]</parameter>)</literal></entry>
       <entry><type>setof record</type></entry>
       <entry>get <type>aclitem</type> array as tuples</entry>
      </row>
      <row>
       <entry><literal><function>makeaclitem</function>(<parameter>grantee</parameter>, <parameter>grantor</parameter>, <parameter>privilege</parameter>, <parameter>grantable</parameter>)</literal></entry>
       <entry><type>aclitem</type></entry>
       <entry>build an <type>aclitem</type> from input</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    <function>acldefault</function> returns the built-in default access
    privileges for an object of type <parameter>type</parameter> belonging to
    role <parameter>ownerId</parameter>.  These represent 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 is a <type>CHAR</type>: write
    '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>

   <para>
    <function>aclexplode</function> returns an <type>aclitem</type> array
    as a set of rows. Output columns are grantor <type>oid</type>,
    grantee <type>oid</type> (<literal>0</literal> for <literal>PUBLIC</literal>),
    granted privilege as <type>text</type> (<literal>SELECT</literal>, ...)
    and whether the privilege is grantable as <type>boolean</type>.
    <function>makeaclitem</function> performs the inverse operation.
   </para>

  <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.  To list the names of all visible tables:
<programlisting>
SELECT relname FROM pg_class WHERE pg_table_is_visible(oid);
</programlisting>
  </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="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry></row>
     </thead>

     <tbody>
      <row>
       <entry><literal><function>pg_collation_is_visible(<parameter>collation_oid</parameter>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is collation visible in search path</entry>
      </row>
      <row>
       <entry><literal><function>pg_conversion_is_visible(<parameter>conversion_oid</parameter>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is conversion visible in search path</entry>
      </row>
      <row>
       <entry><literal><function>pg_function_is_visible(<parameter>function_oid</parameter>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is function visible in search path</entry>
      </row>
      <row>
       <entry><literal><function>pg_opclass_is_visible(<parameter>opclass_oid</parameter>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is operator class visible in search path</entry>
      </row>
      <row>
       <entry><literal><function>pg_operator_is_visible(<parameter>operator_oid</parameter>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is operator visible in search path</entry>
      </row>
      <row>
       <entry><literal><function>pg_opfamily_is_visible(<parameter>opclass_oid</parameter>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is operator family visible in search path</entry>
      </row>
      <row>
       <entry><literal><function>pg_statistics_obj_is_visible(<parameter>stat_oid</parameter>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is statistics object visible in search path</entry>
      </row>
      <row>
       <entry><literal><function>pg_table_is_visible(<parameter>table_oid</parameter>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is table visible in search path</entry>
      </row>
      <row>
       <entry><literal><function>pg_ts_config_is_visible(<parameter>config_oid</parameter>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is text search configuration visible in search path</entry>
      </row>
      <row>
       <entry><literal><function>pg_ts_dict_is_visible(<parameter>dict_oid</parameter>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is text search dictionary visible in search path</entry>
      </row>
      <row>
       <entry><literal><function>pg_ts_parser_is_visible(<parameter>parser_oid</parameter>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is text search parser visible in search path</entry>
      </row>
      <row>
       <entry><literal><function>pg_ts_template_is_visible(<parameter>template_oid</parameter>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is text search template visible in search path</entry>
      </row>
      <row>
       <entry><literal><function>pg_type_is_visible(<parameter>type_oid</parameter>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is type (or domain) visible in search path</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <indexterm>
    <primary>pg_collation_is_visible</primary>
   </indexterm>
   <indexterm>
    <primary>pg_conversion_is_visible</primary>
   </indexterm>
   <indexterm>
    <primary>pg_function_is_visible</primary>
   </indexterm>
   <indexterm>
    <primary>pg_opclass_is_visible</primary>
   </indexterm>
   <indexterm>
    <primary>pg_operator_is_visible</primary>
   </indexterm>
   <indexterm>
    <primary>pg_opfamily_is_visible</primary>
   </indexterm>
   <indexterm>
    <primary>pg_statistics_obj_is_visible</primary>
   </indexterm>
   <indexterm>
    <primary>pg_table_is_visible</primary>
   </indexterm>
   <indexterm>
    <primary>pg_ts_config_is_visible</primary>
   </indexterm>
   <indexterm>
    <primary>pg_ts_dict_is_visible</primary>
   </indexterm>
   <indexterm>
    <primary>pg_ts_parser_is_visible</primary>
   </indexterm>
   <indexterm>
    <primary>pg_ts_template_is_visible</primary>
   </indexterm>
   <indexterm>
    <primary>pg_type_is_visible</primary>
   </indexterm>

   <para>
    Each function performs the visibility check for one type of database
    object.  Note that <function>pg_table_is_visible</function> can also be used
    with views, materialized views, indexes, sequences and foreign tables;
    <function>pg_function_is_visible</function> can also be used with
    procedures and aggregates;
    <function>pg_type_is_visible</function> can also be used with domains.
    For functions and operators, an object in
    the search path is visible if there is no object of the same name
    <emphasis>and argument data type(s)</emphasis> earlier in the path.  For operator
    classes, both name and associated index access method are considered.
   </para>

   <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>

   <indexterm>
    <primary>format_type</primary>
   </indexterm>

   <indexterm>
    <primary>pg_get_constraintdef</primary>
   </indexterm>

   <indexterm>
    <primary>pg_get_expr</primary>
   </indexterm>

   <indexterm>
    <primary>pg_get_functiondef</primary>
   </indexterm>

   <indexterm>
    <primary>pg_get_function_arguments</primary>
   </indexterm>

   <indexterm>
    <primary>pg_get_function_identity_arguments</primary>
   </indexterm>

   <indexterm>
    <primary>pg_get_function_result</primary>
   </indexterm>

   <indexterm>
    <primary>pg_get_indexdef</primary>
   </indexterm>

   <indexterm>
    <primary>pg_get_keywords</primary>
   </indexterm>

   <indexterm>
    <primary>pg_get_ruledef</primary>
   </indexterm>

   <indexterm>
    <primary>pg_get_serial_sequence</primary>
   </indexterm>

   <indexterm>
    <primary>pg_get_statisticsobjdef</primary>
   </indexterm>

   <indexterm>
    <primary>pg_get_triggerdef</primary>
   </indexterm>

   <indexterm>
    <primary>pg_get_userbyid</primary>
   </indexterm>

   <indexterm>
    <primary>pg_get_viewdef</primary>
   </indexterm>

   <indexterm>
    <primary>pg_index_column_has_property</primary>
   </indexterm>

   <indexterm>
    <primary>pg_index_has_property</primary>
   </indexterm>

   <indexterm>
    <primary>pg_indexam_has_property</primary>
   </indexterm>

   <indexterm>
    <primary>pg_options_to_table</primary>
   </indexterm>

   <indexterm>
    <primary>pg_tablespace_databases</primary>
   </indexterm>

   <indexterm>
    <primary>pg_tablespace_location</primary>
   </indexterm>

   <indexterm>
    <primary>pg_typeof</primary>
   </indexterm>

   <indexterm>
    <primary>collation for</primary>
   </indexterm>

   <indexterm>
    <primary>to_regclass</primary>
   </indexterm>

   <indexterm>
    <primary>to_regproc</primary>
   </indexterm>

   <indexterm>
    <primary>to_regprocedure</primary>
   </indexterm>

   <indexterm>
    <primary>to_regoper</primary>
   </indexterm>

   <indexterm>
    <primary>to_regoperator</primary>
   </indexterm>

   <indexterm>
    <primary>to_regtype</primary>
   </indexterm>

   <indexterm>
    <primary>to_regnamespace</primary>
   </indexterm>

   <indexterm>
    <primary>to_regrole</primary>
   </indexterm>

  <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="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry></row>
     </thead>

     <tbody>
      <row>
       <entry><literal><function>format_type(<parameter>type_oid</parameter>, <parameter>typemod</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get SQL name of a data type</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_constraintdef(<parameter>constraint_oid</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get definition of a constraint</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_constraintdef(<parameter>constraint_oid</parameter>, <parameter>pretty_bool</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get definition of a constraint</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_expr(<parameter>pg_node_tree</parameter>, <parameter>relation_oid</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>decompile internal form of an expression, assuming that any Vars
       in it refer to the relation indicated by the second parameter</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_expr(<parameter>pg_node_tree</parameter>, <parameter>relation_oid</parameter>, <parameter>pretty_bool</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>decompile internal form of an expression, assuming that any Vars
       in it refer to the relation indicated by the second parameter</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_functiondef(<parameter>func_oid</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get definition of a function or procedure</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_function_arguments(<parameter>func_oid</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get argument list of function's or procedure's definition (with default values)</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_function_identity_arguments(<parameter>func_oid</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get argument list to identify a function or procedure (without default values)</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_function_result(<parameter>func_oid</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get <literal>RETURNS</literal> clause for function (returns null for a procedure)</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_indexdef(<parameter>index_oid</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get <command>CREATE INDEX</command> command for index</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_indexdef(<parameter>index_oid</parameter>, <parameter>column_no</parameter>, <parameter>pretty_bool</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get <command>CREATE INDEX</command> command for index,
       or definition of just one index column when
       <parameter>column_no</parameter> is not zero</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_keywords()</function></literal></entry>
       <entry><type>setof record</type></entry>
       <entry>get list of SQL keywords and their categories</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_ruledef(<parameter>rule_oid</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get <command>CREATE RULE</command> command for rule</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_ruledef(<parameter>rule_oid</parameter>, <parameter>pretty_bool</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get <command>CREATE RULE</command> command for rule</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_serial_sequence(<parameter>table_name</parameter>, <parameter>column_name</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get name of the sequence that a serial or identity column uses</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_statisticsobjdef(<parameter>statobj_oid</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get <command>CREATE STATISTICS</command> command for extended statistics object</entry>
      </row>
      <row>
       <entry><function>pg_get_triggerdef</function>(<parameter>trigger_oid</parameter>)</entry>
       <entry><type>text</type></entry>
       <entry>get <command>CREATE [ CONSTRAINT ] TRIGGER</command> command for trigger</entry>
      </row>
      <row>
       <entry><function>pg_get_triggerdef</function>(<parameter>trigger_oid</parameter>, <parameter>pretty_bool</parameter>)</entry>
       <entry><type>text</type></entry>
       <entry>get <command>CREATE [ CONSTRAINT ] TRIGGER</command> command for trigger</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_userbyid(<parameter>role_oid</parameter>)</function></literal></entry>
       <entry><type>name</type></entry>
       <entry>get role name with given OID</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_viewdef(<parameter>view_name</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get underlying <command>SELECT</command> command for view or materialized view (<emphasis>deprecated</emphasis>)</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_viewdef(<parameter>view_name</parameter>, <parameter>pretty_bool</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get underlying <command>SELECT</command> command for view or materialized view (<emphasis>deprecated</emphasis>)</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_viewdef(<parameter>view_oid</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get underlying <command>SELECT</command> command for view or materialized view</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_viewdef(<parameter>view_oid</parameter>, <parameter>pretty_bool</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get underlying <command>SELECT</command> command for view or materialized view</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_viewdef(<parameter>view_oid</parameter>, <parameter>wrap_column_int</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get underlying <command>SELECT</command> command for view or
              materialized view; lines with fields are wrapped to specified
              number of columns, pretty-printing is implied</entry>
      </row>
      <row>
       <entry><literal><function>pg_index_column_has_property(<parameter>index_oid</parameter>, <parameter>column_no</parameter>, <parameter>prop_name</parameter>)</function></literal></entry>
       <entry><type>boolean</type></entry>
       <entry>test whether an index column has a specified property</entry>
      </row>
      <row>
       <entry><literal><function>pg_index_has_property(<parameter>index_oid</parameter>, <parameter>prop_name</parameter>)</function></literal></entry>
       <entry><type>boolean</type></entry>
       <entry>test whether an index has a specified property</entry>
      </row>
      <row>
       <entry><literal><function>pg_indexam_has_property(<parameter>am_oid</parameter>, <parameter>prop_name</parameter>)</function></literal></entry>
       <entry><type>boolean</type></entry>
       <entry>test whether an index access method has a specified property</entry>
      </row>
      <row>
       <entry><literal><function>pg_options_to_table(<parameter>reloptions</parameter>)</function></literal></entry>
       <entry><type>setof record</type></entry>
       <entry>get the set of storage option name/value pairs</entry>
      </row>
      <row>
       <entry><literal><function>pg_tablespace_databases(<parameter>tablespace_oid</parameter>)</function></literal></entry>
       <entry><type>setof oid</type></entry>
       <entry>get the set of database OIDs that have objects in the tablespace</entry>
      </row>
      <row>
       <entry><literal><function>pg_tablespace_location(<parameter>tablespace_oid</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get the path in the file system that this tablespace is located in</entry>
      </row>
      <row>
       <entry><literal><function>pg_typeof(<parameter>any</parameter>)</function></literal></entry>
       <entry><type>regtype</type></entry>
       <entry>get the data type of any value</entry>
      </row>
      <row>
       <entry><literal><function>collation for (<parameter>any</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get the collation of the argument</entry>
      </row>
      <row>
       <entry><literal><function>to_regclass(<parameter>rel_name</parameter>)</function></literal></entry>
       <entry><type>regclass</type></entry>
       <entry>get the OID of the named relation</entry>
      </row>
      <row>
       <entry><literal><function>to_regproc(<parameter>func_name</parameter>)</function></literal></entry>
       <entry><type>regproc</type></entry>
       <entry>get the OID of the named function</entry>
      </row>
      <row>
       <entry><literal><function>to_regprocedure(<parameter>func_name</parameter>)</function></literal></entry>
       <entry><type>regprocedure</type></entry>
       <entry>get the OID of the named function</entry>
      </row>
      <row>
       <entry><literal><function>to_regoper(<parameter>operator_name</parameter>)</function></literal></entry>
       <entry><type>regoper</type></entry>
       <entry>get the OID of the named operator</entry>
      </row>
      <row>
       <entry><literal><function>to_regoperator(<parameter>operator_name</parameter>)</function></literal></entry>
       <entry><type>regoperator</type></entry>
       <entry>get the OID of the named operator</entry>
      </row>
      <row>
       <entry><literal><function>to_regtype(<parameter>type_name</parameter>)</function></literal></entry>
       <entry><type>regtype</type></entry>
       <entry>get the OID of the named type</entry>
      </row>
      <row>
       <entry><literal><function>to_regnamespace(<parameter>schema_name</parameter>)</function></literal></entry>
       <entry><type>regnamespace</type></entry>
       <entry>get the OID of the named schema</entry>
      </row>
      <row>
       <entry><literal><function>to_regrole(<parameter>role_name</parameter>)</function></literal></entry>
       <entry><type>regrole</type></entry>
       <entry>get the OID of the named role</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   <function>format_type</function> returns the SQL name of 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>

  <para>
   <function>pg_get_keywords</function> returns a set of records describing
   the SQL keywords recognized by the server. The <structfield>word</structfield> column
   contains the keyword.  The <structfield>catcode</structfield> column contains a
   category code: <literal>U</literal> for unreserved, <literal>C</literal> for column name,
   <literal>T</literal> for type or function name, or <literal>R</literal> for reserved.
   The <structfield>catdesc</structfield> column contains a possibly-localized string
   describing the category.
  </para>

  <para>
   <function>pg_get_constraintdef</function>,
   <function>pg_get_indexdef</function>, <function>pg_get_ruledef</function>,
   <function>pg_get_statisticsobjdef</function>, and
   <function>pg_get_triggerdef</function>, respectively reconstruct the
   creating command for a constraint, index, rule, extended statistics object,
   or trigger. (Note that this is a decompiled reconstruction, not the
   original text of the command.) <function>pg_get_expr</function> decompiles
   the internal form of an individual expression, such as the default value
   for a column.  It can be useful when examining the contents of system
   catalogs.  If the expression might contain Vars, specify the OID of the
   relation they refer to as the second parameter; if no Vars are expected,
   zero is sufficient. <function>pg_get_viewdef</function> reconstructs the
   <command>SELECT</command> query that defines a view. Most of these functions come
   in two variants, one of which can optionally <quote>pretty-print</quote> the
   result.  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>; avoid using pretty-printed output for dump
   purposes.  Passing <literal>false</literal> for the pretty-print parameter yields
   the same result as the variant that does not have the parameter at all.
  </para>

  <para>
   <function>pg_get_functiondef</function> returns a complete
   <command>CREATE OR REPLACE FUNCTION</command> statement for a function.
   <function>pg_get_function_arguments</function> returns the argument list
   of a function, in the form it would need to appear in within
   <command>CREATE FUNCTION</command>.
   <function>pg_get_function_result</function> similarly returns the
   appropriate <literal>RETURNS</literal> clause for the function.
   <function>pg_get_function_identity_arguments</function> returns the
   argument list necessary to identify a function, in the form it
   would need to appear in within <command>ALTER FUNCTION</command>, for
   instance.  This form omits default values.
  </para>

  <para>
   <function>pg_get_serial_sequence</function> 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 the identity 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, this association can be modified or removed with <command>ALTER
   SEQUENCE OWNED BY</command>.  (The function probably should have been called
   <function>pg_get_owned_sequence</function>; its current name reflects the
   fact that it has typically been used with <type>serial</type>
   or <type>bigserial</type> columns.)  The first input parameter is a table name
   with optional schema, and the second parameter is a column name.  Because
   the first parameter is potentially a schema and table, it is not treated as
   a double-quoted identifier, meaning it is lower cased by default, while the
   second parameter, being just a column name, is treated as double-quoted and
   has its case preserved.  The function returns a value suitably formatted
   for passing to sequence functions
   (see <xref linkend="functions-sequence"/>).  A typical use is in reading the
   current value of a sequence for an identity or serial column, for example:
<programlisting>
SELECT currval(pg_get_serial_sequence('sometable', 'id'));
</programlisting>
  </para>

  <para>
   <function>pg_get_userbyid</function> extracts a role's name given
   its OID.
  </para>

  <para>
   <function>pg_index_column_has_property</function>,
   <function>pg_index_has_property</function>, and
   <function>pg_indexam_has_property</function> return whether the
   specified index column, index, or index access method possesses the named
   property. <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.  Refer to
   <xref linkend="functions-info-index-column-props"/> for column properties,
   <xref linkend="functions-info-index-props"/> for index properties, and
   <xref linkend="functions-info-indexam-props"/> for access method properties.
   (Note that extension access methods can define additional property names
   for their indexes.)
  </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>
   <function>pg_options_to_table</function> returns the set of storage
   option name/value pairs
   (<replaceable>option_name</replaceable>/<replaceable>option_value</replaceable>) when passed
   <structname>pg_class</structname>.<structfield>reloptions</structfield> or
   <structname>pg_attribute</structname>.<structfield>attoptions</structfield>.
  </para>

  <para>
   <function>pg_tablespace_databases</function> allows a tablespace to be
   examined. It returns the set of OIDs of databases that have objects stored
   in the tablespace. If this function returns any rows, the tablespace is not
   empty and cannot be dropped. To display the specific objects populating the
   tablespace, you will need to connect to the databases identified by
   <function>pg_tablespace_databases</function> and query their
   <structname>pg_class</structname> catalogs.
  </para>

  <para>
   <function>pg_typeof</function> 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.  For example:
<programlisting>
SELECT pg_typeof(33);

 pg_typeof 
-----------
 integer
(1 row)

SELECT typlen FROM pg_type WHERE oid = pg_typeof(33);
 typlen 
--------
      4
(1 row)
</programlisting>
  </para>

  <para>
   The expression <literal>collation for</literal> returns the collation of the
   value that is passed to it.  Example:
<programlisting>
SELECT collation for (description) FROM pg_description LIMIT 1;
 pg_collation_for 
------------------
 "default"
(1 row)

SELECT collation for ('foo' COLLATE "de_DE");
 pg_collation_for 
------------------
 "de_DE"
(1 row)
</programlisting>
  The value might be quoted and schema-qualified.  If no collation is derived
  for the argument expression, then a null value is returned.  If the argument
  is not of a collatable data type, then an error is raised.
  </para>

  <para>
   The <function>to_regclass</function>, <function>to_regproc</function>,
   <function>to_regprocedure</function>, <function>to_regoper</function>,
   <function>to_regoperator</function>, <function>to_regtype</function>,
   <function>to_regnamespace</function>, and <function>to_regrole</function>
   functions translate relation, function, operator, type, schema, and role
   names (given as <type>text</type>) to objects of
   type <type>regclass</type>, <type>regproc</type>, <type>regprocedure</type>,
   <type>regoper</type>, <type>regoperator</type>, <type>regtype</type>,
   <type>regnamespace</type>, and <type>regrole</type>
   respectively.  These functions differ from a cast from
   text in that they don't accept a numeric OID, and that they return null
   rather than throwing an error if the name is not found (or, for
   <function>to_regproc</function> and <function>to_regoper</function>, if
   the given name matches multiple objects).
  </para>

   <indexterm>
    <primary>pg_describe_object</primary>
   </indexterm>

   <indexterm>
    <primary>pg_identify_object</primary>
   </indexterm>

   <indexterm>
    <primary>pg_identify_object_as_address</primary>
   </indexterm>

   <indexterm>
    <primary>pg_get_object_address</primary>
   </indexterm>

  <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="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry></row>
     </thead>

     <tbody>
      <row>
       <entry><literal><function>pg_describe_object(<parameter>classid</parameter> <type>oid</type>, <parameter>objid</parameter> <type>oid</type>, <parameter>objsubid</parameter> <type>integer</type>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get description of a database object</entry>
      </row>
      <row>
       <entry><literal><function>pg_identify_object(<parameter>classid</parameter> <type>oid</type>, <parameter>objid</parameter> <type>oid</type>, <parameter>objsubid</parameter> <type>integer</type>)</function></literal></entry>
       <entry><parameter>type</parameter> <type>text</type>, <parameter>schema</parameter> <type>text</type>, <parameter>name</parameter> <type>text</type>, <parameter>identity</parameter> <type>text</type></entry>
       <entry>get identity of a database object</entry>
      </row>
      <row>
       <entry><literal><function>pg_identify_object_as_address(<parameter>classid</parameter> <type>oid</type>, <parameter>objid</parameter> <type>oid</type>, <parameter>objsubid</parameter> <type>integer</type>)</function></literal></entry>
       <entry><parameter>type</parameter> <type>text</type>, <parameter>object_names</parameter> <type>text[]</type>, <parameter>object_args</parameter> <type>text[]</type></entry>
       <entry>get external representation of a database object's address</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_object_address(<parameter>type</parameter> <type>text</type>, <parameter>object_names</parameter> <type>text[]</type>, <parameter>object_args</parameter> <type>text[]</type>)</function></literal></entry>
       <entry><parameter>classid</parameter> <type>oid</type>, <parameter>objid</parameter> <type>oid</type>, <parameter>objsubid</parameter> <type>integer</type></entry>
       <entry>get address of a database object from its external representation</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   <function>pg_describe_object</function> returns a textual description of a database
   object specified 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 useful to determine the identity of an object as stored in the
   <structname>pg_depend</structname> catalog.
  </para>

  <para>
   <function>pg_identify_object</function> 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.
  </para>

  <para>
   <function>pg_identify_object_as_address</function> 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.
   This function is the inverse of <function>pg_get_object_address</function>.
  </para>

  <para>
   <function>pg_get_object_address</function> returns a row containing enough
   information to uniquely identify the database object specified by its
   type 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>
   and can be passed to other system functions such as
   <function>pg_identify_object</function> or <function>pg_describe_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>.
  </para>

   <indexterm>
    <primary>col_description</primary>
   </indexterm>

   <indexterm>
    <primary>obj_description</primary>
   </indexterm>

   <indexterm>
    <primary>shobj_description</primary>
   </indexterm>

   <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="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry></row>
     </thead>

     <tbody>
      <row>
       <entry><literal><function>col_description(<parameter>table_oid</parameter>, <parameter>column_number</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get comment for a table column</entry>
      </row>
      <row>
       <entry><literal><function>obj_description(<parameter>object_oid</parameter>, <parameter>catalog_name</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get comment for a database object</entry>
      </row>
      <row>
       <entry><literal><function>obj_description(<parameter>object_oid</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get comment for a database object (<emphasis>deprecated</emphasis>)</entry>
      </row>
      <row>
       <entry><literal><function>shobj_description(<parameter>object_oid</parameter>, <parameter>catalog_name</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>get comment for a shared database object</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    <function>col_description</function> 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>

   <para>
    The two-parameter form of <function>obj_description</function> 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.
    The one-parameter form of <function>obj_description</function> requires only
    the object OID.  It is deprecated since there is no guarantee that
    OIDs are unique across different system catalogs; therefore, the wrong
    comment might be returned.
   </para>

   <para>
    <function>shobj_description</function> is used just like
    <function>obj_description</function> except it is used for retrieving
    comments on shared objects.  Some system catalogs are global to all
    databases within each cluster, and the descriptions for objects in them
    are stored globally as well.
   </para>

   <indexterm>
    <primary>txid_current</primary>
   </indexterm>

   <indexterm>
    <primary>txid_current_if_assigned</primary>
   </indexterm>

   <indexterm>
    <primary>txid_current_snapshot</primary>
   </indexterm>

   <indexterm>
    <primary>txid_snapshot_xip</primary>
   </indexterm>

   <indexterm>
    <primary>txid_snapshot_xmax</primary>
   </indexterm>

   <indexterm>
    <primary>txid_snapshot_xmin</primary>
   </indexterm>

   <indexterm>
    <primary>txid_visible_in_snapshot</primary>
   </indexterm>

   <indexterm>
    <primary>txid_status</primary>
   </indexterm>

   <para>
    The functions shown in <xref linkend="functions-txid-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-txid-snapshot">
    <title>Transaction IDs and Snapshots</title>
    <tgroup cols="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry></row>
     </thead>

     <tbody>
      <row>
       <entry><literal><function>txid_current()</function></literal></entry>
       <entry><type>bigint</type></entry>
       <entry>get current transaction ID, assigning a new one if the current transaction does not have one</entry>
      </row>
      <row>
       <entry><literal><function>txid_current_if_assigned()</function></literal></entry>
       <entry><type>bigint</type></entry>
       <entry>same as <function>txid_current()</function> but returns null instead of assigning a new transaction ID if none is already assigned</entry>
      </row>
      <row>
       <entry><literal><function>txid_current_snapshot()</function></literal></entry>
       <entry><type>txid_snapshot</type></entry>
       <entry>get current snapshot</entry>
      </row>
      <row>
       <entry><literal><function>txid_snapshot_xip(<parameter>txid_snapshot</parameter>)</function></literal></entry>
       <entry><type>setof bigint</type></entry>
       <entry>get in-progress transaction IDs in snapshot</entry>
      </row>
      <row>
       <entry><literal><function>txid_snapshot_xmax(<parameter>txid_snapshot</parameter>)</function></literal></entry>
       <entry><type>bigint</type></entry>
       <entry>get <literal>xmax</literal> of snapshot</entry>
      </row>
      <row>
       <entry><literal><function>txid_snapshot_xmin(<parameter>txid_snapshot</parameter>)</function></literal></entry>
       <entry><type>bigint</type></entry>
       <entry>get <literal>xmin</literal> of snapshot</entry>
      </row>
      <row>
       <entry><literal><function>txid_visible_in_snapshot(<parameter>bigint</parameter>, <parameter>txid_snapshot</parameter>)</function></literal></entry>
       <entry><type>boolean</type></entry>
       <entry>is transaction ID visible in snapshot? (do not use with subtransaction ids)</entry>
      </row>
      <row>
       <entry><literal><function>txid_status(<parameter>bigint</parameter>)</function></literal></entry>
       <entry><type>text</type></entry>
       <entry>report the status of the given transaction: <literal>committed</literal>, <literal>aborted</literal>, <literal>in progress</literal>, or null if the transaction ID is too old</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, these functions
    export a 64-bit format that is extended with an <quote>epoch</quote> counter
    so it will not wrap around during the life of an installation.
    The data type used by these functions, <type>txid_snapshot</type>,
    stores information about transaction ID
    visibility at a particular moment in time.  Its components are
    described in <xref linkend="functions-txid-snapshot-parts"/>.
   </para>

   <table id="functions-txid-snapshot-parts">
    <title>Snapshot Components</title>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>Name</entry>
       <entry>Description</entry>
      </row>
     </thead>

     <tbody>

      <row>
       <entry><type>xmin</type></entry>
       <entry>
         Earliest transaction ID (txid) that is still active.  All earlier
         transactions will either be committed and visible, or rolled
         back and dead.
       </entry>
      </row>

      <row>
       <entry><type>xmax</type></entry>
       <entry>
        First as-yet-unassigned txid.  All txids greater than or equal to this
        are not yet started as of the time of the snapshot, and thus invisible.
       </entry>
      </row>

      <row>
       <entry><type>xip_list</type></entry>
       <entry>
        Active txids at the time of the snapshot.  The list
        includes only those active txids between <literal>xmin</literal>
        and <literal>xmax</literal>; there might be active txids higher
        than <literal>xmax</literal>.  A txid that is <literal>xmin &lt;= txid &lt;
        xmax</literal> and not in this list was already completed
        at the time of the snapshot, and thus either visible or
        dead according to its commit status.  The list does not
        include txids of subtransactions.
       </entry>
      </row>

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

   <para>
    <type>txid_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>

   <para>
    <function>txid_status(bigint)</function> reports the commit status of a recent
    transaction.  Applications may use it to determine whether a transaction
    committed or aborted when the application and database server become
    disconnected while a <literal>COMMIT</literal> is in progress.
    The status of a transaction will be reported as either
    <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 is old enough that no references to that
    transaction survive in the system and the commit status information has
    been discarded, this function will return NULL.  Note that prepared
    transactions are reported as <literal>in progress</literal>; applications must
    check <link
    linkend="view-pg-prepared-xacts"><literal>pg_prepared_xacts</literal></link> if they
    need to determine whether the txid is a prepared transaction.
   </para>

   <para>
    The functions shown in <xref linkend="functions-commit-timestamp"/>
    provide information about transactions that have been already committed.
    These functions mainly provide information about when the transactions
    were committed. They only provide useful data when
    <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</title>
    <tgroup cols="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry></row>
     </thead>

     <tbody>
      <row>
       <entry>
        <indexterm><primary>pg_xact_commit_timestamp</primary></indexterm>
        <literal><function>pg_xact_commit_timestamp(<parameter>xid</parameter>)</function></literal>
       </entry>
       <entry><type>timestamp with time zone</type></entry>
       <entry>get commit timestamp of a transaction</entry>
      </row>

      <row>
       <entry>
        <indexterm><primary>pg_last_committed_xact</primary></indexterm>
        <literal><function>pg_last_committed_xact()</function></literal>
       </entry>
       <entry><parameter>xid</parameter> <type>xid</type>, <parameter>timestamp</parameter> <type>timestamp with time zone</type></entry>
       <entry>get transaction ID and commit timestamp of latest committed transaction</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,
    and not specific to any one database. They provide most of the same
    information, from the same source, as
    <xref linkend="app-pgcontroldata"/>, although in a form better suited
    to <acronym>SQL</acronym> functions.
   </para>

   <table id="functions-controldata">
    <title>Control Data Functions</title>
    <tgroup cols="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry></row>
     </thead>

     <tbody>
      <row>
       <entry>
        <indexterm><primary>pg_control_checkpoint</primary></indexterm>
        <literal><function>pg_control_checkpoint()</function></literal>
       </entry>
       <entry><type>record</type></entry>
       <entry>
        Returns information about current checkpoint state.
       </entry>
      </row>

      <row>
       <entry>
        <indexterm><primary>pg_control_system</primary></indexterm>
        <literal><function>pg_control_system()</function></literal>
       </entry>
       <entry><type>record</type></entry>
       <entry>
        Returns information about current control file state.
       </entry>
      </row>

      <row>
       <entry>
        <indexterm><primary>pg_control_init</primary></indexterm>
        <literal><function>pg_control_init()</function></literal>
       </entry>
       <entry><type>record</type></entry>
       <entry>
        Returns information about cluster initialization state.
       </entry>
      </row>

      <row>
       <entry>
        <indexterm><primary>pg_control_recovery</primary></indexterm>
        <literal><function>pg_control_recovery()</function></literal>
       </entry>
       <entry><type>record</type></entry>
       <entry>
        Returns information about recovery state.
       </entry>
      </row>

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

   <para>
    <function>pg_control_checkpoint</function> returns a record, shown in
    <xref linkend="functions-pg-control-checkpoint"/>
   </para>

   <table id="functions-pg-control-checkpoint">
    <title><function>pg_control_checkpoint</function> Columns</title>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>Column Name</entry>
       <entry>Data Type</entry>
      </row>
     </thead>

     <tbody>

      <row>
       <entry><literal>checkpoint_lsn</literal></entry>
       <entry><type>pg_lsn</type></entry>
      </row>

      <row>
       <entry><literal>redo_lsn</literal></entry>
       <entry><type>pg_lsn</type></entry>
      </row>

      <row>
       <entry><literal>redo_wal_file</literal></entry>
       <entry><type>text</type></entry>
      </row>

      <row>
       <entry><literal>timeline_id</literal></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><literal>prev_timeline_id</literal></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><literal>full_page_writes</literal></entry>
       <entry><type>boolean</type></entry>
      </row>

      <row>
       <entry><literal>next_xid</literal></entry>
       <entry><type>text</type></entry>
      </row>

      <row>
       <entry><literal>next_oid</literal></entry>
       <entry><type>oid</type></entry>
      </row>

      <row>
       <entry><literal>next_multixact_id</literal></entry>
       <entry><type>xid</type></entry>
      </row>

      <row>
       <entry><literal>next_multi_offset</literal></entry>
       <entry><type>xid</type></entry>
      </row>

      <row>
       <entry><literal>oldest_xid</literal></entry>
       <entry><type>xid</type></entry>
      </row>

      <row>
       <entry><literal>oldest_xid_dbid</literal></entry>
       <entry><type>oid</type></entry>
      </row>

      <row>
       <entry><literal>oldest_active_xid</literal></entry>
       <entry><type>xid</type></entry>
      </row>

      <row>
       <entry><literal>oldest_multi_xid</literal></entry>
       <entry><type>xid</type></entry>
      </row>

      <row>
       <entry><literal>oldest_multi_dbid</literal></entry>
       <entry><type>oid</type></entry>
      </row>

      <row>
       <entry><literal>oldest_commit_ts_xid</literal></entry>
       <entry><type>xid</type></entry>
      </row>

      <row>
       <entry><literal>newest_commit_ts_xid</literal></entry>
       <entry><type>xid</type></entry>
      </row>

      <row>
       <entry><literal>checkpoint_time</literal></entry>
       <entry><type>timestamp with time zone</type></entry>
      </row>

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

   <para>
    <function>pg_control_system</function> returns a record, shown in
    <xref linkend="functions-pg-control-system"/>
   </para>

   <table id="functions-pg-control-system">
    <title><function>pg_control_system</function> Columns</title>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>Column Name</entry>
       <entry>Data Type</entry>
      </row>
     </thead>

     <tbody>

      <row>
       <entry><literal>pg_control_version</literal></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><literal>catalog_version_no</literal></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><literal>system_identifier</literal></entry>
       <entry><type>bigint</type></entry>
      </row>

      <row>
       <entry><literal>pg_control_last_modified</literal></entry>
       <entry><type>timestamp with time zone</type></entry>
      </row>

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

   <para>
    <function>pg_control_init</function> returns a record, shown in
    <xref linkend="functions-pg-control-init"/>
   </para>

   <table id="functions-pg-control-init">
    <title><function>pg_control_init</function> Columns</title>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>Column Name</entry>
       <entry>Data Type</entry>
      </row>
     </thead>

     <tbody>

      <row>
       <entry><literal>max_data_alignment</literal></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><literal>database_block_size</literal></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><literal>blocks_per_segment</literal></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><literal>wal_block_size</literal></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><literal>bytes_per_wal_segment</literal></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><literal>max_identifier_length</literal></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><literal>max_index_columns</literal></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><literal>max_toast_chunk_size</literal></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><literal>large_object_chunk_size</literal></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><literal>float8_pass_by_value</literal></entry>
       <entry><type>boolean</type></entry>
      </row>

      <row>
       <entry><literal>data_page_checksum_version</literal></entry>
       <entry><type>integer</type></entry>
      </row>

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

   <para>
    <function>pg_control_recovery</function> returns a record, shown in
    <xref linkend="functions-pg-control-recovery"/>
   </para>

   <table id="functions-pg-control-recovery">
    <title><function>pg_control_recovery</function> Columns</title>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>Column Name</entry>
       <entry>Data Type</entry>
      </row>
     </thead>

     <tbody>

      <row>
       <entry><literal>min_recovery_end_lsn</literal></entry>
       <entry><type>pg_lsn</type></entry>
      </row>

      <row>
       <entry><literal>min_recovery_end_timeline</literal></entry>
       <entry><type>integer</type></entry>
      </row>

      <row>
       <entry><literal>backup_start_lsn</literal></entry>
       <entry><type>pg_lsn</type></entry>
      </row>

      <row>
       <entry><literal>backup_end_lsn</literal></entry>
       <entry><type>pg_lsn</type></entry>
      </row>

      <row>
       <entry><literal>end_of_backup_record_required</literal></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>

   <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="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry></row>
     </thead>

     <tbody>
      <row>
       <entry>
        <indexterm>
         <primary>current_setting</primary>
        </indexterm>
        <literal><function>current_setting(<parameter>setting_name</parameter> [, <parameter>missing_ok</parameter> ])</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>get current value of setting</entry>
      </row>
      <row>
       <entry>
        <indexterm>
         <primary>set_config</primary>
        </indexterm>
        <literal><function>set_config(<parameter>setting_name</parameter>,
                             <parameter>new_value</parameter>,
                             <parameter>is_local</parameter>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>set parameter and return new value</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <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>
    The function <function>current_setting</function> yields the
    current value of the setting <parameter>setting_name</parameter>.
    It corresponds to the <acronym>SQL</acronym> command
    <command>SHOW</command>.  An example:
<programlisting>
SELECT current_setting('datestyle');

 current_setting
-----------------
 ISO, MDY
(1 row)
</programlisting>

    If there is no setting named <parameter>setting_name</parameter>,
    <function>current_setting</function> throws an error
    unless <parameter>missing_ok</parameter> is supplied and is
    <literal>true</literal>.
   </para>

   <para>
    <function>set_config</function> sets the parameter
    <parameter>setting_name</parameter> to
    <parameter>new_value</parameter>.  If
    <parameter>is_local</parameter> is <literal>true</literal>, the
    new value will only apply to the current transaction. If you want
    the new value to apply for the current session, use
    <literal>false</literal> instead. The function corresponds to the
    SQL command <command>SET</command>. An example:
<programlisting>
SELECT set_config('log_statement_stats', 'off', false);

 set_config
------------
 off
(1 row)
</programlisting>
   </para>

  </sect2>

  <sect2 id="functions-admin-signal">
   <title>Server Signaling Functions</title>

   <indexterm>
    <primary>pg_cancel_backend</primary>
   </indexterm>
   <indexterm>
    <primary>pg_reload_conf</primary>
   </indexterm>
   <indexterm>
    <primary>pg_rotate_logfile</primary>
   </indexterm>
   <indexterm>
    <primary>pg_terminate_backend</primary>
   </indexterm>

   <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>

   <table id="functions-admin-signal-table">
    <title>Server Signaling Functions</title>
    <tgroup cols="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>
        <literal><function>pg_cancel_backend(<parameter>pid</parameter> <type>int</type>)</function></literal>
        </entry>
       <entry><type>boolean</type></entry>
       <entry>Cancel a backend's current query.  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.
        </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_reload_conf()</function></literal>
        </entry>
       <entry><type>boolean</type></entry>
       <entry>Cause server processes to reload their configuration files</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_rotate_logfile()</function></literal>
        </entry>
       <entry><type>boolean</type></entry>
       <entry>Rotate server's log file</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_terminate_backend(<parameter>pid</parameter> <type>int</type>)</function></literal>
        </entry>
       <entry><type>boolean</type></entry>
       <entry>Terminate a backend.  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.
       </entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    Each of these functions returns <literal>true</literal> if
    successful and <literal>false</literal> otherwise.
   </para>

   <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>

   <para>
    <function>pg_reload_conf</function> sends a <systemitem>SIGHUP</systemitem> signal
    to the server, causing configuration files
    to be reloaded by all server processes.
   </para>

   <para>
    <function>pg_rotate_logfile</function> 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>

  </sect2>

  <sect2 id="functions-admin-backup">
   <title>Backup Control Functions</title>

   <indexterm>
    <primary>backup</primary>
   </indexterm>
   <indexterm>
    <primary>pg_create_restore_point</primary>
   </indexterm>
   <indexterm>
    <primary>pg_current_wal_flush_lsn</primary>
   </indexterm>
   <indexterm>
    <primary>pg_current_wal_insert_lsn</primary>
   </indexterm>
   <indexterm>
    <primary>pg_current_wal_lsn</primary>
   </indexterm>
   <indexterm>
    <primary>pg_start_backup</primary>
   </indexterm>
   <indexterm>
    <primary>pg_stop_backup</primary>
   </indexterm>
   <indexterm>
    <primary>pg_is_in_backup</primary>
   </indexterm>
   <indexterm>
    <primary>pg_backup_start_time</primary>
   </indexterm>
   <indexterm>
    <primary>pg_switch_wal</primary>
   </indexterm>
   <indexterm>
    <primary>pg_walfile_name</primary>
   </indexterm>
   <indexterm>
    <primary>pg_walfile_name_offset</primary>
   </indexterm>
   <indexterm>
    <primary>pg_wal_lsn_diff</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>

   <table id="functions-admin-backup-table">
    <title>Backup Control Functions</title>
    <tgroup cols="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>
        <literal><function>pg_create_restore_point(<parameter>name</parameter> <type>text</type>)</function></literal>
        </entry>
       <entry><type>pg_lsn</type></entry>
       <entry>Create a named point for performing restore (restricted to superusers by default, but other users can be granted EXECUTE to run the function)</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_current_wal_flush_lsn()</function></literal>
        </entry>
       <entry><type>pg_lsn</type></entry>
       <entry>Get current write-ahead log flush location</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_current_wal_insert_lsn()</function></literal>
        </entry>
       <entry><type>pg_lsn</type></entry>
       <entry>Get current write-ahead log insert location</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_current_wal_lsn()</function></literal>
        </entry>
       <entry><type>pg_lsn</type></entry>
       <entry>Get current write-ahead log write location</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_start_backup(<parameter>label</parameter> <type>text</type> <optional>, <parameter>fast</parameter> <type>boolean</type> <optional>, <parameter>exclusive</parameter> <type>boolean</type> </optional></optional>)</function></literal>
        </entry>
       <entry><type>pg_lsn</type></entry>
       <entry>Prepare for performing on-line backup (restricted to superusers by default, but other users can be granted EXECUTE to run the function)</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_stop_backup()</function></literal>
        </entry>
       <entry><type>pg_lsn</type></entry>
       <entry>Finish performing exclusive on-line backup (restricted to superusers by default, but other users can be granted EXECUTE to run the function)</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_stop_backup(<parameter>exclusive</parameter> <type>boolean</type> <optional>, <parameter>wait_for_archive</parameter> <type>boolean</type> </optional>)</function></literal>
        </entry>
       <entry><type>setof record</type></entry>
       <entry>Finish performing exclusive or non-exclusive on-line backup (restricted to superusers by default, but other users can be granted EXECUTE to run the function)</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_is_in_backup()</function></literal>
        </entry>
       <entry><type>bool</type></entry>
       <entry>True if an on-line exclusive backup is still in progress.</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_backup_start_time()</function></literal>
        </entry>
       <entry><type>timestamp with time zone</type></entry>
       <entry>Get start time of an on-line exclusive backup in progress.</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_switch_wal()</function></literal>
        </entry>
       <entry><type>pg_lsn</type></entry>
       <entry>Force switch to a new write-ahead log file (restricted to superusers by default, but other users can be granted EXECUTE to run the function)</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_walfile_name(<parameter>lsn</parameter> <type>pg_lsn</type>)</function></literal>
        </entry>
       <entry><type>text</type></entry>
       <entry>Convert write-ahead log location to file name</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_walfile_name_offset(<parameter>lsn</parameter> <type>pg_lsn</type>)</function></literal>
        </entry>
       <entry><type>text</type>, <type>integer</type></entry>
       <entry>Convert write-ahead log location to file name and decimal byte offset within file</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_wal_lsn_diff(<parameter>lsn</parameter> <type>pg_lsn</type>, <parameter>lsn</parameter> <type>pg_lsn</type>)</function></literal>
       </entry>
       <entry><type>numeric</type></entry>
       <entry>Calculate the difference between two write-ahead log locations</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    <function>pg_start_backup</function> accepts an arbitrary user-defined label for
    the backup.  (Typically this would be the name under which the backup dump
    file will be stored.) When used in exclusive mode, the 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,
    performs a checkpoint, and then returns the backup's starting write-ahead
    log location as text.  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 written to the backup
    by the caller.

<programlisting>
postgres=# select pg_start_backup('label_goes_here');
 pg_start_backup
-----------------
 0/D4445B8
(1 row)
</programlisting>
    There is an optional second parameter of type <type>boolean</type>.  If <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.
   </para>

   <para>
    In an exclusive backup, <function>pg_stop_backup</function> removes the label file
    and, if it exists, the <filename>tablespace_map</filename> file created by
    <function>pg_start_backup</function>. In a non-exclusive backup, the contents of
    the <filename>backup_label</filename> and <filename>tablespace_map</filename> are returned
    in the result of the function, and should be written to files in the
    backup (and not in the data directory).  There is an optional second
    parameter of type <type>boolean</type>.  If false, the <function>pg_stop_backup</function>
    will return immediately after the backup is completed without waiting for
    WAL to be archived.  This behavior is only useful for backup
    software which independently monitors WAL archiving. Otherwise, WAL
    required to make the backup consistent might be missing and make the backup
    useless.  When this parameter is set to true, <function>pg_stop_backup</function>
    will wait for WAL to be archived when archiving is enabled; on the standby,
    this means that it will wait only when <varname>archive_mode = always</varname>.
    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, the 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.  The return
    value is the backup's ending write-ahead log location (which again
    can be ignored).  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>
    <function>pg_switch_wal</function> moves to the next write-ahead log file, allowing the
    current file to be archived (assuming you are using continuous archiving).
    The return value is the ending write-ahead log location + 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>
    <function>pg_create_restore_point</function> creates a named write-ahead log
    record that can be used as 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>
    <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 flush
    location is the location guaranteed 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 both 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 the results of any of the
    above functions.  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>

   <para>
    <function>pg_wal_lsn_diff</function> calculates the difference in bytes
    between two write-ahead log locations. It can be used with
    <structname>pg_stat_replication</structname> or some functions shown in
    <xref linkend="functions-admin-backup-table"/> to get the replication lag.
   </para>

   <para>
    For details about proper usage of these functions, see
    <xref linkend="continuous-archiving"/>.
   </para>

  </sect2>

  <sect2 id="functions-recovery-control">
   <title>Recovery Control Functions</title>

   <indexterm>
    <primary>pg_is_in_recovery</primary>
   </indexterm>
   <indexterm>
    <primary>pg_last_wal_receive_lsn</primary>
   </indexterm>
   <indexterm>
    <primary>pg_last_wal_replay_lsn</primary>
   </indexterm>
   <indexterm>
    <primary>pg_last_xact_replay_timestamp</primary>
   </indexterm>

   <para>
    The functions shown in <xref
    linkend="functions-recovery-info-table"/> provide information
    about the current status of the standby.
    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="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>
        <literal><function>pg_is_in_recovery()</function></literal>
        </entry>
       <entry><type>bool</type></entry>
       <entry>True if recovery is still in progress.
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_last_wal_receive_lsn()</function></literal>
        </entry>
       <entry><type>pg_lsn</type></entry>
       <entry>Get last write-ahead log location received and synced to disk by
        streaming replication. While streaming replication is in progress
        this will increase monotonically. If recovery has completed this will
        remain static at
        the value 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 NULL.
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_last_wal_replay_lsn()</function></literal>
        </entry>
       <entry><type>pg_lsn</type></entry>
       <entry>Get last write-ahead log location replayed during recovery.
        If recovery is still in progress this will increase monotonically.
        If recovery has completed then this value will remain static at
        the value of the last WAL record applied during that recovery.
        When the server has been started normally without recovery
        the function returns NULL.
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_last_xact_replay_timestamp()</function></literal>
        </entry>
       <entry><type>timestamp with time zone</type></entry>
       <entry>Get time stamp of 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, this function
        returns NULL.  Otherwise, if recovery is still in progress this will
        increase monotonically.  If recovery has completed then this value will
        remain static at the value of the last transaction applied during that
        recovery.  When the server has been started normally without recovery
        the function returns NULL.
       </entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <indexterm>
    <primary>pg_is_wal_replay_paused</primary>
   </indexterm>
   <indexterm>
    <primary>pg_promote</primary>
   </indexterm>
   <indexterm>
    <primary>pg_wal_replay_pause</primary>
   </indexterm>
   <indexterm>
    <primary>pg_wal_replay_resume</primary>
   </indexterm>

   <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="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>
        <literal><function>pg_is_wal_replay_paused()</function></literal>
        </entry>
       <entry><type>bool</type></entry>
       <entry>True if recovery is paused.
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_promote(<parameter>wait</parameter> <type>boolean</type> DEFAULT true, <parameter>wait_seconds</parameter> <type>integer</type> DEFAULT 60)</function></literal>
        </entry>
       <entry><type>boolean</type></entry>
       <entry>
        Promotes a physical standby server.  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
        <literal>SIGUSR1</literal> to the postmaster to trigger the promotion.
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_wal_replay_pause()</function></literal>
        </entry>
       <entry><type>void</type></entry>
       <entry>Pauses recovery immediately (restricted to superusers by default, but other users can be granted EXECUTE to run the function).
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_wal_replay_resume()</function></literal>
        </entry>
       <entry><type>void</type></entry>
       <entry>Restarts recovery if it was paused (restricted to superusers by default, but other users can be granted EXECUTE to run the function).
       </entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    While recovery is paused no further database changes are applied.
    If in hot standby, 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>
    If streaming replication is disabled, the paused state may continue
    indefinitely without problem. While streaming replication is in
    progress 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>

   <indexterm>
     <primary>pg_export_snapshot</primary>
   </indexterm>

   <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="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>
        <literal><function>pg_export_snapshot()</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>Save the current snapshot and return its identifier</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    The function <function>pg_export_snapshot</function> saves the 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.  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>

   <para>
    See  <xref linkend="sql-set-transaction"/> for details of how to use an
    exported snapshot.
   </para>
  </sect2>

  <sect2 id="functions-replication">
   <title>Replication 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 restricted to superusers.
    Use of functions for replication slot 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 <acronym>SQL</acronym> Functions</title>
    <tgroup cols="3">
     <thead>
      <row>
       <entry>Function</entry>
       <entry>Return Type</entry>
       <entry>Description</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>
        <indexterm>
         <primary>pg_create_physical_replication_slot</primary>
        </indexterm>
        <literal><function>pg_create_physical_replication_slot(<parameter>slot_name</parameter> <type>name</type> <optional>, <parameter>immediately_reserve</parameter> <type>boolean</type>, <parameter>temporary</parameter> <type>boolean</type></optional>)</function></literal>
       </entry>
       <entry>
        (<parameter>slot_name</parameter> <type>name</type>, <parameter>lsn</parameter> <type>pg_lsn</type>)
       </entry>
       <entry>
        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 current session. Temporary slots are also
        released upon any error. This function corresponds
        to the replication protocol command <literal>CREATE_REPLICATION_SLOT
        ... PHYSICAL</literal>.
       </entry>
      </row>
      <row>
       <entry>
        <indexterm>
         <primary>pg_drop_replication_slot</primary>
        </indexterm>
        <literal><function>pg_drop_replication_slot(<parameter>slot_name</parameter> <type>name</type>)</function></literal>
       </entry>
       <entry>
        <type>void</type>
       </entry>
       <entry>
        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 when connected to the same database the slot was created on.
       </entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>pg_create_logical_replication_slot</primary>
        </indexterm>
        <literal><function>pg_create_logical_replication_slot(<parameter>slot_name</parameter> <type>name</type>, <parameter>plugin</parameter> <type>name</type> <optional>, <parameter>temporary</parameter> <type>boolean</type></optional>)</function></literal>
       </entry>
       <entry>
        (<parameter>slot_name</parameter> <type>name</type>, <parameter>lsn</parameter> <type>pg_lsn</type>)
       </entry>
       <entry>
        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 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>.
       </entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>pg_copy_physical_replication_slot</primary>
        </indexterm>
        <literal><function>pg_copy_physical_replication_slot(<parameter>src_slot_name</parameter> <type>name</type>, <parameter>dst_slot_name</parameter> <type>name</type> <optional>, <parameter>temporary</parameter> <type>boolean</type></optional>)</function></literal>
       </entry>
       <entry>
        (<parameter>slot_name</parameter> <type>name</type>, <parameter>lsn</parameter> <type>pg_lsn</type>)
       </entry>
       <entry>
        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.
       </entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>pg_copy_logical_replication_slot</primary>
        </indexterm>
        <literal><function>pg_copy_logical_replication_slot(<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>)</function></literal>
       </entry>
       <entry>
        (<parameter>slot_name</parameter> <type>name</type>, <parameter>lsn</parameter> <type>pg_lsn</type>)
       </entry>
       <entry>
        Copies an existing logical replication slot named <parameter>src_slot_name</parameter>
        to a logical replication slot named <parameter>dst_slot_name</parameter>
        while 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 <parameter>temporary</parameter> or <parameter>plugin</parameter> are omitted,
        the same values as the source logical slot are used.
       </entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>pg_logical_slot_get_changes</primary>
        </indexterm>
        <literal><function>pg_logical_slot_get_changes(<parameter>slot_name</parameter> <type>name</type>, <parameter>upto_lsn</parameter> <type>pg_lsn</type>, <parameter>upto_nchanges</parameter> <type>int</type>, VARIADIC <parameter>options</parameter> <type>text[]</type>)</function></literal>
       </entry>
       <entry>
        (<parameter>lsn</parameter> <type>pg_lsn</type>, <parameter>xid</parameter> <type>xid</type>, <parameter>data</parameter> <type>text</type>)
       </entry>
       <entry>
        Returns changes in the slot <parameter>slot_name</parameter>, starting
        from the point at which since 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.
       </entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>pg_logical_slot_peek_changes</primary>
        </indexterm>
        <literal><function>pg_logical_slot_peek_changes(<parameter>slot_name</parameter> <type>name</type>, <parameter>upto_lsn</parameter> <type>pg_lsn</type>, <parameter>upto_nchanges</parameter> <type>int</type>, VARIADIC <parameter>options</parameter> <type>text[]</type>)</function></literal>
       </entry>
       <entry>
        (<parameter>lsn</parameter> <type>pg_lsn</type>, <parameter>xid</parameter> <type>xid</type>, <parameter>data</parameter> <type>text</type>)
       </entry>
       <entry>
        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.
       </entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>pg_logical_slot_get_binary_changes</primary>
        </indexterm>
        <literal><function>pg_logical_slot_get_binary_changes(<parameter>slot_name</parameter> <type>name</type>, <parameter>upto_lsn</parameter> <type>pg_lsn</type>, <parameter>upto_nchanges</parameter> <type>int</type>, VARIADIC <parameter>options</parameter> <type>text[]</type>)</function></literal>
       </entry>
       <entry>
        (<parameter>lsn</parameter> <type>pg_lsn</type>, <parameter>xid</parameter> <type>xid</type>, <parameter>data</parameter> <type>bytea</type>)
       </entry>
       <entry>
        Behaves just like
        the <function>pg_logical_slot_get_changes()</function> function,
        except that changes are returned as <type>bytea</type>.
       </entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>pg_logical_slot_peek_binary_changes</primary>
        </indexterm>
        <literal><function>pg_logical_slot_peek_binary_changes(<parameter>slot_name</parameter> <type>name</type>, <parameter>upto_lsn</parameter> <type>pg_lsn</type>, <parameter>upto_nchanges</parameter> <type>int</type>, VARIADIC <parameter>options</parameter> <type>text[]</type>)</function></literal>
       </entry>
       <entry>
        (<parameter>lsn</parameter> <type>pg_lsn</type>, <parameter>xid</parameter> <type>xid</type>, <parameter>data</parameter> <type>bytea</type>)
       </entry>
       <entry>
        Behaves just like
        the <function>pg_logical_slot_get_changes()</function> function,
        except that changes are returned as <type>bytea</type> and that
        changes are not consumed; that is, they will be returned again
        on future calls.
       </entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>pg_replication_slot_advance</primary>
        </indexterm>
        <literal><function>pg_replication_slot_advance(<parameter>slot_name</parameter> <type>name</type>, <parameter>upto_lsn</parameter> <type>pg_lsn</type>)</function></literal>
       </entry>
       <entry>
        (<parameter>slot_name</parameter> <type>name</type>, <parameter>end_lsn</parameter> <type>pg_lsn</type>)
        <type>bool</type>
       </entry>
       <entry>
        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
        name of the slot and real position to which it was advanced to.
       </entry>
      </row>

      <row>
       <entry id="pg-replication-origin-create">
        <indexterm>
         <primary>pg_replication_origin_create</primary>
        </indexterm>
        <literal><function>pg_replication_origin_create(<parameter>node_name</parameter> <type>text</type>)</function></literal>
       </entry>
       <entry>
        <type>oid</type>
       </entry>
       <entry>
        Create a replication origin with the given external
        name, and return the internal id assigned to it.
       </entry>
      </row>

      <row>
       <entry id="pg-replication-origin-drop">
        <indexterm>
         <primary>pg_replication_origin_drop</primary>
        </indexterm>
        <literal><function>pg_replication_origin_drop(<parameter>node_name</parameter> <type>text</type>)</function></literal>
       </entry>
       <entry>
        <type>void</type>
       </entry>
       <entry>
        Delete a previously created replication origin, including any
        associated replay progress.
       </entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>pg_replication_origin_oid</primary>
        </indexterm>
        <literal><function>pg_replication_origin_oid(<parameter>node_name</parameter> <type>text</type>)</function></literal>
       </entry>
       <entry>
        <type>oid</type>
       </entry>
       <entry>
        Lookup a replication origin by name and return the internal id. If no
        corresponding replication origin is found an error is thrown.
       </entry>
      </row>

      <row>
       <entry id="pg-replication-origin-session-setup">
        <indexterm>
         <primary>pg_replication_origin_session_setup</primary>
        </indexterm>
        <literal><function>pg_replication_origin_session_setup(<parameter>node_name</parameter> <type>text</type>)</function></literal>
       </entry>
       <entry>
        <type>void</type>
       </entry>
       <entry>
        Mark the current session as replaying from the given
        origin, allowing replay progress to be tracked.  Use
        <function>pg_replication_origin_session_reset</function> to revert.
        Can only be used if no previous origin is configured.
       </entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>pg_replication_origin_session_reset</primary>
        </indexterm>
        <literal><function>pg_replication_origin_session_reset()</function></literal>
       </entry>
       <entry>
        <type>void</type>
       </entry>
       <entry>
        Cancel the effects
        of <function>pg_replication_origin_session_setup()</function>.
       </entry>
      </row>

      <row>
       <entry>
        <indexterm>
         <primary>pg_replication_origin_session_is_setup</primary>
        </indexterm>
        <literal><function>pg_replication_origin_session_is_setup()</function></literal>
       </entry>
       <entry>
        <type>bool</type>
       </entry>
       <entry>
        Has a replication origin been configured in the current session?
       </entry>
      </row>

      <row>
       <entry id="pg-replication-origin-session-progress">
        <indexterm>
         <primary>pg_replication_origin_session_progress</primary>
        </indexterm>
        <literal><function>pg_replication_origin_session_progress(<parameter>flush</parameter> <type>bool</type>)</function></literal>
       </entry>
       <entry>
        <type>pg_lsn</type>
       </entry>
       <entry>
        Return the replay location for the replication origin configured 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.
       </entry>
      </row>

      <row>
       <entry id="pg-replication-origin-xact-setup">
        <indexterm>
         <primary>pg_replication_origin_xact_setup</primary>
        </indexterm>
        <literal><function>pg_replication_origin_xact_setup(<parameter>origin_lsn</parameter> <type>pg_lsn</type>, <parameter>origin_timestamp</parameter> <type>timestamptz</type>)</function></literal>
       </entry>
       <entry>
        <type>void</type>
       </entry>
       <entry>
        Mark 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 previously been
        configured using
        <function>pg_replication_origin_session_setup()</function>.
       </entry>
      </row>

      <row>
       <entry id="pg-replication-origin-xact-reset">
        <indexterm>
         <primary>pg_replication_origin_xact_reset</primary>
        </indexterm>
        <literal><function>pg_replication_origin_xact_reset()</function></literal>
       </entry>
       <entry>
        <type>void</type>
       </entry>
       <entry>
        Cancel the effects of
        <function>pg_replication_origin_xact_setup()</function>.
       </entry>
      </row>

      <row>
       <entry id="pg-replication-origin-advance">
        <indexterm>
         <primary>pg_replication_origin_advance</primary>
        </indexterm>
        <literal>pg_replication_origin_advance<function>(<parameter>node_name</parameter> <type>text</type>, <parameter>lsn</parameter> <type>pg_lsn</type>)</function></literal>
       </entry>
       <entry>
        <type>void</type>
       </entry>
       <entry>
        Set replication progress for the given node to the given
        location. This primarily is useful for setting up the initial location
        or a new location after configuration changes and similar. Be aware
        that careless use of this function can lead to inconsistently
        replicated data.
       </entry>
      </row>

      <row>
       <entry id="pg-replication-origin-progress">
        <indexterm>
         <primary>pg_replication_origin_progress</primary>
        </indexterm>
        <literal><function>pg_replication_origin_progress(<parameter>node_name</parameter> <type>text</type>, <parameter>flush</parameter> <type>bool</type>)</function></literal>
       </entry>
       <entry>
        <type>pg_lsn</type>
       </entry>
       <entry>
        Return 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.
       </entry>
      </row>

      <row>
       <entry id="pg-logical-emit-message-text">
        <indexterm>
         <primary>pg_logical_emit_message</primary>
        </indexterm>
        <literal><function>pg_logical_emit_message(<parameter>transactional</parameter> <type>bool</type>, <parameter>prefix</parameter> <type>text</type>, <parameter>content</parameter> <type>text</type>)</function></literal>
       </entry>
       <entry>
        <type>pg_lsn</type>
       </entry>
       <entry>
        Emit text logical decoding message. This can be used to pass generic
        messages to logical decoding plugins through WAL. The parameter
        <parameter>transactional</parameter> specifies if the message should
        be part of current transaction or if it should be written immediately
        and decoded as soon as the logical decoding reads the record. The
        <parameter>prefix</parameter> is textual prefix used by the logical
        decoding plugins to easily recognize interesting messages for them.
        The <parameter>content</parameter> is the text of the message.
       </entry>
      </row>

      <row>
       <entry id="pg-logical-emit-message-bytea">
        <literal><function>pg_logical_emit_message(<parameter>transactional</parameter> <type>bool</type>, <parameter>prefix</parameter> <type>text</type>, <parameter>content</parameter> <type>bytea</type>)</function></literal>
       </entry>
       <entry>
        <type>pg_lsn</type>
       </entry>
       <entry>
        Emit binary logical decoding message. This can be used to pass generic
        messages to logical decoding plugins through WAL. The parameter
        <parameter>transactional</parameter> specifies if the message should
        be part of current transaction or if it should be written immediately
        and decoded as soon as the logical decoding reads the record. The
        <parameter>prefix</parameter> is textual prefix used by the logical
        decoding plugins to easily recognize interesting messages for them.
        The <parameter>content</parameter> is the binary content of the
        message.
       </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.
   </para>

   <indexterm>
    <primary>pg_column_size</primary>
   </indexterm>
   <indexterm>
    <primary>pg_database_size</primary>
   </indexterm>
   <indexterm>
    <primary>pg_indexes_size</primary>
   </indexterm>
   <indexterm>
    <primary>pg_relation_size</primary>
   </indexterm>
   <indexterm>
    <primary>pg_size_bytes</primary>
   </indexterm>
   <indexterm>
    <primary>pg_size_pretty</primary>
   </indexterm>
   <indexterm>
    <primary>pg_table_size</primary>
   </indexterm>
   <indexterm>
    <primary>pg_tablespace_size</primary>
   </indexterm>
   <indexterm>
    <primary>pg_total_relation_size</primary>
   </indexterm>

   <table id="functions-admin-dbsize">
    <title>Database Object Size Functions</title>
    <tgroup cols="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry><literal><function>pg_column_size(<type>any</type>)</function></literal></entry>
       <entry><type>int</type></entry>
       <entry>Number of bytes used to store a particular value (possibly compressed)</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_database_size(<type>oid</type>)</function></literal>
        </entry>
       <entry><type>bigint</type></entry>
       <entry>Disk space used by the database with the specified OID</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_database_size(<type>name</type>)</function></literal>
        </entry>
       <entry><type>bigint</type></entry>
       <entry>Disk space used by the database with the specified name</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_indexes_size(<type>regclass</type>)</function></literal>
        </entry>
       <entry><type>bigint</type></entry>
       <entry>
        Total disk space used by indexes attached to the specified table
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_relation_size(<parameter>relation</parameter> <type>regclass</type>, <parameter>fork</parameter> <type>text</type>)</function></literal>
        </entry>
       <entry><type>bigint</type></entry>
       <entry>
        Disk space used by the specified fork (<literal>'main'</literal>,
        <literal>'fsm'</literal>, <literal>'vm'</literal>, or <literal>'init'</literal>)
        of the specified table or index
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_relation_size(<parameter>relation</parameter> <type>regclass</type>)</function></literal>
        </entry>
       <entry><type>bigint</type></entry>
       <entry>
        Shorthand for <literal>pg_relation_size(..., 'main')</literal>
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_size_bytes(<type>text</type>)</function></literal>
        </entry>
       <entry><type>bigint</type></entry>
       <entry>
         Converts a size in human-readable format with size units into bytes
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_size_pretty(<type>bigint</type>)</function></literal>
        </entry>
       <entry><type>text</type></entry>
       <entry>
         Converts a size in bytes expressed as a 64-bit integer into a
         human-readable format with size units
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_size_pretty(<type>numeric</type>)</function></literal>
        </entry>
       <entry><type>text</type></entry>
       <entry>
         Converts a size in bytes expressed as a numeric value into a
         human-readable format with size units
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_table_size(<type>regclass</type>)</function></literal>
        </entry>
       <entry><type>bigint</type></entry>
       <entry>
        Disk space used by the specified table, excluding indexes
        (but including TOAST, free space map, and visibility map)
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_tablespace_size(<type>oid</type>)</function></literal>
        </entry>
       <entry><type>bigint</type></entry>
       <entry>Disk space used by the tablespace with the specified OID</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_tablespace_size(<type>name</type>)</function></literal>
        </entry>
       <entry><type>bigint</type></entry>
       <entry>Disk space used by the tablespace with the specified name</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_total_relation_size(<type>regclass</type>)</function></literal>
        </entry>
       <entry><type>bigint</type></entry>
       <entry>
        Total disk space used by the specified table,
        including all indexes and <acronym>TOAST</acronym> data
       </entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    <function>pg_column_size</function> shows the space used to store any individual
    data value.
   </para>

   <para>
    <function>pg_total_relation_size</function> accepts the OID or name of a
    table or toast table, and returns the total on-disk space used for
    that table, including all associated indexes.  This function is
    equivalent to <function>pg_table_size</function>
    <literal>+</literal> <function>pg_indexes_size</function>.
   </para>

   <para>
    <function>pg_table_size</function> accepts the OID or name of a table and
    returns the disk space needed for that table, exclusive of indexes.
    (TOAST space, free space map, and visibility map are included.)
   </para>

   <para>
    <function>pg_indexes_size</function> accepts the OID or name of a table and
    returns the total disk space used by all the indexes attached to that
    table.
   </para>

   <para>
    <function>pg_database_size</function> and <function>pg_tablespace_size</function>
    accept the OID or name of a database or tablespace, and return the total
    disk space used therein.  To use <function>pg_database_size</function>,
    you must have <literal>CONNECT</literal> permission on the specified database
    (which is granted by default), or be a member of the <literal>pg_read_all_stats</literal>
    role. To use <function>pg_tablespace_size</function>, you must have
    <literal>CREATE</literal> permission 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>

   <para>
    <function>pg_relation_size</function> accepts the OID or name of a table, index
    or toast table, and returns the on-disk size in bytes of one fork of
    that 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, it 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>

   <para>
    <function>pg_size_pretty</function> can be used to format the result of one of
    the other functions in a human-readable way, using bytes, kB, MB, GB or TB
    as appropriate.
   </para>

   <para>
    <function>pg_size_bytes</function> can be used to get the size in bytes from a
    string in human-readable format. The input may have units of bytes, kB,
    MB, GB or TB, and is parsed case-insensitively. If no units are specified,
    bytes are assumed.
   </para>

   <note>
    <para>
     The units kB, MB, GB and TB used by the functions
     <function>pg_size_pretty</function> and <function>pg_size_bytes</function> are defined
     using 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>
   </note>

   <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>
    If an OID that does not represent an existing object is passed as
    argument to one of the above functions, NULL is returned.
   </para>

   <para>
    The functions shown in <xref linkend="functions-admin-dblocation"/> assist
    in identifying the specific disk files associated with database objects.
   </para>

   <indexterm>
    <primary>pg_relation_filenode</primary>
   </indexterm>
   <indexterm>
    <primary>pg_relation_filepath</primary>
   </indexterm>
   <indexterm>
    <primary>pg_filenode_relation</primary>
   </indexterm>

   <table id="functions-admin-dblocation">
    <title>Database Object Location Functions</title>
    <tgroup cols="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>
        <literal><function>pg_relation_filenode(<parameter>relation</parameter> <type>regclass</type>)</function></literal>
        </entry>
       <entry><type>oid</type></entry>
       <entry>
        Filenode number of the specified relation
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_relation_filepath(<parameter>relation</parameter> <type>regclass</type>)</function></literal>
        </entry>
       <entry><type>text</type></entry>
       <entry>
        File path name of the specified relation
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_filenode_relation(<parameter>tablespace</parameter> <type>oid</type>, <parameter>filenode</parameter> <type>oid</type>)</function></literal>
        </entry>
       <entry><type>regclass</type></entry>
       <entry>
        Find the relation associated with a given tablespace and filenode
       </entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    <function>pg_relation_filenode</function> accepts the OID or name of a table,
    index, sequence, or toast table, and returns the <quote>filenode</quote> number
    currently assigned to it.  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 tables 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>

   <para>
    <function>pg_relation_filepath</function> is similar to
    <function>pg_relation_filenode</function>, but it returns the entire file path name
    (relative to the database cluster's data directory <varname>PGDATA</varname>) of
    the relation.
   </para>

   <para>
    <function>pg_filenode_relation</function> is the reverse of
    <function>pg_relation_filenode</function>. Given a <quote>tablespace</quote> OID and
    a <quote>filenode</quote>, it returns the associated relation's OID. For a table
    in the database's default tablespace, the tablespace can be specified as 0.
   </para>

   <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="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry></row>
     </thead>

     <tbody>
      <row>
       <entry>
        <indexterm><primary>pg_collation_actual_version</primary></indexterm>
        <literal><function>pg_collation_actual_version(<type>oid</type>)</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>Return actual version of collation from operating system</entry>
      </row>
      <row>
       <entry>
        <indexterm><primary>pg_import_system_collations</primary></indexterm>
        <literal><function>pg_import_system_collations(<parameter>schema</parameter> <type>regnamespace</type>)</function></literal>
       </entry>
       <entry><type>integer</type></entry>
       <entry>Import operating system collations</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    <function>pg_collation_actual_version</function> 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 <literal>pg_collation.collversion</literal>, then objects depending on
    the collation might need to be rebuilt.  See also
    <xref linkend="sql-altercollation"/>.
   </para>

   <para>
    <function>pg_import_system_collations</function> adds collations to the system
    catalog <literal>pg_collation</literal> 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 <literal>pg_collation</literal> 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>

   <table id="functions-info-partition">
    <title>Partitioning Information Functions</title>
    <tgroup cols="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry></row>
     </thead>

     <tbody>
      <row>
       <entry>
        <indexterm><primary>pg_partition_tree</primary></indexterm>
        <literal><function>pg_partition_tree(<type>regclass</type>)</function></literal>
       </entry>
       <entry><type>setof record</type></entry>
       <entry>
        List information about tables or indexes in a partition tree for a
        given partitioned table or partitioned index, with one row for each
        partition.  Information provided includes the name of the partition,
        the name of its immediate parent, a boolean value telling if the
        partition is a leaf, and an integer telling its level in the hierarchy.
        The value of level begins at <literal>0</literal> for the input table
        or index in its role as the root of the partition tree,
        <literal>1</literal> for its partitions, <literal>2</literal> for
        their partitions, and so on.
       </entry>
      </row>
      <row>
       <entry>
        <indexterm><primary>pg_partition_ancestors</primary></indexterm>
        <literal><function>pg_partition_ancestors(<type>regclass</type>)</function></literal>
       </entry>
       <entry><type>setof regclass</type></entry>
       <entry>
        List the ancestor relations of the given partition,
        including the partition itself.
       </entry>
      </row>
      <row>
       <entry>
        <indexterm><primary>pg_partition_root</primary></indexterm>
        <literal><function>pg_partition_root(<type>regclass</type>)</function></literal>
       </entry>
       <entry><type>regclass</type></entry>
       <entry>
        Return the top-most parent of a partition tree to which the given
        relation belongs.
       </entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    To check the total size of the data contained in
    <structname>measurement</structname> table described in
    <xref linkend="ddl-partitioning-declarative-example"/>, one could use the
    following query:
   </para>

<programlisting>
=# SELECT pg_size_pretty(sum(pg_relation_size(relid))) AS total_size
     FROM pg_partition_tree('measurement');
 total_size 
------------
 24 kB
(1 row)
</programlisting>

  </sect2>

  <sect2 id="functions-admin-index">
   <title>Index Maintenance Functions</title>

   <indexterm>
    <primary>brin_summarize_new_values</primary>
   </indexterm>

   <indexterm>
    <primary>gin_clean_pending_list</primary>
   </indexterm>

   <indexterm>
    <primary>brin_summarize_range</primary>
   </indexterm>

   <indexterm>
    <primary>brin_desummarize_range</primary>
   </indexterm>

   <para>
    <xref linkend="functions-admin-index-table"/> shows the functions
    available for index maintenance tasks.
    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="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry></row>
     </thead>

     <tbody>
      <row>
       <entry>
        <literal><function>brin_summarize_new_values(<parameter>index</parameter> <type>regclass</type>)</function></literal>
       </entry>
       <entry><type>integer</type></entry>
       <entry>summarize page ranges not already summarized</entry>
      </row>
      <row>
       <entry>
        <literal><function>brin_summarize_range(<parameter>index</parameter> <type>regclass</type>, <parameter>blockNumber</parameter> <type>bigint</type>)</function></literal>
       </entry>
       <entry><type>integer</type></entry>
       <entry>summarize the page range covering the given block, if not already summarized</entry>
      </row>
      <row>
       <entry>
        <literal><function>brin_desummarize_range(<parameter>index</parameter> <type>regclass</type>, <parameter>blockNumber</parameter> <type>bigint</type>)</function></literal>
       </entry>
       <entry><type>integer</type></entry>
       <entry>de-summarize the page range covering the given block, if summarized</entry>
      </row>
      <row>
       <entry>
        <literal><function>gin_clean_pending_list(<parameter>index</parameter> <type>regclass</type>)</function></literal>
       </entry>
       <entry><type>bigint</type></entry>
       <entry>move GIN pending list entries into main index structure</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    <function>brin_summarize_new_values</function> accepts the OID or name of a
    BRIN index and inspects the 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 the table pages.
    It returns the number of new page range summaries that were inserted
    into the index.  <function>brin_summarize_range</function> does the same, except
    it only summarizes the range that covers the given block number.
   </para>

   <para>
    <function>gin_clean_pending_list</function> accepts the OID or name of
    a GIN index and cleans up the pending list of the specified index
    by moving entries in it to the main GIN data structure in bulk.
    It returns the number of pages removed from the pending list.
    Note that if the argument is a GIN index built with
    the <literal>fastupdate</literal> option disabled, no cleanup happens and the
    return value is 0, because the index doesn't have a pending list.
    Please see <xref linkend="gin-fast-update"/> and <xref linkend="gin-tips"/>
    for details of the pending list and <literal>fastupdate</literal> option.
   </para>

  </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 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 which the database can read and
    that those reads bypass all in-database privilege checks.  This means that,
    among other things, a user with this access is able to read the contents of the
    <literal>pg_authid</literal> table where authentication information is contained,
    as well as read any file in the database.  Therefore, granting access to these
    functions should be carefully considered.
   </para>

   <table id="functions-admin-genfile-table">
    <title>Generic File Access Functions</title>
    <tgroup cols="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>
        <literal><function>pg_ls_dir(<parameter>dirname</parameter> <type>text</type> [, <parameter>missing_ok</parameter> <type>boolean</type>, <parameter>include_dot_dirs</parameter> <type>boolean</type>])</function></literal>
       </entry>
       <entry><type>setof text</type></entry>
       <entry>
        List the contents of a directory.  Restricted to superusers by default, but other users can be granted EXECUTE to run the function.
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_ls_logdir()</function></literal>
       </entry>
       <entry><type>setof record</type></entry>
       <entry>
        List the name, size, and last modification time of files in the log
        directory. Access is granted to members of the <literal>pg_monitor</literal>
        role and may be granted to other non-superuser roles.
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_ls_waldir()</function></literal>
       </entry>
       <entry><type>setof record</type></entry>
       <entry>
        List the name, size, and last modification time of files in the WAL
        directory. Access is granted to members of the <literal>pg_monitor</literal>
        role and may be granted to other non-superuser roles.
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_ls_archive_statusdir()</function></literal>
       </entry>
       <entry><type>setof record</type></entry>
       <entry>
        List the name, size, and last modification time of files in the WAL
        archive status directory. Access is granted to members of the
        <literal>pg_monitor</literal> role and may be granted to other
        non-superuser roles.
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_ls_tmpdir(<optional><parameter>tablespace</parameter> <type>oid</type></optional>)</function></literal>
       </entry>
       <entry><type>setof record</type></entry>
       <entry>
        List the name, size, and last modification time of files in the
        temporary directory for <parameter>tablespace</parameter>.  If
        <parameter>tablespace</parameter> is not provided, the
        <literal>pg_default</literal> tablespace is used.  Access is granted
        to members of the <literal>pg_monitor</literal> role and may be
        granted to other non-superuser roles.
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_read_file(<parameter>filename</parameter> <type>text</type> [, <parameter>offset</parameter> <type>bigint</type>, <parameter>length</parameter> <type>bigint</type> [, <parameter>missing_ok</parameter> <type>boolean</type>] ])</function></literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Return the contents of a text file.  Restricted to superusers by default, but other users can be granted EXECUTE to run the function.
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_read_binary_file(<parameter>filename</parameter> <type>text</type> [, <parameter>offset</parameter> <type>bigint</type>, <parameter>length</parameter> <type>bigint</type> [, <parameter>missing_ok</parameter> <type>boolean</type>] ])</function></literal>
       </entry>
       <entry><type>bytea</type></entry>
       <entry>
        Return the contents of a file.  Restricted to superusers by default, but other users can be granted EXECUTE to run the function.
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_stat_file(<parameter>filename</parameter> <type>text</type>[, <parameter>missing_ok</parameter> <type>boolean</type>])</function></literal>
       </entry>
       <entry><type>record</type></entry>
       <entry>
        Return information about a file.  Restricted to superusers by default, but other users can be granted EXECUTE to run the function.
       </entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <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 NULL (except
    <function>pg_ls_dir</function>, which returns an empty result set). If
    <literal>false</literal>, an error is raised. The default is <literal>false</literal>.
   </para>

   <indexterm>
    <primary>pg_ls_dir</primary>
   </indexterm>
   <para>
    <function>pg_ls_dir</function> returns the names of all files (and directories
    and other special files) in the specified directory. The <parameter>
    include_dot_dirs</parameter> indicates whether <quote>.</quote> and <quote>..</quote> are
    included in the result set. The default is to exclude them
    (<literal>false</literal>), but including them can be useful when
    <parameter>missing_ok</parameter> is <literal>true</literal>, to distinguish an
    empty directory from an non-existent directory.
   </para>

   <indexterm>
    <primary>pg_ls_logdir</primary>
   </indexterm>
   <para>
    <function>pg_ls_logdir</function> returns the name, size, and last modified time
    (mtime) of each file in the log directory. By default, only superusers
    and members of the <literal>pg_monitor</literal> role can use this function.
    Access may be granted to others using <command>GRANT</command>.
   </para>

   <indexterm>
    <primary>pg_ls_waldir</primary>
   </indexterm>
   <para>
    <function>pg_ls_waldir</function> returns the name, size, and last modified time
    (mtime) of each file in the write ahead log (WAL) directory. By
    default only superusers and members of the <literal>pg_monitor</literal> role
    can use this function. Access may be granted to others using
    <command>GRANT</command>.
   </para>

   <indexterm>
    <primary>pg_ls_archive_statusdir</primary>
   </indexterm>
   <para>
    <function>pg_ls_archive_statusdir</function> returns the name, size, and
    last modified time (mtime) of each file in the WAL archive status
    directory <filename>pg_wal/archive_status</filename>. By default only
    superusers and members of the <literal>pg_monitor</literal> role can
    use this function. Access may be granted to others using
    <command>GRANT</command>.
   </para>

   <indexterm>
    <primary>pg_ls_tmpdir</primary>
   </indexterm>
   <para>
    <function>pg_ls_tmpdir</function> returns the name, size, and last modified
    time (mtime) of each 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 used.  By
    default only superusers and members of the <literal>pg_monitor</literal>
    role can use this function.  Access may be granted to others using
    <command>GRANT</command>.
   </para>

   <indexterm>
    <primary>pg_read_file</primary>
   </indexterm>
   <para>
    <function>pg_read_file</function> returns part of a text file, starting
    at the given <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 server encoding; an error is thrown if they are not valid in that
    encoding.
   </para>

   <indexterm>
    <primary>pg_read_binary_file</primary>
   </indexterm>
   <para>
    <function>pg_read_binary_file</function> is similar to
    <function>pg_read_file</function>, except that the result is a <type>bytea</type> value;
    accordingly, no encoding checks are performed.
    In combination with the <function>convert_from</function> function, this function
    can be used to read a file in a specified encoding:
<programlisting>
SELECT convert_from(pg_read_binary_file('file_in_utf8.txt'), 'UTF8');
</programlisting>
   </para>

   <indexterm>
    <primary>pg_stat_file</primary>
   </indexterm>
   <para>
    <function>pg_stat_file</function> returns a record containing the file
    size, last accessed time stamp, last modified time stamp,
    last file status change time stamp (Unix platforms only),
    file creation time stamp (Windows only), and a <type>boolean</type>
    indicating if it is a directory.  Typical usages include:
<programlisting>
SELECT * FROM pg_stat_file('filename');
SELECT (pg_stat_file('filename')).modification;
</programlisting>
   </para>

  </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>

   <table id="functions-advisory-locks-table">
    <title>Advisory Lock Functions</title>
    <tgroup cols="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>
        <literal><function>pg_advisory_lock(<parameter>key</parameter> <type>bigint</type>)</function></literal>
       </entry>
       <entry><type>void</type></entry>
       <entry>Obtain exclusive session level advisory lock</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_advisory_lock(<parameter>key1</parameter> <type>int</type>, <parameter>key2</parameter> <type>int</type>)</function></literal>
       </entry>
       <entry><type>void</type></entry>
       <entry>Obtain exclusive session level advisory lock</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_advisory_lock_shared(<parameter>key</parameter> <type>bigint</type>)</function></literal>
       </entry>
       <entry><type>void</type></entry>
       <entry>Obtain shared session level advisory lock</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_advisory_lock_shared(<parameter>key1</parameter> <type>int</type>, <parameter>key2</parameter> <type>int</type>)</function></literal>
       </entry>
       <entry><type>void</type></entry>
       <entry>Obtain shared session level advisory lock</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_advisory_unlock(<parameter>key</parameter> <type>bigint</type>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Release an exclusive session level advisory lock</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_advisory_unlock(<parameter>key1</parameter> <type>int</type>, <parameter>key2</parameter> <type>int</type>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Release an exclusive session level advisory lock</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_advisory_unlock_all()</function></literal>
       </entry>
       <entry><type>void</type></entry>
       <entry>Release all session level advisory locks held by the current session</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_advisory_unlock_shared(<parameter>key</parameter> <type>bigint</type>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Release a shared session level advisory lock</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_advisory_unlock_shared(<parameter>key1</parameter> <type>int</type>, <parameter>key2</parameter> <type>int</type>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Release a shared session level advisory lock</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_advisory_xact_lock(<parameter>key</parameter> <type>bigint</type>)</function></literal>
       </entry>
       <entry><type>void</type></entry>
       <entry>Obtain exclusive transaction level advisory lock</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_advisory_xact_lock(<parameter>key1</parameter> <type>int</type>, <parameter>key2</parameter> <type>int</type>)</function></literal>
       </entry>
       <entry><type>void</type></entry>
       <entry>Obtain exclusive transaction level advisory lock</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_advisory_xact_lock_shared(<parameter>key</parameter> <type>bigint</type>)</function></literal>
       </entry>
       <entry><type>void</type></entry>
       <entry>Obtain shared transaction level advisory lock</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_advisory_xact_lock_shared(<parameter>key1</parameter> <type>int</type>, <parameter>key2</parameter> <type>int</type>)</function></literal>
       </entry>
       <entry><type>void</type></entry>
       <entry>Obtain shared transaction level advisory lock</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_try_advisory_lock(<parameter>key</parameter> <type>bigint</type>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Obtain exclusive session level advisory lock if available</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_try_advisory_lock(<parameter>key1</parameter> <type>int</type>, <parameter>key2</parameter> <type>int</type>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Obtain exclusive session level advisory lock if available</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_try_advisory_lock_shared(<parameter>key</parameter> <type>bigint</type>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Obtain shared session level advisory lock if available</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_try_advisory_lock_shared(<parameter>key1</parameter> <type>int</type>, <parameter>key2</parameter> <type>int</type>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Obtain shared session level advisory lock if available</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_try_advisory_xact_lock(<parameter>key</parameter> <type>bigint</type>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Obtain exclusive transaction level advisory lock if available</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_try_advisory_xact_lock(<parameter>key1</parameter> <type>int</type>, <parameter>key2</parameter> <type>int</type>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Obtain exclusive transaction level advisory lock if available</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_try_advisory_xact_lock_shared(<parameter>key</parameter> <type>bigint</type>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Obtain shared transaction level advisory lock if available</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_try_advisory_xact_lock_shared(<parameter>key1</parameter> <type>int</type>, <parameter>key2</parameter> <type>int</type>)</function></literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Obtain shared transaction level advisory lock if available</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <indexterm>
    <primary>pg_advisory_lock</primary>
   </indexterm>
   <para>
    <function>pg_advisory_lock</function> locks an application-defined resource,
    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 lock on the same resource identifier,
    this function will wait until the resource becomes available.  The lock
    is exclusive.  Multiple lock requests stack, so that if the same resource
    is locked three times it must then be unlocked three times to be
    released for other sessions' use.
   </para>

   <indexterm>
    <primary>pg_advisory_lock_shared</primary>
   </indexterm>
   <para>
    <function>pg_advisory_lock_shared</function> works the same as
    <function>pg_advisory_lock</function>,
    except the lock can be shared with other sessions requesting shared locks.
    Only would-be exclusive lockers are locked out.
   </para>

   <indexterm>
    <primary>pg_try_advisory_lock</primary>
   </indexterm>
   <para>
    <function>pg_try_advisory_lock</function> is similar to
    <function>pg_advisory_lock</function>, except the function will not wait for the
    lock to become available.  It will either obtain the lock immediately and
    return <literal>true</literal>, or return <literal>false</literal> if the lock cannot be
    acquired immediately.
   </para>

   <indexterm>
    <primary>pg_try_advisory_lock_shared</primary>
   </indexterm>
   <para>
    <function>pg_try_advisory_lock_shared</function> works the same as
    <function>pg_try_advisory_lock</function>, except it attempts to acquire
    a shared rather than an exclusive lock.
   </para>

   <indexterm>
    <primary>pg_advisory_unlock</primary>
   </indexterm>
   <para>
    <function>pg_advisory_unlock</function> will release a previously-acquired
    exclusive session level advisory lock.  It
    returns <literal>true</literal> if the lock is successfully released.
    If the lock was not held, it will return <literal>false</literal>,
    and in addition, an SQL warning will be reported by the server.
   </para>

   <indexterm>
    <primary>pg_advisory_unlock_shared</primary>
   </indexterm>
   <para>
    <function>pg_advisory_unlock_shared</function> works the same as
    <function>pg_advisory_unlock</function>,
    except it releases a shared session level advisory lock.
   </para>

   <indexterm>
    <primary>pg_advisory_unlock_all</primary>
   </indexterm>
   <para>
    <function>pg_advisory_unlock_all</function> will release 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>

   <indexterm>
    <primary>pg_advisory_xact_lock</primary>
   </indexterm>
   <para>
    <function>pg_advisory_xact_lock</function> works the same as
    <function>pg_advisory_lock</function>, except the lock is automatically released
    at the end of the current transaction and cannot be released explicitly.
   </para>

   <indexterm>
    <primary>pg_advisory_xact_lock_shared</primary>
   </indexterm>
   <para>
    <function>pg_advisory_xact_lock_shared</function> works the same as
    <function>pg_advisory_lock_shared</function>, except the lock is automatically released
    at the end of the current transaction and cannot be released explicitly.
   </para>

   <indexterm>
    <primary>pg_try_advisory_xact_lock</primary>
   </indexterm>
   <para>
    <function>pg_try_advisory_xact_lock</function> works the same as
    <function>pg_try_advisory_lock</function>, except the lock, if acquired,
    is automatically released at the end of the current transaction and
    cannot be released explicitly.
   </para>

   <indexterm>
    <primary>pg_try_advisory_xact_lock_shared</primary>
   </indexterm>
   <para>
    <function>pg_try_advisory_xact_lock_shared</function> works the same as
    <function>pg_try_advisory_lock_shared</function>, except the lock, if acquired,
    is automatically released at the end of the current transaction and
    cannot be released explicitly.
   </para>

  </sect2>

  </sect1>

  <sect1 id="functions-trigger">
   <title>Trigger Functions</title>

   <indexterm>
     <primary>suppress_redundant_updates_trigger</primary>
   </indexterm>

   <para>
      Currently <productname>PostgreSQL</productname> provides one built in trigger
      function, <function>suppress_redundant_updates_trigger</function>,
      which will prevent any update
      that does not actually change the data in the row from taking place, in
      contrast to the normal behavior which always performs the 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 normally 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 an update are actually changed,
      use of this trigger will actually make the update run slower.
    </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 would want to fire this trigger last for each row.
      Bearing in mind that triggers fire in name order, you would then
      choose a trigger name that comes after the name of any other trigger
      you might have on the table.
    </para>
    <para>
       For more information about creating triggers, see
        <xref linkend="sql-createtrigger"/>.
    </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>

   <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>bool</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>

   <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,
    <function>pg_event_trigger_dropped_objects</function> raises an error.
    <function>pg_event_trigger_dropped_objects</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>bool</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>bool</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>bool</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</title>
    <tgroup cols="3">
     <thead>
      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry></row>
     </thead>

     <tbody>
      <row>
       <entry>
        <indexterm><primary>pg_event_trigger_table_rewrite_oid</primary></indexterm>
        <literal><function>pg_event_trigger_table_rewrite_oid()</function></literal>
       </entry>
       <entry><type>Oid</type></entry>
       <entry>The OID of the table about to be rewritten.</entry>
      </row>

      <row>
       <entry>
        <indexterm><primary>pg_event_trigger_table_rewrite_reason</primary></indexterm>
        <literal><function>pg_event_trigger_table_rewrite_reason()</function></literal>
       </entry>
       <entry><type>int</type></entry>
       <entry>
        The reason code(s) explaining the reason for rewriting. The exact
        meaning of the codes is release dependent.
       </entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    The <function>pg_event_trigger_table_rewrite_oid</function> function 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>
     <secondary>pg_mcv_list</secondary>
   </indexterm>

   <para>
    <function>pg_mcv_list_items</function> returns a list of all items
    stored in a multi-column <acronym>MCV</acronym> list, and 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>int</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> can be obtained only from the
     <literal>pg_statistic_ext_data.stxdmcv</literal> column.
   </para>
  </sect2>

  </sect1>

</chapter>
