<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.470 2009/01/15 18:19:58 heikki Exp $ -->

 <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 show the list of all actually
   available functions and operators, respectively.
  </para>

  <para>
   If you are concerned about portability then take 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 the 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</></member>
     <member><literal>OR</></member>
     <member><literal>NOT</></member>
    </simplelist>

    <acronym>SQL</acronym> uses a three-valued Boolean logic where the null value 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 Operators</title>

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

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

   <table id="functions-comparison-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 data types where this
    makes sense.  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>
    <indexterm>
     <primary>BETWEEN</primary>
    </indexterm>
    In addition to the comparison operators, the special
    <token>BETWEEN</token> construct is available.
<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>
    Similarly,
<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>
    There is no difference between the two respective forms apart from
    the <acronym>CPU</acronym> cycles required to rewrite the first one
    into the second one internally.
    <indexterm>
     <primary>BETWEEN SYMMETRIC</primary>
    </indexterm>
    <token>BETWEEN SYMMETRIC</> is the same as <literal>BETWEEN</>
    except there is no requirement that the argument to the left of <literal>AND</> be less than
    or equal to the argument on the right;  the proper range is automatically determined.
   </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 constructs
<synopsis>
<replaceable>expression</replaceable> IS NULL
<replaceable>expression</replaceable> IS NOT NULL
</synopsis>
    or the equivalent, but nonstandard, constructs
<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</> is not <quote>equal to</quote>
    <literal>NULL</>.  (The null value represents an unknown value,
    and it is not known whether two unknown values are equal.) This
    behavior conforms to the SQL standard.
   </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>.  This was
    the default behavior in <productname>PostgreSQL</productname>
    releases 6.5 through 7.1.
   </para>
  </tip>

  <note>
   <para>
    If the <replaceable>expression</replaceable> is row-valued, then
    <literal>IS NULL</> is true when the row expression itself is null
    or when all the row's fields are null, while
    <literal>IS NOT NULL</> 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</> and <literal>IS NOT NULL</> do not always return
    inverse results for row-valued expressions, i.e. a row-valued
    expression that contains both NULL and non-null values will return false
    for both tests.
    This definition conforms to the SQL standard, and is a change from the
    inconsistent behavior exhibited by <productname>PostgreSQL</productname>
    versions prior to 8.2.
   </para>
  </note>

   <para>
    <indexterm>
     <primary>IS DISTINCT FROM</primary>
    </indexterm>
    <indexterm>
     <primary>IS NOT DISTINCT FROM</primary>
    </indexterm>
    The ordinary comparison operators yield null (signifying <quote>unknown</>)
    when either input is null.  Another way to do comparisons is with the
    <literal>IS <optional> NOT </> DISTINCT FROM</literal> construct:
<synopsis>
<replaceable>expression</replaceable> IS DISTINCT FROM <replaceable>expression</replaceable>
<replaceable>expression</replaceable> IS NOT DISTINCT FROM <replaceable>expression</replaceable>
</synopsis>
    For non-null inputs, <literal>IS DISTINCT FROM</literal> is
    the same as the <literal>&lt;&gt;</> operator.  However, when both
    inputs are null it will return false, and when just one input is
    null it will return 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 constructs effectively act as though null
    were a normal data value, rather than <quote>unknown</>.
   </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 constructs
<synopsis>
<replaceable>expression</replaceable> IS TRUE
<replaceable>expression</replaceable> IS NOT TRUE
<replaceable>expression</replaceable> IS FALSE
<replaceable>expression</replaceable> IS NOT FALSE
<replaceable>expression</replaceable> IS UNKNOWN
<replaceable>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</>.
    Notice that <literal>IS UNKNOWN</> and <literal>IS NOT UNKNOWN</> 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
    constructs
<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>
-->

  </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
    common mathematical conventions for all possible permutations 
    (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 results)</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</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>

   <indexterm>
    <primary>abs</primary>
   </indexterm>
   <indexterm>
    <primary>cbrt</primary>
   </indexterm>
   <indexterm>
    <primary>ceiling</primary>
   </indexterm>
   <indexterm>
    <primary>degrees</primary>
   </indexterm>
   <indexterm>
    <primary>div</primary>
   </indexterm>
   <indexterm>
    <primary>exp</primary>
   </indexterm>
   <indexterm>
    <primary>floor</primary>
   </indexterm>
   <indexterm>
    <primary>ln</primary>
   </indexterm>
   <indexterm>
    <primary>log</primary>
   </indexterm>
   <indexterm>
    <primary>mod</primary>
   </indexterm>
   <indexterm>
    <primary>pi</primary>
   </indexterm>
   <indexterm>
    <primary>power</primary>
   </indexterm>
   <indexterm>
    <primary>radians</primary>
   </indexterm>
   <indexterm>
    <primary>random</primary>
   </indexterm>
   <indexterm>
    <primary>round</primary>
   </indexterm>
   <indexterm>
    <primary>setseed</primary>
   </indexterm>
   <indexterm>
    <primary>sign</primary>
   </indexterm>
   <indexterm>
    <primary>sqrt</primary>
   </indexterm>
   <indexterm>
    <primary>trunc</primary>
   </indexterm>
   <indexterm>
    <primary>width_bucket</primary>
   </indexterm>

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

      <row>
       <entry><literal><function>cbrt</function>(<type>dp</type>)</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><literal><function>ceil</function>(<type>dp</type> or <type>numeric</type>)</literal></entry>
       <entry>(same as input)</entry>
       <entry>smallest integer not less than argument</entry>
       <entry><literal>ceil(-42.8)</literal></entry>
       <entry><literal>-42</literal></entry>
      </row>

      <row>
       <entry><literal><function>ceiling</function>(<type>dp</type> or <type>numeric</type>)</literal></entry>
       <entry>(same as input)</entry>
       <entry>smallest integer not less than argument (alias for <function>ceil</function>)</entry>
       <entry><literal>ceiling(-95.3)</literal></entry>
       <entry><literal>-95</literal></entry>
      </row>

      <row>
       <entry><literal><function>degrees</function>(<type>dp</type>)</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><literal><function>div</function>(<parameter>y</parameter> <type>numeric</>,
        <parameter>x</parameter> <type>numeric</>)</literal></entry>
       <entry><type>numeric</></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><literal><function>exp</function>(<type>dp</type> or <type>numeric</type>)</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><literal><function>floor</function>(<type>dp</type> or <type>numeric</type>)</literal></entry>
       <entry>(same as input)</entry>
       <entry>largest integer not greater than argument</entry>
       <entry><literal>floor(-42.8)</literal></entry>
       <entry><literal>-43</literal></entry>
      </row>

      <row>
       <entry><literal><function>ln</function>(<type>dp</type> or <type>numeric</type>)</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><literal><function>log</function>(<type>dp</type> or <type>numeric</type>)</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><literal><function>log</function>(<parameter>b</parameter> <type>numeric</type>,
        <parameter>x</parameter> <type>numeric</type>)</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><literal><function>mod</function>(<parameter>y</parameter>,
        <parameter>x</parameter>)</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><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><literal><function>power</function>(<parameter>a</parameter> <type>dp</type>,
        <parameter>b</parameter> <type>dp</type>)</literal></entry>
       <entry><type>dp</type></entry>
       <entry><parameter>a</> 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</function>(<parameter>a</parameter> <type>numeric</type>,
        <parameter>b</parameter> <type>numeric</type>)</literal></entry>
       <entry><type>numeric</type></entry>
       <entry><parameter>a</> 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>radians</function>(<type>dp</type>)</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><literal><function>random</function>()</literal></entry>
       <entry><type>dp</type></entry>
       <entry>random value between 0.0 and 1.0</entry>
       <entry><literal>random()</literal></entry>
       <entry></entry>
      </row>

      <row>
       <entry><literal><function>round</function>(<type>dp</type> or <type>numeric</type>)</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</function>(<parameter>v</parameter> <type>numeric</type>, <parameter>s</parameter> <type>int</type>)</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><literal><function>setseed</function>(<type>dp</type>)</literal></entry>
       <entry><type>void</type></entry>
       <entry>set seed for subsequent <literal>random()</literal> calls (value between -1.0 and 1.0)</entry>
       <entry><literal>setseed(0.54823)</literal></entry>
       <entry></entry>
      </row>

      <row>
       <entry><literal><function>sign</function>(<type>dp</type> or <type>numeric</type>)</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><literal><function>sqrt</function>(<type>dp</type> or <type>numeric</type>)</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><literal><function>trunc</function>(<type>dp</type> or <type>numeric</type>)</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</function>(<parameter>v</parameter> <type>numeric</type>, <parameter>s</parameter> <type>int</type>)</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><literal><function>width_bucket</function>(<parameter>op</parameter> <type>numeric</type>, <parameter>b1</parameter> <type>numeric</type>, <parameter>b2</parameter> <type>numeric</type>, <parameter>count</parameter> <type>int</type>)</literal></entry>
       <entry><type>int</type></entry>
       <entry>return the bucket to which <parameter>operand</> would
       be assigned in an equidepth histogram with <parameter>count</>
       buckets, in the range <parameter>b1</> to <parameter>b2</></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</function>(<parameter>op</parameter> <type>dp</type>, <parameter>b1</parameter> <type>dp</type>, <parameter>b2</parameter> <type>dp</type>, <parameter>count</parameter> <type>int</type>)</literal></entry>
       <entry><type>int</type></entry>
       <entry>return the bucket to which <parameter>operand</> would
       be assigned in an equidepth histogram with <parameter>count</>
       buckets, in the range <parameter>b1</> to <parameter>b2</></entry>
       <entry><literal>width_bucket(5.35, 0.024, 10.06, 5)</literal></entry>
       <entry><literal>3</literal></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   Finally, <xref linkend="functions-math-trig-table"> shows the
   available trigonometric functions.  All trigonometric functions
   take arguments and return values of type <type>double
   precision</type>.
  </para>

   <indexterm>
    <primary>acos</primary>
   </indexterm>
   <indexterm>
    <primary>asin</primary>
   </indexterm>
   <indexterm>
    <primary>atan</primary>
   </indexterm>
   <indexterm>
    <primary>atan2</primary>
   </indexterm>
   <indexterm>
    <primary>cos</primary>
   </indexterm>
   <indexterm>
    <primary>cot</primary>
   </indexterm>
   <indexterm>
    <primary>sin</primary>
   </indexterm>
   <indexterm>
    <primary>tan</primary>
   </indexterm>

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

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

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

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

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

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

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

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

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

      <row>
       <entry><literal><function>tan</function>(<replaceable>x</replaceable>)</literal></entry>
       <entry>tangent</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 with a special syntax
    wherein certain key words rather than commas are used to separate the
    arguments.  Details are in <xref linkend="functions-string-sql">.
    These functions are also implemented using the regular syntax for
    function invocation.  (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</>.  Those coercions have been removed because they frequently
     caused surprising behaviors.  However, the string concatenation operator
     (<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</> if you need to duplicate the previous behavior.
    </para>
   </note>

   <indexterm>
    <primary>bit_length</primary>
   </indexterm>
   <indexterm>
    <primary>char_length</primary>
   </indexterm>
   <indexterm>
    <primary>lower</primary>
   </indexterm>
   <indexterm>
    <primary>octet_length</primary>
   </indexterm>
   <indexterm>
    <primary>overlay</primary>
   </indexterm>
   <indexterm>
    <primary>position</primary>
   </indexterm>
   <indexterm>
    <primary>substring</primary>
   </indexterm>
   <indexterm>
    <primary>trim</primary>
   </indexterm>
   <indexterm>
    <primary>upper</primary>
   </indexterm>

   <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><literal><function>bit_length</function>(<parameter>string</parameter>)</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><literal><function>char_length</function>(<parameter>string</parameter>)</literal> or <literal><function>character_length</function>(<parameter>string</parameter>)</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><literal><function>lower</function>(<parameter>string</parameter>)</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><literal><function>octet_length</function>(<parameter>string</parameter>)</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><literal><function>overlay</function>(<parameter>string</parameter> placing <parameter>string</parameter> from <type>int</type> <optional>for <type>int</type></optional>)</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><literal><function>position</function>(<parameter>substring</parameter> in <parameter>string</parameter>)</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><literal><function>substring</function>(<parameter>string</parameter> <optional>from <type>int</type></optional> <optional>for <type>int</type></optional>)</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</function>(<parameter>string</parameter> from <replaceable>pattern</replaceable>)</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</function>(<parameter>string</parameter> from <replaceable>pattern</replaceable> for <replaceable>escape</replaceable>)</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>
        <literal><function>trim</function>(<optional>leading | trailing | both</optional>
        <optional><parameter>characters</parameter></optional> from
        <parameter>string</parameter>)</literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Remove the longest string containing only the
        <parameter>characters</parameter> (a space by default) from the
        start/end/both ends of the <parameter>string</parameter>
       </entry>
       <entry><literal>trim(both 'x' from 'xTomxx')</literal></entry>
       <entry><literal>Tom</literal></entry>
      </row>

      <row>
       <entry><literal><function>upper</function>(<parameter>string</parameter>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>Convert string to uppercase</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>

   <indexterm>
    <primary>ascii</primary>
   </indexterm>
   <indexterm>
    <primary>btrim</primary>
   </indexterm>
   <indexterm>
    <primary>chr</primary>
   </indexterm>
   <indexterm>
    <primary>convert</primary>
   </indexterm>
   <indexterm>
    <primary>convert_from</primary>
   </indexterm>
   <indexterm>
    <primary>convert_to</primary>
   </indexterm>
   <indexterm>
    <primary>decode</primary>
   </indexterm>
   <indexterm>
    <primary>encode</primary>
   </indexterm>
   <indexterm>
    <primary>initcap</primary>
   </indexterm>
   <indexterm>
    <primary>lpad</primary>
   </indexterm>
   <indexterm>
    <primary>ltrim</primary>
   </indexterm>
   <indexterm>
    <primary>md5</primary>
   </indexterm>
   <indexterm>
    <primary>pg_client_encoding</primary>
   </indexterm>
   <indexterm>
    <primary>quote_ident</primary>
   </indexterm>
   <indexterm>
    <primary>quote_literal</primary>
   </indexterm>
   <indexterm>
    <primary>quote_nullable</primary>
   </indexterm>
   <indexterm>
    <primary>repeat</primary>
   </indexterm>
   <indexterm>
    <primary>replace</primary>
   </indexterm>
   <indexterm>
    <primary>rpad</primary>
   </indexterm>
   <indexterm>
    <primary>rtrim</primary>
   </indexterm>
   <indexterm>
    <primary>split_part</primary>
   </indexterm>
   <indexterm>
    <primary>strpos</primary>
   </indexterm>
   <indexterm>
    <primary>substr</primary>
   </indexterm>
   <indexterm>
    <primary>to_ascii</primary>
   </indexterm>
   <indexterm>
    <primary>to_hex</primary>
   </indexterm>
   <indexterm>
    <primary>translate</primary>
   </indexterm>

   <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><literal><function>ascii</function>(<parameter>string</parameter>)</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 a strictly <acronym>ASCII</acronym> character.
       </entry>
       <entry><literal>ascii('x')</literal></entry>
       <entry><literal>120</literal></entry>
      </row>

      <row>
       <entry><literal><function>btrim</function>(<parameter>string</parameter> <type>text</type>
       <optional>, <parameter>characters</parameter> <type>text</type></optional>)</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', 'xy')</literal></entry>
       <entry><literal>trim</literal></entry>
      </row>

      <row>
       <entry><literal><function>chr</function>(<type>int</type>)</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 a strictly
        <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>
        <literal><function>convert</function>(<parameter>string</parameter> <type>bytea</type>,
        <parameter>src_encoding</parameter> <type>name</type>,
        <parameter>dest_encoding</parameter> <type>name</type>)</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 ISO 8859-1 encoding</entry>
      </row>

      <row>
       <entry>
        <literal><function>convert_from</function>(<parameter>string</parameter> <type>bytea</type>,
        <parameter>src_encoding</parameter> <type>name</type>)</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>
        <literal><function>convert_to</function>(<parameter>string</parameter> <type>text</type>,
        <parameter>dest_encoding</parameter> <type>name</type>)</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>
        <literal><function>decode</function>(<parameter>string</parameter> <type>text</type>,
        <parameter>type</parameter> <type>text</type>)</literal>
       </entry>
       <entry><type>bytea</type></entry>
       <entry>
        Decode binary data from <parameter>string</parameter> previously 
        encoded with <function>encode</>.  Parameter type is same as in <function>encode</>.
       </entry>
       <entry><literal>decode('MTIzAAE=', 'base64')</literal></entry>
       <entry><literal>123\000\001</literal></entry>
      </row>       

      <row>
       <entry>
        <literal><function>encode</function>(<parameter>data</parameter> <type>bytea</type>,
        <parameter>type</parameter> <type>text</type>)</literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>
        Encode binary data to different representation.  Supported
        types are: <literal>base64</>, <literal>hex</>, <literal>escape</>.
        <literal>Escape</> merely outputs null bytes as <literal>\000</> and
        doubles backslashes.
       </entry>
       <entry><literal>encode(E'123\\000\\001', 'base64')</literal></entry>
       <entry><literal>MTIzAAE=</literal></entry>
      </row>       

      <row>
       <entry><literal><function>initcap</function>(<parameter>string</parameter>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>
        Convert the first letter of each word to uppercase and the
        rest to lowercase. 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><literal><function>length</function>(<parameter>string</parameter>)</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</function>(<parameter>string</parameter><type>bytea</type>,
        <parameter>encoding</parameter> <type>name</type> )</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>
        <literal><function>lpad</function>(<parameter>string</parameter> <type>text</type>,
        <parameter>length</parameter> <type>int</type>
        <optional>, <parameter>fill</parameter> <type>text</type></optional>)</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><literal><function>ltrim</function>(<parameter>string</parameter> <type>text</type>
        <optional>, <parameter>characters</parameter> <type>text</type></optional>)</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('zzzytrim', 'xyz')</literal></entry>
       <entry><literal>trim</literal></entry>
      </row>

      <row>
       <entry><literal><function>md5</function>(<parameter>string</parameter>)</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><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><literal><function>quote_ident</function>(<parameter>string</parameter> <type>text</type>)</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><literal><function>quote_literal</function>(<parameter>string</parameter> <type>text</type>)</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('O\'Reilly')</literal></entry>
       <entry><literal>'O''Reilly'</literal></entry>
      </row>

      <row>
       <entry><literal><function>quote_literal</function>(<parameter>value</parameter> <type>anyelement</type>)</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><literal><function>quote_nullable</function>(<parameter>string</parameter> <type>text</type>)</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</>.
        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</function>(<parameter>value</parameter> <type>anyelement</type>)</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</>.
        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><literal><function>regexp_matches</function>(<parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type> [, <parameter>flags</parameter> <type>text</type>])</literal></entry>
       <entry><type>setof text[]</type></entry>
       <entry>
        Return all captured substrings resulting from matching a POSIX regular
        expression against the <parameter>string</parameter>. See
        <xref linkend="functions-posix-regexp"> for more information.
       </entry>
       <entry><literal>regexp_matches('foobarbequebaz', '(bar)(beque)')</literal></entry>
       <entry><literal>{bar,beque}</literal></entry>
      </row>

      <row>
       <entry><literal><function>regexp_replace</function>(<parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type>, <parameter>replacement</parameter> <type>text</type> [, <parameter>flags</parameter> <type>text</type>])</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><literal><function>regexp_split_to_array</function>(<parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type> [, <parameter>flags</parameter> <type>text</type> ])</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', E'\\s+')</literal></entry>
       <entry><literal>{hello,world}</literal></entry>
      </row>

      <row>
       <entry><literal><function>regexp_split_to_table</function>(<parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type> [, <parameter>flags</parameter> <type>text</type>])</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', E'\\s+')</literal></entry>
       <entry><literal>hello</literal><para><literal>world</literal></para> (2 rows)</entry>
      </row>

      <row>
       <entry><literal><function>repeat</function>(<parameter>string</parameter> <type>text</type>, <parameter>number</parameter> <type>int</type>)</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><literal><function>replace</function>(<parameter>string</parameter> <type>text</type>,
       <parameter>from</parameter> <type>text</type>,
       <parameter>to</parameter> <type>text</type>)</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>
        <literal><function>rpad</function>(<parameter>string</parameter> <type>text</type>,
        <parameter>length</parameter> <type>int</type>
        <optional>, <parameter>fill</parameter> <type>text</type></optional>)</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><literal><function>rtrim</function>(<parameter>string</parameter> <type>text</type>
        <optional>, <parameter>characters</parameter> <type>text</type></optional>)</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('trimxxxx', 'x')</literal></entry>
       <entry><literal>trim</literal></entry>
      </row>

      <row>
       <entry><literal><function>split_part</function>(<parameter>string</parameter> <type>text</type>,
       <parameter>delimiter</parameter> <type>text</type>,
       <parameter>field</parameter> <type>int</type>)</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><literal><function>strpos</function>(<parameter>string</parameter>, <parameter>substring</parameter>)</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><literal><function>substr</function>(<parameter>string</parameter>, <parameter>from</parameter> <optional>, <parameter>count</parameter></optional>)</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><literal><function>to_ascii</function>(<parameter>string</parameter> <type>text</type>
        <optional>, <parameter>encoding</parameter> <type>text</type></optional>)</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>LATIN2</>, <literal>LATIN9</>,
       and <literal>WIN1250</> encodings)
       </entry>

       <entry><literal>to_ascii('Karel')</literal></entry>
       <entry><literal>Karel</literal></entry>
      </row>

      <row>
       <entry><literal><function>to_hex</function>(<parameter>number</parameter> <type>int</type>
       or <type>bigint</type>)</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>
        <literal><function>translate</function>(<parameter>string</parameter> <type>text</type>,
        <parameter>from</parameter> <type>text</type>,
        <parameter>to</parameter> <type>text</type>)</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
       </entry>
       <entry><literal>translate('12345', '14', 'ax')</literal></entry>
       <entry><literal>a23x5</literal></entry>
      </row>       
      
     </tbody>
    </tgroup>
   </table>


   <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 equally 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>ascii_to_mic</literal></entry>
       <entry><literal>SQL_ASCII</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
      </row>

      <row>
       <entry><literal>ascii_to_utf8</literal></entry>
       <entry><literal>SQL_ASCII</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <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>KOI8</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>KOI8</literal></entry>
       <entry><literal>ISO_8859_5</literal></entry>
      </row>

      <row>
       <entry><literal>koi8_r_to_mic</literal></entry>
       <entry><literal>KOI8</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
      </row>

      <row>
       <entry><literal>koi8_r_to_utf8</literal></entry>
       <entry><literal>KOI8</literal></entry>
       <entry><literal>UTF8</literal></entry>
      </row>

      <row>
       <entry><literal>koi8_r_to_windows_1251</literal></entry>
       <entry><literal>KOI8</literal></entry>
       <entry><literal>WIN1251</literal></entry>
      </row>

      <row>
       <entry><literal>koi8_r_to_windows_866</literal></entry>
       <entry><literal>KOI8</literal></entry>
       <entry><literal>WIN866</literal></entry>
      </row>

      <row>
       <entry><literal>mic_to_ascii</literal></entry>
       <entry><literal>MULE_INTERNAL</literal></entry>
       <entry><literal>SQL_ASCII</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>KOI8</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>tcvn_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_ascii</literal></entry>
       <entry><literal>UTF8</literal></entry>
       <entry><literal>SQL_ASCII</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>KOI8</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_tcvn</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>KOI8</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>KOI8</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>ut8_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>ut8_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>

  </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 with a
    special syntax where 
    certain key words rather than commas are used to separate the
    arguments.  Details are in
    <xref linkend="functions-binarystring-sql">.
    Some functions are also implemented using the regular syntax for
    function invocation.
    (See <xref linkend="functions-binarystring-other">.)
   </para>

   <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>E'\\\\Post'::bytea || E'\\047gres\\000'::bytea</literal></entry>
       <entry><literal>\\Post'gres\000</literal></entry>
      </row>

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

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

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

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

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

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

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

      <row>
       <entry>
        <literal><function>trim</function>(<optional>both</optional>
        <parameter>bytes</parameter> from
        <parameter>string</parameter>)</literal>
       </entry>
       <entry><type>bytea</type></entry>
       <entry>
        Remove the longest string containing only the bytes in
        <parameter>bytes</parameter> from the start
        and end of <parameter>string</parameter>
       </entry>
       <entry><literal>trim(E'\\000'::bytea from E'\\000Tom\\000'::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><literal><function>btrim</function>(<parameter>string</parameter>
        <type>bytea</type>, <parameter>bytes</parameter> <type>bytea</type>)</literal></entry>
       <entry><type>bytea</type></entry>
       <entry>
        Remove the longest string consisting only of bytes
        in <parameter>bytes</parameter> from the start and end of
        <parameter>string</parameter>
      </entry>
      <entry><literal>btrim(E'\\000trim\\000'::bytea, E'\\000'::bytea)</literal></entry>
      <entry><literal>trim</literal></entry>
     </row>

     <row>
      <entry>
       <literal><function>decode</function>(<parameter>string</parameter> <type>text</type>,
              <parameter>type</parameter> <type>text</type>)</literal>
      </entry>
      <entry><type>bytea</type></entry>
      <entry>
       Decode binary string from <parameter>string</parameter> previously 
       encoded with <function>encode</>.  Parameter type is same as in <function>encode</>.
      </entry>
      <entry><literal>decode(E'123\\000456', 'escape')</literal></entry>
      <entry><literal>123\000456</literal></entry>
     </row>

     <row>
      <entry>
       <literal><function>encode</function>(<parameter>string</parameter> <type>bytea</type>,
              <parameter>type</parameter> <type>text</type>)</literal>
      </entry>
      <entry><type>text</type></entry>
      <entry>
       Encode binary string to <acronym>ASCII</acronym>-only representation.  Supported
       types are: <literal>base64</>, <literal>hex</>, <literal>escape</>.
      </entry>
      <entry><literal>encode(E'123\\000456'::bytea, 'escape')</literal></entry>
      <entry><literal>123\000456</literal></entry>
     </row>

     <row>
      <entry><literal><function>length</function>(<parameter>string</parameter>)</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(E'jo\\000se'::bytea)</literal></entry>
      <entry><literal>5</literal></entry>
     </row>

     <row>
      <entry><literal><function>md5</function>(<parameter>string</parameter>)</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(E'Th\\000omas'::bytea)</literal></entry>
      <entry><literal>8ab2d3c9689aaf18 b4958c334c82d8b1</literal></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </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>.
   </para>

   <para>
    In addition, it is possible to cast integral values to and from type
    <type>bit</>.
    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</> means casting to
    <literal>bit(1)</>, and so it will deliver only the least significant
    bit of the integer.
   </para>

    <note>
     <para>
      Prior to <productname>PostgreSQL</productname> 8.0, casting an
      integer to <type>bit(n)</> would copy the leftmost <literal>n</>
      bits of the integer, whereas now it copies the rightmost <literal>n</>
      bits.  Also, 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?</> operators, functions are available to extract
    or replace matching substrings and to split a string at the matches.
   </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>

  <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>
     Every <replaceable>pattern</replaceable> defines a set of strings.
     The <function>LIKE</function> expression returns true if the
     <replaceable>string</replaceable> is contained in the set of
     strings represented by <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 underscore, 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 string
     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 matches always cover the entire
    string.  To match a sequence anywhere within a string, the
    pattern must therefore 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>

   <para>
    Note that the backslash already has a special meaning in string literals,
    so to write a pattern constant that contains a backslash you must write two
    backslashes in an SQL statement (assuming escape string syntax is used, see
    <xref linkend="sql-syntax-strings">).  Thus, writing a pattern that
    actually matches a literal backslash means writing four backslashes in the
    statement.  You can avoid this by selecting a different escape character
    with <literal>ESCAPE</literal>; then a backslash is not special to
    <function>LIKE</function> anymore. (But it is still special to the string
    literal parser, so you still need two of them.)
   </para>

   <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>
    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>
  </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 much like <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 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 practice, wherein the pattern
    can match any part of the string.
    Also like
    <function>LIKE</function>, <function>SIMILAR TO</function> uses
    <literal>_</> and <literal>%</> as wildcard characters denoting
    any single character and any string, respectively (these are
    comparable to <literal>.</> and <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>
      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 bounded repetition (<literal>?</> and <literal>{...}</>)
    are not provided, though they exist in POSIX.  Also, the dot (<literal>.</>)
    is not a metacharacter.
   </para>

   <para>
    As with <function>LIKE</>, a backslash disables the special meaning
    of any of these metacharacters; or a different escape character can
    be specified with <literal>ESCAPE</>.
   </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>
</programlisting>
   </para>

   <para>
    The <function>substring</> function with three parameters,
    <function>substring(<replaceable>string</replaceable> from
    <replaceable>pattern</replaceable> for
    <replaceable>escape-character</replaceable>)</function>, provides
    extraction of a substring that matches an SQL
    regular expression pattern.  As with <literal>SIMILAR TO</>, the
    specified pattern must match to the entire data string, else the
    function fails and returns null.  To indicate the part of the
    pattern that should be returned on success, the pattern must contain
    two occurrences of the escape character followed by a double quote
    (<literal>"</>). <!-- " font-lock sanity -->
    The text matching the portion of the pattern
    between these markers is returned.
   </para>

   <para>
    Some examples:
<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_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</> 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 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 provides substitution of
     new text for substrings that match POSIX regular expression patterns.
     It has the syntax
     <function>regexp_replace</function>(<replaceable>source</>,
     <replaceable>pattern</>, <replaceable>replacement</>
     <optional>, <replaceable>flags</> </optional>).
     The <replaceable>source</> string is returned unchanged if
     there is no match to the <replaceable>pattern</>.  If there is a
     match, the <replaceable>source</> string is returned with the
     <replaceable>replacement</> string substituted for the matching
     substring.  The <replaceable>replacement</> string can contain
     <literal>\</><replaceable>n</>, where <replaceable>n</> is <literal>1</>
     through <literal>9</>, to indicate that the source substring matching the
     <replaceable>n</>'th parenthesized subexpression of the pattern should be
     inserted, and it can contain <literal>\&amp;</> to indicate that the
     substring matching the entire pattern should be inserted.  Write
     <literal>\\</> if you need to put a literal backslash in the replacement
     text.  (As always, remember to double backslashes written in literal
     constant strings, assuming escape string syntax is used.)
     The <replaceable>flags</> parameter is an optional text
     string containing zero or more single-letter flags that change the
     function's behavior.  Flag <literal>i</> specifies case-insensitive
     matching, while flag <literal>g</> specifies replacement of each matching
     substring rather than only the first one.  Other supported flags 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(..)', E'X\\1Y', 'g')
                                   <lineannotation>fooXarYXazY</lineannotation>
</programlisting>
   </para>

    <para>
     The <function>regexp_matches</> function returns all of the captured
     substrings resulting from matching a POSIX regular expression pattern.
     It has the syntax
     <function>regexp_matches</function>(<replaceable>string</>, <replaceable>pattern</>
     <optional>, <replaceable>flags</> </optional>).
     If there is no match to the <replaceable>pattern</>, the function returns
     no rows.  If there is a match, the function returns a text array whose
     <replaceable>n</>'th element is the substring matching the
     <replaceable>n</>'th parenthesized subexpression of the pattern
     (not counting <quote>non-capturing</> parentheses; see below for
     details).  If the pattern does not contain any parenthesized
     subexpressions, then the result is a single-element text array containing
     the substring matching the whole pattern.
     The <replaceable>flags</> parameter is an optional text
     string containing zero or more single-letter flags that change the
     function's behavior.  Flag <literal>g</> causes the function to find
     each match in the string, not only the first one, and return a row for
     each such match.  Other supported
     flags are described in <xref linkend="posix-embedded-options-table">.
    </para>

   <para>
    Some examples:
<programlisting>
SELECT regexp_matches('foobarbequebaz', '(bar)(beque)');
 regexp_matches 
----------------
 {bar,beque}
(1 row)

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

SELECT regexp_matches('foobarbequebaz', 'barbeque');
 regexp_matches 
----------------
 {barbeque}
(1 row)
</programlisting>
   </para>

    <para>
     The <function>regexp_split_to_table</> 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>pattern</>
     <optional>, <replaceable>flags</> </optional>).
     If there is no match to the <replaceable>pattern</>, the function returns the
     <replaceable>string</>.  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</> 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 behaves the same as
     <function>regexp_split_to_table</>, except that <function>regexp_split_to_array</>
     returns its result as an array of <type>text</>.  It has the syntax
     <function>regexp_split_to_array</function>(<replaceable>string</>, <replaceable>pattern</>
     <optional>, <replaceable>flags</> </optional>).
     The parameters are the same as for <function>regexp_split_to_table</>.
    </para>

   <para>
    Some examples:
<programlisting>

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

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

SELECT foo FROM regexp_split_to_table('the quick brown fox', E'\\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_matches</>, 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 package written by Henry Spencer.  Much of
    the description of regular expressions below is copied verbatim from his
    manual entry.
   </para>

   <para>
    Regular expressions (<acronym>RE</acronym>s), as defined in
    <acronym>POSIX</acronym> 1003.2, come in two forms:
    <firstterm>extended</> <acronym>RE</acronym>s or <acronym>ERE</>s
    (roughly those of <command>egrep</command>), and
    <firstterm>basic</> <acronym>RE</acronym>s or <acronym>BRE</>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 anyway
    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</> <acronym>RE</acronym>s or <acronym>ARE</>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>
     The form of regular expressions accepted by
     <productname>PostgreSQL</> can be chosen by setting the <xref
     linkend="guc-regex-flavor"> run-time parameter.  The usual
     setting is <literal>advanced</>, but one might choose
     <literal>extended</> for maximum backwards compatibility with
     pre-7.4 releases of <productname>PostgreSQL</>.
    </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</> or
    <firstterm>constraints</>, 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</> possibly followed
    by a single <firstterm>quantifier</>.
    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</> 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>(</><replaceable>re</><literal>)</> </entry>
       <entry> (where <replaceable>re</> is any regular expression)
       matches a match for
       <replaceable>re</>, with the match noted for possible reporting </entry>
       </row>

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

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

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

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

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

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

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

   <para>
    An RE cannot end with <literal>\</>.
   </para>

   <note>
    <para>
     Remember that the backslash (<literal>\</literal>) already has a special
     meaning in <productname>PostgreSQL</> string literals.
     To write a pattern constant that contains a backslash,
     you must write two backslashes in the statement, assuming escape
     string syntax is used (see <xref linkend="sql-syntax-strings">).
    </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>*</> </entry>
       <entry> a sequence of 0 or more matches of the atom </entry>
       </row>

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

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

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

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

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

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

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

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

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

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

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

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

    <para>
     <firstterm>Non-greedy</> quantifiers (available in AREs only) match the
     same possibilities as their corresponding normal (<firstterm>greedy</>)
     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.
     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>^</> </entry>
       <entry> matches at the beginning of the string </entry>
       </row>

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

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

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

   <para>
    Lookahead constraints cannot contain <firstterm>back references</>
    (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</> 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 (following a possible <literal>^</literal>).  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
    a single element of the bracket expression's list.  A bracket
    expression containing a multiple-character collating element can thus
    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</> currently has no multicharacter 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 equivalence
    class, 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.  Standard
    character class names are: <literal>alnum</literal>,
    <literal>alpha</literal>, <literal>blank</literal>,
    <literal>cntrl</literal>, <literal>digit</literal>,
    <literal>graph</literal>, <literal>lower</literal>,
    <literal>print</literal>, <literal>punct</literal>,
    <literal>space</literal>, <literal>upper</literal>,
    <literal>xdigit</literal>.  These stand for the character classes
    defined in
    <citerefentry><refentrytitle>ctype</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
    A locale can provide others.  A character class cannot be used as
    an endpoint of a range.
   </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</> character (as
    defined by
    <citerefentry><refentrytitle>ctype</refentrytitle><manvolnum>3</manvolnum></citerefentry>)
    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 certainly easier to type).
   </para>
   </sect3>

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

   <para>
    <firstterm>Escapes</> are special sequences beginning with <literal>\</>
    followed by an alphanumeric character. Escapes come in several varieties:
    character entry, class shorthands, constraint escapes, and back references.
    A <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>\</> followed by an alphanumeric character merely stands for
    that character as an ordinary character, and inside a bracket expression,
    <literal>\</> is an ordinary character.
    (The latter is the one actual incompatibility between EREs and AREs.)
   </para>

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

   <para>
    <firstterm>Class-shorthand escapes</> 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</> 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</> (<literal>\</><replaceable>n</>) matches the
    same string matched by the previous parenthesized subexpression specified
    by the number <replaceable>n</>
    (see <xref linkend="posix-constraint-backref-table">).  For example,
    <literal>([bc])\1</> matches <literal>bb</> or <literal>cc</>
    but not <literal>bc</> or <literal>cb</>.
    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>

   <note>
    <para>
     Keep in mind that an escape's leading <literal>\</> will need to be
     doubled when entering the pattern as an SQL string constant.  For example:
<programlisting>
'123' ~ E'^\\d{3}' <lineannotation>true</lineannotation>
</programlisting>
    </para>
   </note>

   <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</> </entry>
       <entry> alert (bell) character, as in C </entry>
       </row>

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

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

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

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

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

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

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

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

       <row>
       <entry> <literal>\u</><replaceable>wxyz</> </entry>
       <entry> (where <replaceable>wxyz</> is exactly four hexadecimal digits)
       the UTF16 (Unicode, 16-bit) character <literal>U+</><replaceable>wxyz</>
       in the local byte ordering </entry>
       </row>

       <row>
       <entry> <literal>\U</><replaceable>stuvwxyz</> </entry>
       <entry> (where <replaceable>stuvwxyz</> is exactly eight hexadecimal
       digits)
       reserved for a somewhat-hypothetical Unicode extension to 32 bits
       </entry> 
       </row>

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

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

       <row>
       <entry> <literal>\0</> </entry>
       <entry> the character whose value is <literal>0</> </entry>
       </row>

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

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

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

   <para>
    The character-entry escapes are always taken as ordinary characters.
    For example, <literal>\135</> is <literal>]</> in ASCII, but
    <literal>\135</> 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</> </entry>
       <entry> <literal>[[:digit:]]</> </entry>
       </row>

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

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

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

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

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

   <para>
    Within bracket expressions, <literal>\d</>, <literal>\s</>,
    and <literal>\w</> lose their outer brackets,
    and <literal>\D</>, <literal>\S</>, and <literal>\W</> are illegal.
    (So, for example, <literal>[a-c\d]</> is equivalent to
    <literal>[a-c[:digit:]]</>.
    Also, <literal>[a-c\D]</>, which is equivalent to
    <literal>[a-c^[:digit:]]</>, 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</> </entry>
       <entry> matches only at the beginning of the string
       (see <xref linkend="posix-matching-rules"> for how this differs from
       <literal>^</>) </entry>
       </row>

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

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

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

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

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

   <para>
    A word is defined as in the specification of
    <literal>[[:&lt;:]]</> and <literal>[[:&gt;:]]</> 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>\</><replaceable>m</> </entry>
       <entry> (where <replaceable>m</> is a nonzero digit)
       a back reference to the <replaceable>m</>'th subexpression </entry>
       </row>

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

   <note>
    <para>
     There is an inherent historical ambiguity between octal character-entry 
     escapes and back references, which is resolved by 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 multidigit 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>
    Normally the flavor of RE being used is determined by
    <varname>regex_flavor</>.
    However, this can be overridden by a <firstterm>director</> prefix.
    If an RE begins with <literal>***:</>,
    the rest of the RE is taken as an ARE regardless of
    <varname>regex_flavor</>.
    If an RE begins with <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</>:
    a sequence <literal>(?</><replaceable>xyz</><literal>)</>
    (where <replaceable>xyz</> is one or more alphabetic characters)
    specifies options affecting the rest of the RE.
    These options override any previously determined options (including
    both the RE flavor and case sensitivity).
    The available option letters are
    shown in <xref linkend="posix-embedded-options-table">.
   </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</> </entry>
       <entry> rest of RE is a BRE </entry>
       </row>

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

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

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

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

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

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

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

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

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

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

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

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

   <para>
    In addition to the usual (<firstterm>tight</>) RE syntax, in which all
    characters are significant, there is an <firstterm>expanded</> syntax,
    available by specifying the embedded <literal>x</> option.
    In the expanded syntax,
    white-space characters in the RE are ignored, as are
    all characters between a <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>#</> preceded by <literal>\</> is
       retained
      </para>
     </listitem>
     <listitem>
      <para>
       white space or <literal>#</> within a bracket expression is retained
      </para>
     </listitem>
     <listitem>
      <para>
       white space and comments cannot appear within multicharacter symbols,
       such as <literal>(?:</>
      </para>
     </listitem>
    </itemizedlist>

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

   <para>
    Finally, in an ARE, outside bracket expressions, the sequence
    <literal>(?#</><replaceable>ttt</><literal>)</>
    (where <replaceable>ttt</> is any text not containing a <literal>)</>)
    is a comment, completely ignored.
    Again, this is not allowed between the characters of
    multicharacter symbols, like <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</> of these metasyntax extensions is available if
    an initial <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</> or
    <firstterm>non-greedy</>.
   </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>{</><replaceable>m</><literal>}</>
       or
       <literal>{</><replaceable>m</><literal>}?</>)
       has the same greediness (possibly none) as the atom itself.
      </para>
     </listitem>
     <listitem>
      <para>
       A quantified atom with other normal quantifiers (including
       <literal>{</><replaceable>m</><literal>,</><replaceable>n</><literal>}</>
       with <replaceable>m</> equal to <replaceable>n</>)
       is greedy (prefers longest match).
      </para>
     </listitem>
     <listitem>
      <para>
       A quantified atom with a non-greedy quantifier (including
       <literal>{</><replaceable>m</><literal>,</><replaceable>n</><literal>}?</>
       with <replaceable>m</> equal to <replaceable>n</>)
       is non-greedy (prefers shortest match).
      </para>
     </listitem>
     <listitem>
      <para>
       A branch &mdash; that is, an RE that has no top-level
       <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>|</> 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</>.  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*</>
    is greedy.  It can match beginning at the <literal>Y</>, and it matches
    the longest possible string starting there, i.e., <literal>Y123</>.
    The output is the parenthesized part of that, or <literal>123</>.
    In the second case, the RE as a whole is non-greedy because <literal>Y*?</>
    is non-greedy.  It can match beginning at the <literal>Y</>, and it matches
    the shortest possible string starting there, i.e., <literal>Y1</>.
    The subexpression <literal>[0-9]{1,3}</> is greedy but it cannot change
    the decision as to the overall match length; so it is forced to match
    just <literal>1</>.
   </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</> relative to each other.
   </para>

   <para>
    The quantifiers <literal>{1,1}</> and <literal>{1,1}?</>
    can be used to force greediness or non-greediness, respectively,
    on a subexpression or a whole RE.
   </para>

   <para>
    Match lengths are measured in characters, not collating elements.
    An empty string is considered longer than no match at all.
    For example:
    <literal>bb*</>
    matches the three middle characters of <literal>abbbc</>;
    <literal>(week|wee)(night|knights)</>
    matches all ten characters of <literal>weeknights</>;
    when <literal>(.*).*</>
    is matched against <literal>abc</> the parenthesized subexpression
    matches all three characters; and when
    <literal>(a*)*</> is matched against <literal>bc</>
    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</> becomes <literal>[xX]</>.
    When it appears inside a bracket expression, all case counterparts
    of it are added to the bracket expression, e.g.
    <literal>[x]</> becomes <literal>[xX]</>
    and <literal>[^x]</> becomes <literal>[^xX]</>.
   </para>

   <para>
    If newline-sensitive matching is specified, <literal>.</>
    and bracket expressions using <literal>^</>
    will never match the newline character
    (so that matches will never cross newlines unless the RE
    explicitly arranges it)
    and <literal>^</>and <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</> and <literal>\Z</>
    continue to match beginning or end of string <emphasis>only</>.
   </para>

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

   <para>
    If inverse partial newline-sensitive matching is specified,
    this affects <literal>^</> and <literal>$</>
    as with newline-sensitive matching, but not <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>\</> 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>***</> 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>\B</>,
    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
    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</>:

    <itemizedlist>
     <listitem>
      <para>
       In AREs, <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>\</> remains a special character within
       <literal>[]</>, so a literal <literal>\</> within a bracket
       expression must be written <literal>\\</>.
      </para>
     </listitem>
    </itemizedlist>

    While these differences are unlikely to create a problem for most
    applications, you can avoid them if necessary by
    setting <varname>regex_flavor</> to <literal>extended</>.
   </para>
   </sect3>

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

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

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

  </sect2>
 </sect1>


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

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

   <indexterm>
    <primary>to_char</primary>
   </indexterm>
   <indexterm>
    <primary>to_date</primary>
   </indexterm>
   <indexterm>
    <primary>to_number</primary>
   </indexterm>
   <indexterm>
    <primary>to_timestamp</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>
   <para>
    The <function>to_timestamp</function> function can also take a single 
    <type>double precision</type> argument to convert from Unix epoch to 
    <type>timestamp with time zone</type>.
    (<type>Integer</type> Unix epochs are implicitly cast to 
    <type>double precision</type>.)
   </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><literal><function>to_char</function>(<type>timestamp</type>, <type>text</type>)</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</function>(<type>interval</type>, <type>text</type>)</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</function>(<type>int</type>, <type>text</type>)</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</function>(<type>numeric</type>, <type>text</type>)</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><literal><function>to_date</function>(<type>text</type>, <type>text</type>)</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><literal><function>to_number</function>(<type>text</type>, <type>text</type>)</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><literal><function>to_timestamp</function>(<type>text</type>, <type>text</type>)</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>
       <row>
        <entry><literal><function>to_timestamp</function>(<type>double precision</type>)</literal></entry>
        <entry><type>timestamp with time zone</type></entry>
        <entry>convert UNIX epoch to time stamp</entry>
        <entry><literal>to_timestamp(200120400)</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    In an output template string (for <function>to_char</>), there are certain patterns that are
    recognized and replaced with appropriately-formatted data from the value
    to be formatted.  Any text that is not a template pattern is simply
    copied verbatim.  Similarly, in an input template string (for anything but <function>to_char</>), template patterns
    identify the parts of the input data string to be looked at and the
    values to be found there.
   </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 (01-12)</entry>
       </row>
       <row>
        <entry><literal>HH12</literal></entry>
        <entry>hour of day (01-12)</entry>
       </row>       
       <row>
        <entry><literal>HH24</literal></entry>
        <entry>hour of day (00-23)</entry>
       </row>       
       <row>
        <entry><literal>MI</literal></entry>
        <entry>minute (00-59)</entry>
       </row>   
       <row>
        <entry><literal>SS</literal></entry>
        <entry>second (00-59)</entry>
       </row>
       <row>
        <entry><literal>MS</literal></entry>
        <entry>millisecond (000-999)</entry>
       </row>
       <row>
        <entry><literal>US</literal></entry>
        <entry>microsecond (000000-999999)</entry>
       </row>
       <row>
        <entry><literal>SSSS</literal></entry>
        <entry>seconds past midnight (0-86399)</entry>
       </row>
       <row>
        <entry><literal>AM</literal> or <literal>A.M.</literal> or
        <literal>PM</literal> or <literal>P.M.</literal></entry>
        <entry>meridian indicator (uppercase)</entry>
       </row>
       <row>
        <entry><literal>am</literal> or <literal>a.m.</literal> or
        <literal>pm</literal> or <literal>p.m.</literal></entry>
        <entry>meridian indicator (lowercase)</entry>
       </row>
       <row>
        <entry><literal>Y,YYY</literal></entry>
        <entry>year (4 and more digits) with comma</entry>
       </row>
       <row>
        <entry><literal>YYYY</literal></entry>
        <entry>year (4 and 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 year (4 and more digits)</entry>
       </row>
       <row>
        <entry><literal>IYY</literal></entry>
        <entry>last 3 digits of ISO year</entry>
       </row>
       <row>
        <entry><literal>IY</literal></entry>
        <entry>last 2 digits of ISO year</entry>
       </row>
       <row>
        <entry><literal>I</literal></entry>
        <entry>last digit of ISO year</entry>
       </row>
       <row>
        <entry><literal>BC</literal> or <literal>B.C.</literal> or
        <literal>AD</literal> or <literal>A.D.</literal></entry>
        <entry>era indicator (uppercase)</entry>
       </row>
       <row>
        <entry><literal>bc</literal> or <literal>b.c.</literal> or
        <literal>ad</literal> or <literal>a.d.</literal></entry>
        <entry>era indicator (lowercase)</entry>
       </row>
       <row>
        <entry><literal>MONTH</literal></entry>
        <entry>full uppercase month name (blank-padded to 9 chars)</entry>
       </row>
       <row>
        <entry><literal>Month</literal></entry>
        <entry>full mixed-case month name (blank-padded to 9 chars)</entry>
       </row>
       <row>
        <entry><literal>month</literal></entry>
        <entry>full lowercase month name (blank-padded to 9 chars)</entry>
       </row>
       <row>
        <entry><literal>MON</literal></entry>
        <entry>abbreviated uppercase month name (3 chars in English, localized lengths vary)</entry>
       </row>
       <row>
        <entry><literal>Mon</literal></entry>
        <entry>abbreviated mixed-case month name (3 chars in English, localized lengths vary)</entry>
       </row>
       <row>
        <entry><literal>mon</literal></entry>
        <entry>abbreviated lowercase month name (3 chars in English, localized lengths vary)</entry>
       </row>
       <row>
        <entry><literal>MM</literal></entry>
        <entry>month number (01-12)</entry>
       </row>
       <row>
        <entry><literal>DAY</literal></entry>
        <entry>full uppercase day name (blank-padded to 9 chars)</entry>
       </row>
       <row>
        <entry><literal>Day</literal></entry>
        <entry>full mixed-case day name (blank-padded to 9 chars)</entry>
       </row>
       <row>
        <entry><literal>day</literal></entry>
        <entry>full lowercase day name (blank-padded to 9 chars)</entry>
       </row>
       <row>
        <entry><literal>DY</literal></entry>
        <entry>abbreviated uppercase day name (3 chars in English, localized lengths vary)</entry>
       </row>
       <row>
        <entry><literal>Dy</literal></entry>
        <entry>abbreviated mixed-case day name (3 chars in English, localized lengths vary)</entry>
       </row>
       <row>
        <entry><literal>dy</literal></entry>
        <entry>abbreviated lowercase day name (3 chars in English, localized lengths vary)</entry>
       </row>
       <row>
        <entry><literal>DDD</literal></entry>
        <entry>day of year (001-366)</entry>
       </row>
       <row>
        <entry><literal>IDDD</literal></entry>
        <entry>ISO day of year (001-371; day 1 of the year is Monday of the first ISO week.)</entry>
       </row>
       <row>
        <entry><literal>DD</literal></entry>
        <entry>day of month (01-31)</entry>
       </row>
       <row>
        <entry><literal>D</literal></entry>
        <entry>day of the week, Sunday(<literal>1</>) to Saturday(<literal>7</>)</entry>
       </row>
       <row>
        <entry><literal>ID</literal></entry>
        <entry>ISO day of the week, Monday(<literal>1</>) to Sunday(<literal>7</>)</entry>
       </row>
       <row>
        <entry><literal>W</literal></entry>
        <entry>week of month (1-5) (The first week starts on the first day of the month.)</entry>
       </row> 
       <row>
        <entry><literal>WW</literal></entry>
        <entry>week number of year (1-53) (The first week starts on the first day of the year.)</entry>
       </row>
       <row>
        <entry><literal>IW</literal></entry>
        <entry>ISO week number of year (1 - 53; the first Thursday of the new 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 (days since November 24, 4714 BC at midnight)</entry>
       </row>
       <row>
        <entry><literal>Q</literal></entry>
        <entry>quarter</entry>
       </row>
       <row>
        <entry><literal>RM</literal></entry>
        <entry>month in Roman numerals (I-XII; I=January) (uppercase)</entry>
       </row>
       <row>
        <entry><literal>rm</literal></entry>
        <entry>month in Roman numerals (i-xii; i=January) (lowercase)</entry>
       </row>
       <row>
        <entry><literal>TZ</literal></entry>
        <entry>time-zone name (uppercase)</entry>
       </row>
       <row>
        <entry><literal>tz</literal></entry>
        <entry>time-zone name (lowercase)</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    Certain 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 padding blanks and zeroes)</entry>
        <entry><literal>FMMonth</literal></entry>
       </row>
       <row>
        <entry><literal>TH</literal> suffix</entry>
        <entry>uppercase ordinal number suffix</entry>
        <entry><literal>DDTH</literal></entry>
       </row>   
       <row>
        <entry><literal>th</literal> suffix</entry>
        <entry>lowercase ordinal number suffix</entry>
        <entry><literal>DDth</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 yet 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.
      </para>
     </listitem>

     <listitem>
      <para>
       <literal>TM</literal> does not include trailing blanks.
      </para>
     </listitem>

     <listitem>
      <para>
       <function>to_timestamp</function> and <function>to_date</function>
       skip multiple blank spaces in the input string if the <literal>FX</literal> option 
       is not used. <literal>FX</literal> must be specified as the first item
       in the template.  For example 
       <literal>to_timestamp('2000&nbsp;&nbsp;&nbsp;&nbsp;JUN', 'YYYY MON')</literal> is correct, but
       <literal>to_timestamp('2000&nbsp;&nbsp;&nbsp;&nbsp;JUN', 'FXYYYY MON')</literal> returns an error,
       because <function>to_timestamp</function> expects one space only.
      </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 pattern key words.  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.
      </para>
     </listitem>

     <listitem>
      <para>
       If you want to have a double quote in the output you must
       precede it with a backslash, for example <literal>E'\\"YYYY
       Month\\"'</literal>. <!-- "" font-lock sanity :-) -->
       (Two backslashes are necessary because the backslash already
       has a special meaning when using the escape string syntax.)
      </para>
     </listitem>

     <listitem>
      <para>
       The <literal>YYYY</literal> conversion from string to <type>timestamp</type> or
       <type>date</type> has a restriction if you use a year 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 conversions from string to <type>timestamp</type> or
       <type>date</type>, the <literal>CC</literal> field is 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 year is computed
       as <literal>(CC-1)*100+YY</literal>.
      </para>
     </listitem>

     <listitem>
      <para>
       An ISO week date (as distinct from a Gregorian date) can be specified to <function>to_timestamp</function> and <function>to_date</function> in one of two ways:
       <itemizedlist>
        <listitem>
         <para>
          Year, week 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 construct a date using a mixture of ISO week and
       Gregorian date fields is nonsensical, and will cause an error.  In the
       context of an ISO year, the concept of a <quote>month</> or <quote>day
       of month</> has no meaning.  In the context of a Gregorian year, the
       ISO week has no meaning.  Users should take care to keep Gregorian and
       ISO date specifications separate.
      </para>
     </listitem>

     <listitem>
      <para>
       Millisecond (<literal>MS</literal>) and microsecond (<literal>US</literal>)
       values in a conversion from string to <type>timestamp</type> are used as part of the
       seconds after the decimal point. For example 
       <literal>to_timestamp('12:3', 'SS:MS')</literal> is not 3 milliseconds,
       but 300, because the conversion counts it as 12 + 0.3 seconds.
       This means 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 use
       <literal>12:003</literal>, which the conversion counts as
       12 + 0.003 = 12.003 seconds.
      </para>

      <para>
       Here is a more 
       complex example: 
       <literal>to_timestamp('15:12:02.020.001230', 'HH: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', ...)</function> function, but
        <function>to_char(..., 'D')</function>'s does not match
        <function>extract('dow', ...)</function>'s day numbering.
      </para>
     </listitem>

     <listitem>
      <para><function>to_char(interval)</function> formats <literal>HH</> and 
        <literal>HH12</> as hours in a single day, while <literal>HH24</>
        can output hours exceeding a single day, e.g. &gt;24.
      </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>value with the specified number of digits</entry>
       </row>
       <row>
        <entry><literal>0</literal></entry>
        <entry>value with leading zeros</entry>
       </row>
       <row>
        <entry><literal>.</literal> (period)</entry>
        <entry>decimal point</entry>
       </row>       
       <row>
        <entry><literal>,</literal> (comma)</entry>
        <entry>group (thousand) 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>scientific notation (not implemented yet)</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    Usage notes for numeric formatting:

    <itemizedlist>
     <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, 'S9999')</literal> produces <literal>'&nbsp;&nbsp;-12'</literal>,
       but <literal>to_char(-12, 'MI9999')</literal> produces <literal>'-&nbsp;&nbsp;12'</literal>.
       The Oracle implementation does not allow the use of
       <literal>MI</literal> ahead of <literal>9</literal>, but rather
       requires that <literal>9</literal> precede
       <literal>MI</literal>.
      </para>
     </listitem>

     <listitem>
      <para>
       <literal>9</literal> results in a value with the same number of 
       digits as there are <literal>9</literal>s. If a digit is
       not available it outputs a space.
      </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>
       <literal>V</literal> effectively
       multiplies the input values by
       <literal>10^<replaceable>n</replaceable></literal>, where
       <replaceable>n</replaceable> is the number of digits following
       <literal>V</literal>. 
       <function>to_char</function> does not support the use of
       <literal>V</literal> combined with a decimal point.
       (E.g., <literal>99.9V99</literal> is not allowed.)
      </para>
     </listitem>
    </itemizedlist>
   </para>   

   <para>
    Certain modifiers can be applied to any template pattern to alter its
    behavior.  For example, <literal>FM9999</literal>
    is the <literal>9999</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 padding blanks and zeroes)</entry>
        <entry><literal>FM9999</literal></entry>
       </row>
       <row>
        <entry><literal>TH</literal> suffix</entry>
        <entry>uppercase ordinal number suffix</entry>
        <entry><literal>999TH</literal></entry>
       </row>   
       <row>
        <entry><literal>th</literal> suffix</entry>
        <entry>lowercase 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, '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>
      </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>+</> and <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></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>

   <indexterm>
    <primary>age</primary>
   </indexterm>
   <indexterm>
    <primary>clock_timestamp</primary>
   </indexterm>
   <indexterm>
    <primary>current_date</primary>
   </indexterm>
   <indexterm>
    <primary>current_time</primary>
   </indexterm>
   <indexterm>
    <primary>current_timestamp</primary>
   </indexterm>
   <indexterm>
    <primary>date_part</primary>
   </indexterm>
   <indexterm>
    <primary>date_trunc</primary>
   </indexterm>
   <indexterm>
    <primary>extract</primary>
   </indexterm>
   <indexterm>
    <primary>isfinite</primary>
   </indexterm>
   <indexterm>
    <primary>justify_days</primary>
   </indexterm>
   <indexterm>
    <primary>justify_hours</primary>
   </indexterm>
   <indexterm>
    <primary>justify_interval</primary>
   </indexterm>
   <indexterm>
    <primary>localtime</primary>
   </indexterm>
   <indexterm>
    <primary>localtimestamp</primary>
   </indexterm>
   <indexterm>
    <primary>now</primary>
   </indexterm>
   <indexterm>
    <primary>statement_timestamp</primary>
   </indexterm>
   <indexterm>
    <primary>timeofday</primary>
   </indexterm>
   <indexterm>
    <primary>transaction_timestamp</primary>
   </indexterm>

    <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><literal><function>age</function>(<type>timestamp</type>, <type>timestamp</type>)</literal></entry>
        <entry><type>interval</type></entry>
        <entry>Subtract arguments, producing a <quote>symbolic</> result that
        uses years and months</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</function>(<type>timestamp</type>)</literal></entry>
        <entry><type>interval</type></entry>
        <entry>Subtract from <function>current_date</function></entry>
        <entry><literal>age(timestamp '1957-06-13')</literal></entry>
        <entry><literal>43 years 8 mons 3 days</literal></entry>
       </row>

       <row>
        <entry><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><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><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><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><literal><function>date_part</function>(<type>text</type>, <type>timestamp</type>)</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</function>(<type>text</type>, <type>interval</type>)</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><literal><function>date_trunc</function>(<type>text</type>, <type>timestamp</type>)</literal></entry>
        <entry><type>timestamp</type></entry>
        <entry>Truncate to specified precision; see also <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>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><literal><function>isfinite</function>(<type>date</type>)</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</function>(<type>timestamp</type>)</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</function>(<type>interval</type>)</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><literal><function>justify_days</function>(<type>interval</type>)</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 '30 days')</literal></entry>
        <entry><literal>1 month</literal></entry>
       </row>

       <row>
        <entry><literal><function>justify_hours</function>(<type>interval</type>)</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 '24 hours')</literal></entry>
        <entry><literal>1 day</literal></entry>
       </row>

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

       <row>
        <entry><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>
      </tbody>
     </tgroup>
    </table>

   <para>
    In addition to these functions, the SQL <literal>OVERLAPS</> 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.
   </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>
</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 (with the session time zone 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</> returned by
   <function>age</> because different months have a different number of
   days.  <productname>PostgreSQL</>'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')</> uses April to yield
   <literal>1 mon 1 day</>, while using May would yield <literal>1 mon 2
   days</> because May has 31 days, while April has only 30.
  </para>

  <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> will
    be 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 to 1.

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

       <para>
        <productname>PostgreSQL</productname> releases before 8.0 did not
        follow the conventional numbering of centuries, but just returned
        the year field divided by 100.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>day</literal></term>
      <listitem>
       <para>
        The day (of the month) field (1 - 31)
       </para>

<screen>
SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>16</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</>) to
        Saturday(<literal>6</>)
       </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
        is different 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 - 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>date</type> and <type>timestamp</type> values, the
        number of seconds since 1970-01-01 00:00:00-00 (can be negative);
        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 20:38:40-08');
<lineannotation>Result: </lineannotation><computeroutput>982384720</computeroutput>

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

       <para>
        Here is how you can convert an epoch value back to a time
        stamp:
       </para>

<screen>
SELECT TIMESTAMP WITH TIME ZONE 'epoch' + 982384720 * INTERVAL '1 second';
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>hour</literal></term>
      <listitem>
       <para>
        The hour field (0 - 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</>) to
        Sunday(<literal>7</>)
       </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</> except for Sunday.  This
        matches the <acronym>ISO</> 8601 day of the week numbering.
       </para>

      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>isoyear</literal></term>
      <listitem>
       <para>
        The <acronym>ISO</acronym> 8601 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> 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 starts January 1, 2001.
       </para>

       <para>
        <productname>PostgreSQL</productname> releases before 8.0 did not
        follow the conventional numbering of millennia, but just returned
        the year field divided by 1000.
       </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 - 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 - 12) ; for <type>interval</type> values
        the number of months, modulo 12 (0 - 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 - 4) that the day 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.
       </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 week of the year that the day is in.  By definition
        (<acronym>ISO</acronym> 8601), the first week of a year
        contains January 4 of that year.  (The <acronym>ISO</acronym>-8601
        week starts on Monday.)  In other words, the first Thursday of
        a year is in week 1 of that year.
       </para>
       <para>
        Because of this, it is possible for early January dates to be part of the 
        52nd or 53rd week of the previous year.  For example, <literal>2005-01-01</>
        is part of the 53rd week of year 2004, and <literal>2006-01-01</> is part of 
        the 52nd week of year 2005.
       </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</>, so subtracting 
        <literal>BC</> years from <literal>AD</> 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>

   <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>)
</synopsis>
    <replaceable>source</replaceable> is a value expression of type
    <type>timestamp</type> or <type>interval</>.
    (Values of type <type>date</type> and
    <type>time</type> are cast automatically, to <type>timestamp</type> or
    <type>interval</> respectively.)
    <replaceable>field</replaceable> selects to which precision to
    truncate the input value.  The return value is of type
    <type>timestamp</type> or <type>interval</>
    with 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>
    Examples:
<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>
</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> construct allows conversions
    of time stamps 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</></literal>
        </entry>
        <entry><type>timestamp with time zone</type></entry>
        <entry>Treat given time stamp <emphasis>without time zone</> as located in the specified time zone</entry>
       </row>

       <row>
        <entry>
         <literal><type>timestamp with time zone</type> AT TIME ZONE <replaceable>zone</></literal>
        </entry>
        <entry><type>timestamp without time zone</type></entry>
        <entry>Convert given time stamp <emphasis>with time zone</> to the new time zone</entry>
       </row>

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

   <para>
    In these expressions, the desired time zone <replaceable>zone</> can be
    specified either as a text string (e.g., <literal>'PST'</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 (supposing that the local time zone is <literal>PST8PDT</>):
<screen>
SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'MST';
<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 'MST';
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 18:38:40</computeroutput>
</screen>
    The first example takes a time stamp without time zone and interprets it as MST time
    (UTC-7), which is then converted to PST (UTC-8) for display.  The second example takes 
    a time stamp specified in EST (UTC-5) and converts it to local time in MST (UTC-7).
   </para>

   <para>
    The function <literal><function>timezone</function>(<replaceable>zone</>,
    <replaceable>timestamp</>)</literal> is equivalent to the SQL-conforming construct
    <literal><replaceable>timestamp</> AT TIME ZONE
    <replaceable>zone</></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 be given
     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>
now()
transaction_timestamp()
statement_timestamp()
clock_timestamp()
timeofday()
</synopsis>
   </para>

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

   <para>
    The following function is available to delay execution of the server
    process:
<synopsis>
pg_sleep(<replaceable>seconds</replaceable>)
</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</>, so fractional-second delays can be specified.
    For example:

<programlisting>
SELECT pg_sleep(1.5);
</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.
     </para>
   </note>

   <warning>
     <para>
      Make sure that your session does not hold more locks than necessary
      when calling <function>pg_sleep</function>.  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><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><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><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</>,
    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</> operator, <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>=</> operator, but
     <literal>=</> compares
     for equal <emphasis>areas</> only.  The other scalar comparison operators
     (<literal>&lt;=</> 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>'((1,-1),(-1,1))' # '((1,1),(-1,-1))'</literal></entry>
       </row>
       <row>
        <entry> <literal>#</literal> </entry>
        <entry>Number of points in path or polygon</entry>
        <entry><literal># '((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?</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;</> and <literal>&lt;@</> were respectively
     called <literal>~</> and <literal>@</>.  These names are still
     available, but are deprecated and will eventually be retired.
    </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</function>(<replaceable>object</>)</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</function>(<replaceable>object</>)</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</function>(<type>circle</>)</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</function>(<type>box</>)</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</function>(<type>path</>)</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</function>(<type>path</>)</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</function>(<replaceable>object</>)</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</function>(<type>path</>)</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</function>(<type>polygon</>)</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</function>(<type>path</>)</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>
<![IGNORE[
<!-- Not defined by this name. Implements the intersection operator '#' -->
       <row>
        <entry><literal><function>point</function>(<type>lseg</>, <type>lseg</>)</literal></entry>
        <entry><type>point</type></entry>
        <entry>intersection</entry>
        <entry><literal>point(lseg '((-1,0),(1,0))',lseg '((-2,-2),(2,2))')</literal></entry>
       </row>
]]>
       <row>
        <entry><literal><function>popen</function>(<type>path</>)</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</function>(<type>circle</type>)</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</function>(<type>box</>)</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><literal><function>box</function>(<type>circle</type>)</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</function>(<type>point</type>, <type>point</type>)</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</function>(<type>polygon</type>)</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>circle</function>(<type>box</type>)</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</function>(<type>point</type>, <type>double precision</type>)</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</function>(<type>polygon</type>)</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>lseg</function>(<type>box</type>)</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</function>(<type>point</type>, <type>point</type>)</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><literal><function>path</function>(<type>polygon</type>)</literal></entry>
        <entry><type>point</type></entry>
        <entry>polygon to path</entry>
        <entry><literal>path(polygon '((0,0),(1,1),(2,0))')</literal></entry>
       </row>
       <row>
        <entry><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</function>(<type>box</type>)</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</function>(<type>circle</type>)</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</function>(<type>lseg</type>)</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</function>(<type>polygon</type>)</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><literal><function>polygon</function>(<type>box</type>)</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</function>(<type>circle</type>)</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</function>(<replaceable class="parameter">npts</replaceable>, <type>circle</type>)</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</function>(<type>path</type>)</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</>
     as though it were an array with indices 0 and 1.  For example, if
     <literal>t.p</> is a <type>point</> column then
     <literal>SELECT p[0] FROM t</> retrieves the X coordinate and
     <literal>UPDATE t SET p[1] = ...</> changes the Y coordinate.
     In the same way, a value of type <type>box</> or <type>lseg</> can be treated
     as an array of two <type>point</> 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>
     won't 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>, and
   <literal>&gt;&gt;=</literal> test for subnet inclusion.  They
   consider only the network parts of the two addresses, ignoring any
   host part, and determine whether one network part 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 within</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 within 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>~</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>host</function>,
   <function>text</function>, and <function>abbrev</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><literal><function>abbrev</function>(<type>inet</type>)</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</function>(<type>cidr</type>)</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><literal><function>broadcast</function>(<type>inet</type>)</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><literal><function>family</function>(<type>inet</type>)</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><literal><function>host</function>(<type>inet</type>)</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><literal><function>hostmask</function>(<type>inet</type>)</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><literal><function>masklen</function>(<type>inet</type>)</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><literal><function>netmask</function>(<type>inet</type>)</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><literal><function>network</function>(<type>inet</type>)</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><literal><function>set_masklen</function>(<type>inet</type>, <type>int</type>)</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</function>(<type>cidr</type>, <type>int</type>)</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><literal><function>text</function>(<type>inet</type>)</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>
      </tbody>
     </tgroup>
    </table>

  <para>
   Any <type>cidr</> value can be cast to <type>inet</> implicitly
   or explicitly; therefore, the functions shown above as operating on
   <type>inet</> also work on <type>cidr</> values.  (Where there are
   separate functions for <type>inet</> and <type>cidr</>, it is because
   the behavior should be different for the two cases.)
   Also, it is permitted to cast an <type>inet</> value to <type>cidr</>.
   When this is done, any bits to the right of the netmask are silently zeroed
   to create a valid <type>cidr</> value.
   In addition,
   you can cast a text value to <type>inet</> or <type>cidr</>
   using normal casting syntax: for example,
   <literal>inet(<replaceable>expression</>)</literal> or
   <literal><replaceable>colname</>::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</function>(<type>macaddr</type>)</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><literal><function>trunc</function>(<type>macaddr</type>)</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.
   </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="4">
      <thead>
       <row>
        <entry>Operator</entry>
        <entry>Description</entry>
        <entry>Example</entry>
        <entry>Result</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry> <literal>@@</literal> </entry>
        <entry><type>tsvector</> matches <type>tsquery</> ?</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>deprecated synonym for <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>concatenate <type>tsvector</>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>AND <type>tsquery</>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>OR <type>tsquery</>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>negate a <type>tsquery</></entry>
        <entry><literal>!! 'cat'::tsquery</literal></entry>
        <entry><literal>!'cat'</literal></entry>
       </row>
       <row>
        <entry> <literal>@&gt;</literal> </entry>
        <entry><type>tsquery</> 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>tsquery</> 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</> 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>&lt;</>, etc) are defined
     for types <type>tsvector</> and <type>tsquery</>.  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><literal><function>to_tsvector</function>(<optional> <replaceable class="PARAMETER">config</> <type>regconfig</> , </optional> <replaceable class="PARAMETER">document</> <type>text</type>)</literal></entry>
        <entry><type>tsvector</type></entry>
        <entry>reduce document text to <type>tsvector</></entry>
        <entry><literal>to_tsvector('english', 'The Fat Rats')</literal></entry>
        <entry><literal>'fat':2 'rat':3</literal></entry>
       </row>
       <row>
        <entry><literal><function>length</function>(<type>tsvector</>)</literal></entry>
        <entry><type>integer</type></entry>
        <entry>number of lexemes in <type>tsvector</></entry>
        <entry><literal>length('fat:2,4 cat:3 rat:5A'::tsvector)</literal></entry>
        <entry><literal>3</literal></entry>
       </row>
       <row>
        <entry><literal><function>setweight</function>(<type>tsvector</>, <type>"char"</>)</literal></entry>
        <entry><type>tsvector</type></entry>
        <entry>assign weight to each element of <type>tsvector</></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><literal><function>strip</function>(<type>tsvector</>)</literal></entry>
        <entry><type>tsvector</type></entry>
        <entry>remove positions and weights from <type>tsvector</></entry>
        <entry><literal>strip('fat:2,4 cat:3 rat:5A'::tsvector)</literal></entry>
        <entry><literal>'cat' 'fat' 'rat'</literal></entry>
       </row>
       <row>
        <entry><literal><function>to_tsquery</function>(<optional> <replaceable class="PARAMETER">config</> <type>regconfig</> , </optional> <replaceable class="PARAMETER">query</> <type>text</type>)</literal></entry>
        <entry><type>tsquery</type></entry>
        <entry>normalize words and convert to <type>tsquery</></entry>
        <entry><literal>to_tsquery('english', 'The &amp; Fat &amp; Rats')</literal></entry>
        <entry><literal>'fat' &amp; 'rat'</literal></entry>
       </row>
       <row>
        <entry><literal><function>plainto_tsquery</function>(<optional> <replaceable class="PARAMETER">config</> <type>regconfig</> , </optional> <replaceable class="PARAMETER">query</> <type>text</type>)</literal></entry>
        <entry><type>tsquery</type></entry>
        <entry>produce <type>tsquery</> ignoring punctuation</entry>
        <entry><literal>plainto_tsquery('english', 'The Fat Rats')</literal></entry>
        <entry><literal>'fat' &amp; 'rat'</literal></entry>
       </row>
       <row>
        <entry><literal><function>numnode</function>(<type>tsquery</>)</literal></entry>
        <entry><type>integer</type></entry>
        <entry>number of lexemes plus operators in <type>tsquery</></entry>
        <entry><literal> numnode('(fat &amp; rat) | cat'::tsquery)</literal></entry>
        <entry><literal>5</literal></entry>
       </row>
       <row>
        <entry><literal><function>querytree</function>(<replaceable class="PARAMETER">query</replaceable> <type>tsquery</>)</literal></entry>
        <entry><type>text</type></entry>
        <entry>get indexable part of a <type>tsquery</></entry>
        <entry><literal>querytree('foo &amp; ! bar'::tsquery)</literal></entry>
        <entry><literal>'foo'</literal></entry>
       </row>
       <row>
        <entry><literal><function>ts_rank</function>(<optional> <replaceable class="PARAMETER">weights</replaceable> <type>float4[]</>, </optional> <replaceable class="PARAMETER">vector</replaceable> <type>tsvector</>, <replaceable class="PARAMETER">query</replaceable> <type>tsquery</> <optional>, <replaceable class="PARAMETER">normalization</replaceable> <type>integer</> </optional>)</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><literal><function>ts_rank_cd</function>(<optional> <replaceable class="PARAMETER">weights</replaceable> <type>float4[]</>, </optional> <replaceable class="PARAMETER">vector</replaceable> <type>tsvector</>, <replaceable class="PARAMETER">query</replaceable> <type>tsquery</> <optional>, <replaceable class="PARAMETER">normalization</replaceable> <type>integer</> </optional>)</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><literal><function>ts_headline</function>(<optional> <replaceable class="PARAMETER">config</replaceable> <type>regconfig</>, </optional> <replaceable class="PARAMETER">document</replaceable> <type>text</>, <replaceable class="PARAMETER">query</replaceable> <type>tsquery</> <optional>, <replaceable class="PARAMETER">options</replaceable> <type>text</> </optional>)</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_rewrite</function>(<replaceable class="PARAMETER">query</replaceable> <type>tsquery</>, <replaceable class="PARAMETER">target</replaceable> <type>tsquery</>, <replaceable class="PARAMETER">substitute</replaceable> <type>tsquery</>)</literal></entry>
        <entry><type>tsquery</type></entry>
        <entry>replace target with substitute 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</function>(<replaceable class="PARAMETER">query</replaceable> <type>tsquery</>, <replaceable class="PARAMETER">select</replaceable> <type>text</>)</literal></entry>
        <entry><type>tsquery</type></entry>
        <entry>replace using targets and substitutes from a <command>SELECT</> 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><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><literal><function>tsvector_update_trigger</function>()</literal></entry>
        <entry><type>trigger</type></entry>
        <entry>trigger function for automatic <type>tsvector</> 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><literal><function>tsvector_update_trigger_column</function>()</literal></entry>
        <entry><type>trigger</type></entry>
        <entry>trigger function for automatic <type>tsvector</> column update</entry>
        <entry><literal>CREATE TRIGGER ... tsvector_update_trigger_column(tsvcol, configcol, title, body)</literal></entry>
        <entry><literal></literal></entry>
        <entry><literal></literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

  <note>
   <para>
    All the text search functions that accept an optional <type>regconfig</>
    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><literal><function>ts_debug</function>(<optional> <replaceable class="PARAMETER">config</replaceable> <type>regconfig</>, </optional> <replaceable class="PARAMETER">document</replaceable> <type>text</>, OUT <replaceable class="PARAMETER">alias</> <type>text</>, OUT <replaceable class="PARAMETER">description</> <type>text</>, OUT <replaceable class="PARAMETER">token</> <type>text</>, OUT <replaceable class="PARAMETER">dictionaries</> <type>regdictionary[]</>, OUT <replaceable class="PARAMETER">dictionary</> <type>regdictionary</>, OUT <replaceable class="PARAMETER">lexemes</> <type>text[]</>)</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><literal><function>ts_lexize</function>(<replaceable class="PARAMETER">dict</replaceable> <type>regdictionary</>, <replaceable class="PARAMETER">token</replaceable> <type>text</>)</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><literal><function>ts_parse</function>(<replaceable class="PARAMETER">parser_name</replaceable> <type>text</>, <replaceable class="PARAMETER">document</replaceable> <type>text</>, OUT <replaceable class="PARAMETER">tokid</> <type>integer</>, OUT <replaceable class="PARAMETER">token</> <type>text</>)</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</function>(<replaceable class="PARAMETER">parser_oid</replaceable> <type>oid</>, <replaceable class="PARAMETER">document</replaceable> <type>text</>, OUT <replaceable class="PARAMETER">tokid</> <type>integer</>, OUT <replaceable class="PARAMETER">token</> <type>text</>)</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><literal><function>ts_token_type</function>(<replaceable class="PARAMETER">parser_name</> <type>text</>, OUT <replaceable class="PARAMETER">tokid</> <type>integer</>, OUT <replaceable class="PARAMETER">alias</> <type>text</>, OUT <replaceable class="PARAMETER">description</> <type>text</>)</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</function>(<replaceable class="PARAMETER">parser_oid</> <type>oid</>, OUT <replaceable class="PARAMETER">tokid</> <type>integer</>, OUT <replaceable class="PARAMETER">alias</> <type>text</>, OUT <replaceable class="PARAMETER">description</> <type>text</>)</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><literal><function>ts_stat</function>(<replaceable class="PARAMETER">sqlquery</replaceable> <type>text</>, <optional> <replaceable class="PARAMETER">weights</replaceable> <type>text</>, </optional> OUT <replaceable class="PARAMETER">word</replaceable> <type>text</>, OUT <replaceable class="PARAMETER">ndoc</replaceable> <type>integer</>, OUT <replaceable class="PARAMETER">nentry</replaceable> <type>integer</>)</literal></entry>
        <entry><type>setof record</type></entry>
        <entry>get statistics of a <type>tsvector</> 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-xml">
  <title>XML Functions</title>

  <para>
   The functions and function-like expressions described in this
   section operate on values of type <type>xml</type>.  Check <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 not repeated here.  Use of many of these
   functions requires the installation to have been built
   with <command>configure --with-libxml</>.
  </para>

  <sect2>
   <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 <literal>--</literal> or end with a
     <literal>-</literal> 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 attribute name by default.  In any other case, 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
     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
     SQL and PostgreSQL data types with the XML Schema specification,
     at which point a more precise description will appear.
    </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,
     this replaces the value in the version declaration, if a
     standalone value is specified, this replaces the value in the
     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,
     like <function>xmlconcat</function> does.
     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, something like the
     following approach can be used:

<screen><![CDATA[
SELECT xmlagg(x) FROM (SELECT * FROM test ORDER BY y DESC) AS tab;
        xmlagg
----------------------
 <bar/><foo>abc</foo>
]]></screen>

     Again, see <xref linkend="functions-aggregate"> for additional
     information.
    </para>
   </sect3>

   <sect3>
    <title>XML Predicates</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>
  </sect2>

  <sect2 id="functions-xml-processing">
   <title>Processing XML</title>

   <indexterm>
    <primary>XPath</primary>
   </indexterm>

   <para>
    To process values of data type <type>xml</type>, PostgreSQL offers
    the function <function>xpath</function>, which evaluates XPath 1.0
    expressions.
   </para>

<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
    expression <replaceable>xpath</replaceable> against the XML value
    <replaceable>xml</replaceable>.  It returns an array of XML values
    corresponding to the node set produced by the XPath expression.
   </para>

   <para>
    The third argument of the function is an array of namespace
    mappings.  This array should be a two-dimensional 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, the
    second the namespace URI.
   </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>
  </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 recommendable 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 made by the data mappings produced 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 may 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 for 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 result data of 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-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 <productname>PostgreSQL</productname>'s
   functions for operating on <firstterm>sequence objects</firstterm>.
   Sequence objects (also called sequence generators or just
   sequences) are special single-row tables created with <xref
   linkend="sql-createsequence" endterm="sql-createsequence-title">.
   A sequence object is usually 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</function>(<type>regclass</type>)</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</function>(<type>regclass</type>)</literal></entry>
        <entry><type>bigint</type></entry>
        <entry>Advance sequence and return new value</entry>
      </row>
      <row>
        <entry><literal><function>setval</function>(<type>regclass</type>, <type>bigint</type>)</literal></entry>
        <entry><type>bigint</type></entry>
        <entry>Set sequence's current value</entry>
      </row>
      <row>
        <entry><literal><function>setval</function>(<type>regclass</type>, <type>bigint</type>, <type>boolean</type>)</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 call is specified by
   a <type>regclass</> argument, which is just the OID of the sequence in the
   <structname>pg_class</> system catalog.  You do not have to look up the
   OID by hand, however, since the <type>regclass</> 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.  To
   achieve some compatibility with the handling of ordinary
   <acronym>SQL</acronym> names, the string will be converted to lowercase
   unless it contains double quotes around the sequence name.  Thus:
<programlisting>
nextval('foo')      <lineannotation>operates on sequence <literal>foo</literal></>
nextval('FOO')      <lineannotation>operates on sequence <literal>foo</literal></>
nextval('"Foo"')    <lineannotation>operates on sequence <literal>Foo</literal></>
</programlisting>
   The sequence name can be schema-qualified if necessary:
<programlisting>
nextval('myschema.foo')     <lineannotation>operates on <literal>myschema.foo</literal></>
nextval('"myschema".foo')   <lineannotation>same as above</lineannotation>
nextval('foo')              <lineannotation>searches search path for <literal>foo</literal></>
</programlisting>
   See <xref linkend="datatype-oid"> for more information about
   <type>regclass</>.
  </para>

  <note>
   <para>
    Before <productname>PostgreSQL</productname> 8.1, the arguments of the
    sequence functions were of type <type>text</>, not <type>regclass</>, and
    the above-described conversion from a text string to an OID value would
    happen at run time during each call.  For backwards compatibility, this
    facility still exists, but internally it is now handled as an implicit
    coercion from <type>text</> to <type>regclass</> 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</>.
    Since this is really just an OID, it will track the originally
    identified sequence despite later renaming, schema reassignment,
    etc.  This <quote>early binding</> behavior is usually desirable for
    sequence references in column defaults and views.  But sometimes you will
    want <quote>late binding</> where the sequence reference is resolved
    at run time.  To get late-binding behavior, force the constant to be
    stored as a <type>text</> constant instead of <type>regclass</>:
<programlisting>
nextval('foo'::text)      <lineannotation><literal>foo</literal> is looked up at runtime</>
</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>
      </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.)  Notice that 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>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><function>lastval</function></term>
      <listitem>
       <para>
        Return the value most recently returned by
        <function>nextval</> in the current session. This function is
        identical to <function>currval</function>, except that instead
        of taking the sequence name as an argument it fetches the
        value of the last sequence that <function>nextval</function>
        was used on 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>
      </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</> is
        also set to the specified value.  In the three-parameter form,
        <literal>is_called</literal> can be set either <literal>true</literal>
        or <literal>false</literal>.  <literal>true</> has the same effect as
        the two-parameter form. If it's 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</> is not changed in this case (this is a change
        from pre-8.3 behavior).  For example,

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

        The result returned by <function>setval</function> is just the value of its
        second argument.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
  </para>

  <para>
   If a sequence object has been created with default parameters,
   <function>nextval</function> calls on it will return successive values
   beginning with 1.  Other behaviors can be obtained by using
   special parameters in the <xref linkend="sql-createsequence" endterm="sql-createsequence-title"> command;
   see its command reference page for more information.
  </para>

  <important>
   <para>
    To avoid blocking of 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, even if the
    transaction that did the <function>nextval</function> later aborts.  This means
    that aborted transactions might leave unused <quote>holes</quote> in the
    sequence of assigned values.  <function>setval</function> operations are never
    rolled back, either.
   </para>
  </important>

 </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 stored procedure
    in a more expressive programming language.
   </para>
  </tip>

  <sect2>
   <title><literal>CASE</></title>

  <para>
   The <acronym>SQL</acronym> <token>CASE</token> expression is a
   generic conditional expression, similar to if/else statements in
   other 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.  <replaceable>condition</replaceable> is an
   expression that returns a <type>boolean</type> result.  If the result is true
   then the value of the <token>CASE</token> expression is the
   <replaceable>result</replaceable> that follows the condition.  If the result is false any
   subsequent <token>WHEN</token> clauses are searched in the same
   manner.  If no <token>WHEN</token>
   <replaceable>condition</replaceable> is true then the value of the
   case expression is the <replaceable>result</replaceable> in the
   <token>ELSE</token> clause.  If the <token>ELSE</token> clause is
   omitted and no condition matches, 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 detail.
  </para>

  <para>
   The following <quote>simple</quote> <token>CASE</token> expression is a
   specialized 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
   <replaceable>expression</replaceable> is computed and compared to
   all the <replaceable>value</replaceable> specifications in the
   <token>WHEN</token> clauses until one is found that is equal.  If
   no match is found, the <replaceable>result</replaceable> in 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>
  </sect2>

  <sect2>
   <title><literal>COALESCE</></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>
  </para>

   <para>
    Like a <token>CASE</token> expression, <function>COALESCE</function> will
    not evaluate arguments that are not 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</> and <function>IFNULL</>, which are used in some other
    database systems.
   </para>
  </sect2>

  <sect2>
   <title><literal>NULLIF</></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> and <replaceable>value2</replaceable>
   are equal;  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>
   If <replaceable>value1</replaceable> is <literal>(none)</>, return a null,
   otherwise return <replaceable>value1</replaceable>.
  </para>

  </sect2>

  <sect2>
   <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</> and <function>LEAST</> 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</> and <function>LEAST</> 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]</literal></entry>
        <entry><literal>t</literal></entry>
       </row>

       <row>
        <entry> <literal>&lt;@</literal> </entry>
        <entry>is contained by</entry>
        <entry><literal>ARRAY[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>
   Array comparisons compare the array contents element-by-element,
   using the default B-Tree comparison function for the element data type.
   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</> 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>
   See <xref linkend="arrays"> for more details about array operator
   behavior.
  </para>

  <para>
   <xref linkend="array-functions-table"> shows the functions
   available for use with array types. See <xref linkend="arrays">
   for more discussion and examples of the use of these functions.
  </para>

    <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_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_to_string</function>(<type>anyarray</type>, <type>text</type>)
         </literal>
        </entry>
        <entry><type>text</type></entry>
        <entry>concatenates array elements using provided delimiter</entry>
        <entry><literal>array_to_string(ARRAY[1, 2, 3], '~^~')</literal></entry>
        <entry><literal>1~^~2~^~3</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[1,2,3,4], 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 length of the first dimension of the array
        (special case of <function>array_length</function> for SQL
        compatibility)</entry>
        <entry><literal>cardinality(array[1,2,3])</literal></entry>
        <entry><literal>3</literal></entry>
       </row>
       <row>
        <entry>
         <literal>
          <function>string_to_array</function>(<type>text</type>, <type>text</type>)
         </literal>
        </entry>
        <entry><type>text[]</type></entry>
        <entry>splits string into array elements using provided delimiter</entry>
        <entry><literal>string_to_array('xx~^~yy~^~zz', '~^~')</literal></entry>
        <entry><literal>{xx,yy,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><literal>1</literal><para><literal>2</literal></para> (2 rows)</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    See also <xref linkend="functions-aggregate"> about the aggregate
    function <function>array_agg</function> for use with arrays.
   </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
   value from a set of input values.  The built-in aggregate functions
   are listed in
   <xref linkend="functions-aggregate-table"> and
   <xref linkend="functions-aggregate-statistics-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="4">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Argument Type</entry>
      <entry>Return Type</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
      </entry>
      <entry>
       array of the argument type
      </entry>
      <entry>input values concatenated into an array</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>average</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>the average (arithmetic mean) of all 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>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>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>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>true if at least one input value is true, otherwise false</entry>
     </row>

     <row>
      <entry><function>count(*)</function></entry>
      <entry></entry>
      <entry><type>bigint</type></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>
       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>equivalent to <function>bool_and</function></entry>
     </row>

     <row>
      <entry><function>max(<replaceable class="parameter">expression</replaceable>)</function></entry>
      <entry>any array, numeric, string, or date/time type</entry>
      <entry>same as argument type</entry>
      <entry>
       maximum value of <replaceable
       class="parameter">expression</replaceable> across all input
       values
      </entry>
     </row>

     <row>
      <entry><function>min(<replaceable class="parameter">expression</replaceable>)</function></entry>
      <entry>any array, numeric, string, or date/time type</entry>
      <entry>same as argument type</entry>
      <entry>
       minimum value of <replaceable
       class="parameter">expression</replaceable> across all input
       values
      </entry>
     </row>

     <row>
      <entry><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>, or
       <type>interval</type>
      </entry>
      <entry>
       <type>bigint</type> for <type>smallint</type> or
       <type>int</type> arguments, <type>numeric</type> for
       <type>bigint</type> arguments, <type>double precision</type>
       for floating-point arguments, otherwise the same as the
       argument data type
      </entry>
      <entry>sum of <replaceable class="parameter">expression</replaceable> across all 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>concatenation of 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>

  <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 both as leading
      to a subquery or as an aggregate if the select expression returns 1 row.
      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 surprised 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 be executed by <productname>PostgreSQL</productname> using a
    sequential scan of the entire table.
   </para>
  </note>

  <para>
   The aggregate functions <function>array_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.  In the current
   implementation, the order of the input is in principle unspecified.
   Supplying the input values from a sorted subquery
   will usually work, however.  For example:

<screen><![CDATA[
SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab;
]]></screen>

   But this syntax is not allowed in the SQL standard, and is
   not portable to other database systems.  A future version of
   <productname>PostgreSQL</> might provide an additional feature to control
   the order in a better-defined way (<literal>xmlagg(expr ORDER BY expr, expr,
   ...)</literal>).
  </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="4">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Argument Type</entry>
      <entry>Return Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>

     <row>
      <entry>
       <indexterm>
        <primary>correlation</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>correlation coefficient</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>covariance</primary>
        <secondary>population</secondary>
       </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>population covariance</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>covariance</primary>
        <secondary>sample</secondary>
       </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>sample covariance</entry>
     </row>

     <row>
      <entry>
       <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>average of the independent variable
      (<literal>sum(<replaceable class="parameter">X</replaceable>)/<replaceable class="parameter">N</replaceable></literal>)</entry>
     </row>

     <row>
      <entry>
       <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>average of the dependent variable
      (<literal>sum(<replaceable class="parameter">Y</replaceable>)/<replaceable class="parameter">N</replaceable></literal>)</entry>
     </row>

     <row>
      <entry>
       <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>number of input rows in which both expressions are nonnull</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>regression 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>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>
       <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>square of the correlation coefficient</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>regression 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>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>
       <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><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>
       <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><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>
       <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><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>
       <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>historical alias for <function>stddev_samp</function></entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>standard deviation</primary>
        <secondary>population</secondary>
       </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>population standard deviation of the input values</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>standard deviation</primary>
        <secondary>sample</secondary>
       </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>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>historical alias for <function>var_samp</function></entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>variance</primary>
        <secondary>population</secondary>
       </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>population variance of the input values (square of the population standard deviation)</entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>variance</primary>
        <secondary>sample</secondary>
       </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>sample variance of the input values (square of the sample standard deviation)</entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </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.
  </para>

  <para>
   The built-in window functions are listed in
   <xref linkend="functions-window-table">.  Note that these functions
   <emphasis>must</> be invoked using window function syntax; that is an
   <literal>OVER</> clause is required.
  </para>

  <para>
   In addition to these functions, any built-in or user-defined aggregate
   function 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</>
   clause follows the call; otherwise they act as regular aggregates.
  </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</> 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</> - 1) / (total rows - 1)</entry>
     </row>

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

     <row>
      <entry>
       <indexterm>
        <primary>ntile</primary>
       </indexterm>
       <function>ntile(<replaceable class="parameter">num_buckets</replaceable> <type>integer</>)</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>any</>
             [, <replaceable class="parameter">offset</replaceable> <type>integer</>
             [, <replaceable class="parameter">default</replaceable> <type>any</> ]])
       </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>.
       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>any</>
              [, <replaceable class="parameter">offset</replaceable> <type>integer</>
              [, <replaceable class="parameter">default</replaceable> <type>any</> ]])
       </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>.
       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</>)</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</>)</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</>, <replaceable class="parameter">nth</replaceable> <type>integer</>)
       </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</> clause of the associated window
   definition.  Rows that are not distinct in the <literal>ORDER BY</>
   ordering are said to be <firstterm>peers</>; the four ranking functions
   are defined so that they give the same answer for any two peer rows.
  </para>

  <para>
   Note that <function>first_value</>, <function>last_value</>, and
   <function>nth_value</> consider only the rows within the <quote>window
   frame</>, 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>nth_value</> and
   particularly <function>last_value</>.  You can redefine the frame as
   being the whole partition by adding <literal>ROWS BETWEEN UNBOUNDED
   PRECEDING AND UNBOUNDED FOLLOWING</> to the <literal>OVER</> clause.
   See <xref linkend="syntax-window-functions"> for more information.
  </para>

  <para>
   When an aggregate function is used as a window function, it aggregates
   over the rows within the current row's window frame.  To obtain
   aggregation over the whole partition, omit <literal>ORDER BY</> or use
   <literal>ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING</>.
   An aggregate used with <literal>ORDER BY</> and the default window frame
   definition produces a <quote>running sum</> type of behavior, which may or
   may not be what's wanted.
  </para>

  <note>
   <para>
    The SQL standard defines a <literal>RESPECT NULLS</> or
    <literal>IGNORE NULLS</> option for <function>lead</>, <function>lag</>,
    <function>first_value</>, <function>last_value</>, and
    <function>nth_value</>.  This is not implemented in
    <productname>PostgreSQL</productname>: the behavior is always the
    same as the standard's default, namely <literal>RESPECT NULLS</>.
    Likewise, the standard's <literal>FROM FIRST</> or <literal>FROM LAST</>
    option for <function>nth_value</> is not implemented: only the
    default <literal>FROM FIRST</> behavior is supported.  (You can achieve
    the result of <literal>FROM LAST</> by reversing the <literal>ORDER BY</>
    ordering.)
   </para>
  </note>

 </sect1>

 <sect1 id="functions-subquery">
  <title>Subquery Expressions</title>

  <indexterm>
   <primary>EXISTS</primary>
  </indexterm>

  <indexterm>
   <primary>IN</primary>
  </indexterm>

  <indexterm>
   <primary>NOT IN</primary>
  </indexterm>

  <indexterm>
   <primary>ANY</primary>
  </indexterm>

  <indexterm>
   <primary>ALL</primary>
  </indexterm>

  <indexterm>
   <primary>SOME</primary>
  </indexterm>

  <indexterm>
   <primary>subquery</primary>
  </indexterm>

  <para>
   This section describes the <acronym>SQL</acronym>-compliant subquery
   expressions available in <productname>PostgreSQL</productname>.
   All of the expression forms documented in this section return
   Boolean (true/false) results.
  </para>

  <sect2>
   <title><literal>EXISTS</literal></title>

<synopsis>
EXISTS (<replaceable>subquery</replaceable>)
</synopsis>

  <para>
   The argument of <token>EXISTS</token> is an arbitrary <command>SELECT</> 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</>; if the subquery returns no rows, the result of <token>EXISTS</token> 
   is <quote>false</>.
  </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 far 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 any side effects (such as
   calling sequence functions); whether the side effects occur or not
   might be difficult to predict.
  </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 uninteresting.  A common coding convention is
   to write all <literal>EXISTS</> 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</>, but
   it produces at most one output row for each <literal>tab1</> row,
   even if there are multiple matching <literal>tab2</> rows:
<screen>
SELECT col1 FROM tab1
    WHERE EXISTS(SELECT 1 FROM tab2 WHERE col2 = tab1.col2);
</screen>
  </para>
  </sect2>

  <sect2>
   <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</> if any equal subquery row is found.
   The result is <quote>false</> if no equal row is found (including the special
   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</> if any equal subquery row is found.
   The result is <quote>false</> if no equal row is found (including the special
   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>
   <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</> if only unequal subquery rows
   are found (including the special case where the subquery returns no rows).
   The result is <quote>false</> 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</> if only unequal subquery rows
   are found (including the special case where the subquery returns no rows).
   The result is <quote>false</> 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>
   <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</> if any true result is obtained.
   The result is <quote>false</> if no true result is found (including the special
   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</> ANY (<replaceable>subquery</replaceable>)
<replaceable>row_constructor</replaceable> <replaceable>operator</> 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</> if the comparison
   returns true for any subquery row.
   The result is <quote>false</> if the comparison returns false for every
   subquery row (including the special case where the subquery returns no
   rows).
   The result is NULL if the comparison does not return true for any row,
   and it returns NULL for at least one row.
  </para>

  <para>
   See <xref linkend="row-wise-comparison"> for details about the meaning
   of a row-wise comparison.
  </para>
  </sect2>

  <sect2>
   <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</> if all rows yield true
   (including the special case where the subquery returns no rows).
   The result is <quote>false</> if any false result is found.
   The result is NULL if the comparison does not return false for any row,
   and it returns NULL for at least one row.
  </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</> if the comparison
   returns true for all subquery rows (including the special
   case where the subquery returns no rows).
   The result is <quote>false</> if the comparison returns false for any
   subquery row.
   The result is NULL if the comparison does not return false for any
   subquery row, and it returns NULL for at least one row.
  </para>

  <para>
   See <xref linkend="row-wise-comparison"> for details about the meaning
   of a row-wise comparison.
  </para>
  </sect2>

  <sect2>
   <title>Row-wise 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-wise 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>row-wise comparison</primary>
  </indexterm>

  <indexterm>
   <primary>comparison</primary>
   <secondary>row-wise</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>
   <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</> 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's 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</> if any true result is obtained.
   The result is <quote>false</> if no true result is found (including the special
   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</> if all comparisons yield true
   (including the special case where the array has zero elements).
   The result is <quote>false</> 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-wise 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 comparisons
   are allowed when the <replaceable>operator</replaceable> is
   <literal>=</>,
   <literal>&lt;&gt;</>,
   <literal>&lt;</>,
   <literal>&lt;=</>,
   <literal>&gt;</> or
   <literal>&gt;=</>,
   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>=</> member of a B-Tree operator
   class.)
  </para>

  <para>
   The <literal>=</> and <literal>&lt;&gt;</> 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>&lt;=</>, <literal>&gt;</> and
   <literal>&gt;=</> 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)</>
   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>&lt;=</>, <literal>&gt;</> and <literal>&gt;=</>
    cases were not handled per SQL specification.  A comparison like
    <literal>ROW(a,b) &lt; ROW(c,d)</>
    was implemented as
    <literal>a &lt; c AND b &lt; d</>
    whereas the correct behavior is equivalent to
    <literal>a &lt; c OR (a = c AND b &lt; d)</>.
   </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 always
   be either 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>

  <note>
   <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 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>
  </note>

  </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.
   Currently the only functions in this class are series generating functions,
   as detailed in <xref linkend="functions-srf-series"> and
   <xref linkend="functions-srf-subscripts">.
  </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</function>(<parameter>start</parameter>, <parameter>stop</parameter>)</literal></entry>
      <entry><type>int</type> or <type>bigint</type></entry>
      <entry><type>setof int</type> or <type>setof bigint</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</function>(<parameter>start</parameter>, <parameter>stop</parameter>, <parameter>step</parameter>)</literal></entry>
      <entry><type>int</type> or <type>bigint</type></entry>
      <entry><type>setof int</type> or <type>setof bigint</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</function>(<parameter>start</parameter>, <parameter>stop</parameter>, <parameter>step</parameter> <type>interval</>)</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)

-- 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</function>(<parameter>array anyarray</parameter>, <parameter>dim int</parameter>)</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</function>(<parameter>array anyarray</parameter>, <parameter>dim int</parameter>, <parameter>reverse boolean</parameter>)</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</> 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}
(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} |         1 |   100
 {100,200} |         2 |   200
(4 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
postgres=# select * from unnest2(array[[1,2],[3,4]]);
 unnest2 
---------
       1
       2
       3
       4
(4 rows)
</programlisting>
  </para>

 </sect1>

 <sect1 id="functions-info">
  <title>System Information Functions</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_schema</function>[()]</literal></entry>
       <entry><type>name</type></entry>
       <entry>name of current schema</entry>
      </row>

      <row>
       <entry><literal><function>current_schemas</function>(<type>boolean</type>)</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>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>
       <!-- 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>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>
       <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</function>(<type>oid</type>)</literal></entry>
       <entry><type>boolean</type></entry>
       <entry>is schema another session's temporary schema?</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_conf_load_time</function>()</literal></entry>
       <entry><type>timestamp with time zone</type></entry>
       <entry>configuration load time</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</> version information</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

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

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

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

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

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

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

   <indexterm>
    <primary>current_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" endterm="sql-set-session-authorization-title">.
    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" endterm="sql-set-role-title">.
    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>.
   </para>

   <note>
    <para>
     <function>current_catalog</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 (optional in PostgreSQL in the case
     of <function>current_schema</function>).
    </para>
   </note>

   <para>
    <function>current_schema</function> returns the name of the schema that is
    at the front of 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</> are included in the search 
    path returned.
   </para>

   <note>
    <para>
     The search path can be altered at run time.  The command is:
<programlisting>
SET search_path TO <replaceable>schema</> <optional>, <replaceable>schema</>, ...</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_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 0 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 any other session's temporary schema.
    (This can be useful, for example, to exclude other sessions' temporary
    tables from a catalog display.)
   </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_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>version</primary>
   </indexterm>

   <para>
    <function>version</function> returns a string describing the
    <productname>PostgreSQL</productname> server's 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_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_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>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>
     </tbody>
    </tgroup>
   </table>

   <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_table_privilege</primary>
   </indexterm>
   <indexterm>
    <primary>has_tablespace_privilege</primary>
   </indexterm>
   <indexterm>
    <primary>pg_has_role</primary>
   </indexterm>

   <para>
    <function>has_database_privilege</function> checks whether a user
    can access a database 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
    <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.  The possibilities for its
    arguments 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</> 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.  The possibilities for its
    arguments 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.  The possibilities
    for its arguments 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.  The possibilities for its
    arguments are analogous to <function>has_table_privilege</function>.
    The desired access privilege type must evaluate to
    <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.  The possibilities for its
    arguments are analogous to <function>has_table_privilege</function>.
    The desired access privilege type must evaluate to
    <literal>USAGE</literal>.
   </para>

   <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 or by OID
    (<literal>pg_authid.oid</literal>), 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>REFERENCES</literal>, or <literal>TRIGGER</literal>.
    (Case of the string is not significant, however.)
    An example is:
<programlisting>
SELECT has_table_privilege('myschema.mytable', 'select');
</programlisting>
   </para>

   <para>
    <function>has_tablespace_privilege</function> checks whether a user
    can access a tablespace 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
    <literal>CREATE</literal>.
   </para>

   <para>
    <function>pg_has_role</function> checks whether a user
    can access a role 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
    <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</>), while
    <literal>USAGE</literal> denotes whether the privileges of the role
    are immediately available without doing <command>SET ROLE</>.
   </para>

  <para>
   To test whether a user holds a grant option on the privilege,
   append <literal>WITH GRANT OPTION</literal> to the privilege key
   word; for example <literal>'UPDATE WITH GRANT OPTION'</literal>.
  </para>

  <para>
   <xref linkend="functions-info-schema-table"> shows functions that
   determine whether a certain object is <firstterm>visible</> 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>

   <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_conversion_is_visible</function>(<parameter>conversion_oid</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is conversion visible in search path</entry>
      </row>
      <row>
       <entry><literal><function>pg_function_is_visible</function>(<parameter>function_oid</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is function visible in search path</entry>
      </row>
      <row>
       <entry><literal><function>pg_operator_is_visible</function>(<parameter>operator_oid</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is operator visible in search path</entry>
      </row>
      <row>
       <entry><literal><function>pg_opclass_is_visible</function>(<parameter>opclass_oid</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is operator class visible in search path</entry>
      </row>
      <row>
       <entry><literal><function>pg_table_is_visible</function>(<parameter>table_oid</parameter>)</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</function>(<parameter>config_oid</parameter>)</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</function>(<parameter>dict_oid</parameter>)</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</function>(<parameter>parser_oid</parameter>)</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</function>(<parameter>template_oid</parameter>)</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</function>(<parameter>type_oid</parameter>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>is type (or domain) visible in search path</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <indexterm>
    <primary>pg_conversion_is_visible</primary>
   </indexterm>
   <indexterm>
    <primary>pg_function_is_visible</primary>
   </indexterm>
   <indexterm>
    <primary>pg_operator_is_visible</primary>
   </indexterm>
   <indexterm>
    <primary>pg_opclass_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, indexes and sequences; <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)</> 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>regtype</>,
    <type>regprocedure</>, <type>regoperator</>, <type>regconfig</>,
    or <type>regdictionary</>),
    for example:
<programlisting>
SELECT pg_type_is_visible('myschema.widget'::regtype);
</programlisting>
    Note that it would not make much sense to test an unqualified 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_keywords</primary>
   </indexterm>

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

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

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

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

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

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

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

   <indexterm>
    <primary>pg_typeof</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</function>(<parameter>type_oid</parameter>, <parameter>typemod</>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get SQL name of a data type</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_constraintdef</function>(<parameter>constraint_oid</parameter>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get definition of a constraint</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_constraintdef</function>(<parameter>constraint_oid</parameter>, <parameter>pretty_bool</>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get definition of a constraint</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_expr</function>(<parameter>expr_text</parameter>, <parameter>relation_oid</>)</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</function>(<parameter>expr_text</parameter>, <parameter>relation_oid</>, <parameter>pretty_bool</>)</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</function>(<parameter>func_oid</parameter>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get definition of a function</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_function_arguments</function>(<parameter>func_oid</parameter>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get argument list of function's definition (with default values)</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_function_identity_arguments</function>(<parameter>func_oid</parameter>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get argument list to identify a function (without default values)</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_function_result</function>(<parameter>func_oid</parameter>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get <literal>RETURNS</> clause for function</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_indexdef</function>(<parameter>index_oid</parameter>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get <command>CREATE INDEX</> command for index</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_indexdef</function>(<parameter>index_oid</parameter>, <parameter>column_no</>, <parameter>pretty_bool</>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get <command>CREATE INDEX</> command for index,
       or definition of just one index column when
       <parameter>column_no</> is not zero</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_ruledef</function>(<parameter>rule_oid</parameter>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get <command>CREATE RULE</> command for rule</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_ruledef</function>(<parameter>rule_oid</parameter>, <parameter>pretty_bool</>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get <command>CREATE RULE</> command for rule</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_serial_sequence</function>(<parameter>table_name</parameter>, <parameter>column_name</parameter>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get name of the sequence that a <type>serial</type> or <type>bigserial</type> column
       uses</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 for trigger</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_userbyid</function>(<parameter>roleid</parameter>)</literal></entry>
       <entry><type>name</type></entry>
       <entry>get role name with given ID</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_viewdef</function>(<parameter>view_name</parameter>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get underlying <command>SELECT</command> command for view (<emphasis>deprecated</emphasis>)</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_viewdef</function>(<parameter>view_name</parameter>, <parameter>pretty_bool</>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get underlying <command>SELECT</command> command for view (<emphasis>deprecated</emphasis>)</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_viewdef</function>(<parameter>view_oid</parameter>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get underlying <command>SELECT</command> command for view</entry>
      </row>
      <row>
       <entry><literal><function>pg_get_viewdef</function>(<parameter>view_oid</parameter>, <parameter>pretty_bool</>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get underlying <command>SELECT</command> command for view</entry>
      </row>
      <row>
       <entry><literal><function>pg_tablespace_databases</function>(<parameter>tablespace_oid</parameter>)</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_typeof</function>(<parameter>any</parameter>)</literal></entry>
       <entry><type>regtype</type></entry>
       <entry>get the data type of any value</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</> column
   contains the keyword.  The <structfield>catcode</> column contains a
   category code: <literal>U</> for unreserved, <literal>C</> for column name,
   <literal>T</> for type or function name, or <literal>R</> for reserved.
   The <structfield>catdesc</> 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>,
   and <function>pg_get_triggerdef</function>, respectively reconstruct the
   creating command for a constraint, index, rule, 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.
   <function>pg_get_viewdef</function> reconstructs the <command>SELECT</>
   query that defines a view. Most of these functions come in two variants,
   one of which can optionally <quote>pretty-print</> 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</>; avoid using pretty-printed output for dump
   purposes.  Passing <literal>false</> 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</> returns a complete
   <command>CREATE OR REPLACE FUNCTION</> 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</>.
   <function>pg_get_function_result</function> similarly returns the
   appropriate <literal>RETURNS</> 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</>, 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.  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 lowercased 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 the sequence functions (see <xref
   linkend="functions-sequence">).  This association can be modified or
   removed with <command>ALTER SEQUENCE OWNED BY</>.  (The function
   probably should have been called
   <function>pg_get_owned_sequence</function>; its name reflects the fact
   that it's typically used with <type>serial</> or <type>bigserial</>
   columns.)
  </para>

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

   <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"
    endterm="sql-comment-title"> command.  A null value is returned if no
    comment could be found matching 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</function>(<parameter>table_oid</parameter>, <parameter>column_number</parameter>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get comment for a table column</entry>
      </row>
      <row>
       <entry><literal><function>obj_description</function>(<parameter>object_oid</parameter>, <parameter>catalog_name</parameter>)</literal></entry>
       <entry><type>text</type></entry>
       <entry>get comment for a database object</entry>
      </row>
      <row>
       <entry><literal><function>obj_description</function>(<parameter>object_oid</parameter>)</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</function>(<parameter>object_oid</parameter>, <parameter>catalog_name</parameter>)</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 a table with OID 123456.
    The one-parameter form of <function>obj_description</function> requires only
    the object OID.  It is now deprecated since there is no guarantee that
    OIDs are unique across different system catalogs; therefore, the wrong
    comment could be returned.
   </para>

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

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

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

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

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

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

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

   <para>
    The functions shown in <xref linkend="functions-txid-snapshot">
    export server internal transaction information to user level.  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</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_xmin</function>(<parameter>txid_snapshot</parameter>)</literal></entry>
       <entry><type>bigint</type></entry>
       <entry>get xmin of snapshot</entry>
      </row>
      <row>
       <entry><literal><function>txid_snapshot_xmax</function>(<parameter>txid_snapshot</parameter>)</literal></entry>
       <entry><type>bigint</type></entry>
       <entry>get xmax of snapshot</entry>
      </row>
      <row>
       <entry><literal><function>txid_snapshot_xip</function>(<parameter>txid_snapshot</parameter>)</literal></entry>
       <entry><type>setof bigint</type></entry>
       <entry>get in-progress transaction IDs in snapshot</entry>
      </row>
      <row>
       <entry><literal><function>txid_visible_in_snapshot</function>(<parameter>bigint</parameter>, <parameter>txid_snapshot</parameter>)</literal></entry>
       <entry><type>boolean</type></entry>
       <entry>is transaction ID visible in snapshot? (do not use with subtransaction ids)</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    The internal transaction ID type (<type>xid</>) is 32 bits wide and so
    it wraps around every 4 billion transactions.  However, these functions
    export a 64-bit format that is extended with an <quote>epoch</> counter
    so that it will not wrap around for 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 later than this one 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</>
        and <literal>xmax</>; there might be active txids higher
        than xmax.  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</>'s textual representation is
    <literal><replaceable>xmin</>:<replaceable>xmax</>:<replaceable>xip_list</></literal>.
    For example <literal>10:20:10,14,15</literal> means
    <literal>xmin=10, xmax=20, xip_list=10, 14, 15</literal>.
   </para>
  </sect1>

  <sect1 id="functions-admin">
   <title>System Administration 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>
        <literal><function>current_setting</function>(<parameter>setting_name</parameter>)</literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>current value of setting</entry>
      </row>
      <row>
       <entry>
        <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>
   </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>

   <indexterm>
    <primary>pg_cancel_backend</primary>
   </indexterm>
   <indexterm>
    <primary>pg_terminate_backend</primary>
   </indexterm>
   <indexterm>
    <primary>pg_reload_conf</primary>
   </indexterm>
   <indexterm>
    <primary>pg_rotate_logfile</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.
   </para>

   <table id="functions-admin-signal-table">
    <title>Server Signalling 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</function>(<parameter>pid</parameter> <type>int</>)</literal>
        </entry>
       <entry><type>boolean</type></entry>
       <entry>Cancel a backend's current query</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_terminate_backend</function>(<parameter>pid</parameter> <type>int</>)</literal>
        </entry>
       <entry><type>boolean</type></entry>
       <entry>Terminate a backend</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>
     </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</> and <function>pg_terminate_backend</>
    send signals (<systemitem>SIGINT</> or <systemitem>SIGTERM</>
    respectively) to backend processes identified by process ID.
    The process ID of an active backend can be found from
    the <structfield>procpid</structfield> column in the
    <structname>pg_stat_activity</structname> view, or by listing the
    <command>postgres</command> processes on the server with
    <application>ps</>.
   </para>

   <para>
    <function>pg_reload_conf</> sends a <systemitem>SIGHUP</> signal
    to the server, causing the configuration files
    to be reloaded by all server processes.
   </para>

   <para>
    <function>pg_rotate_logfile</> 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>

   <indexterm>
    <primary>pg_start_backup</primary>
   </indexterm>
   <indexterm>
    <primary>pg_stop_backup</primary>
   </indexterm>
   <indexterm>
    <primary>pg_switch_xlog</primary>
   </indexterm>
   <indexterm>
    <primary>pg_current_xlog_location</primary>
   </indexterm>
   <indexterm>
    <primary>pg_current_xlog_insert_location</primary>
   </indexterm>
   <indexterm>
    <primary>pg_xlogfile_name_offset</primary>
   </indexterm>
   <indexterm>
    <primary>pg_xlogfile_name</primary>
   </indexterm>
   <indexterm>
    <primary>backup</primary>
   </indexterm>

   <para>
    The functions shown in <xref
    linkend="functions-admin-backup-table"> assist in making on-line backups.
    Use of the first three functions is restricted to superusers.
   </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_start_backup</function>(<parameter>label</> <type>text</>)</literal>
        </entry>
       <entry><type>text</type></entry>
       <entry>Set up for performing on-line backup</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_stop_backup</function>()</literal>
        </entry>
       <entry><type>text</type></entry>
       <entry>Finish performing on-line backup</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_switch_xlog</function>()</literal>
        </entry>
       <entry><type>text</type></entry>
       <entry>Force switch to a new transaction log file</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_current_xlog_location</function>()</literal>
        </entry>
       <entry><type>text</type></entry>
       <entry>Get current transaction log write location</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_current_xlog_insert_location</function>()</literal>
        </entry>
       <entry><type>text</type></entry>
       <entry>Get current transaction log insert location</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_xlogfile_name_offset</function>(<parameter>location</> <type>text</>)</literal>
        </entry>
       <entry><type>text</>, <type>integer</></entry>
       <entry>Convert transaction log location string to file name and decimal byte offset within file</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_xlogfile_name</function>(<parameter>location</> <type>text</>)</literal>
        </entry>
       <entry><type>text</type></entry>
       <entry>Convert transaction log location string to file name</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    <function>pg_start_backup</> accepts a single parameter which is an
    arbitrary user-defined label for the backup.  (Typically this would be
    the name under which the backup dump file will be stored.)  The function
    writes a backup label file into the database cluster's data directory,
    and then returns the backup's starting transaction log location as text.  The user
    need not pay any attention to this result value, but it is provided in
    case it is of use. 
<programlisting>
postgres=# select pg_start_backup('label_goes_here');
 pg_start_backup
-----------------
 0/D4445B8
(1 row)
</programlisting>
   </para>

   <para>
    <function>pg_stop_backup</> removes the label file created by
    <function>pg_start_backup</>, and instead creates a backup history file in
    the transaction log archive area.  The history file includes the label given to
    <function>pg_start_backup</>, the starting and ending transaction log locations for
    the backup, and the starting and ending times of the backup.  The return
    value is the backup's ending transaction log location (which again might be of little
    interest).  After noting the ending location, the current transaction log insertion
    point is automatically advanced to the next transaction log file, so that the
    ending transaction log file can be archived immediately to complete the backup.
   </para>

   <para>
    <function>pg_switch_xlog</> moves to the next transaction log file, allowing the 
    current file to be archived (assuming you are using continuous archiving).
    The result is the ending transaction log location + 1 within the just-completed transaction log file.
    If there has been no transaction log activity since the last transaction log switch,
    <function>pg_switch_xlog</> does nothing and returns the start location
    of the transaction log file currently in use.
   </para>

   <para>
    <function>pg_current_xlog_location</> displays the current transaction log write
    location in the same format used by the above functions.  Similarly,
    <function>pg_current_xlog_insert_location</> displays the current transaction log
    insertion point.  The insertion point is the <quote>logical</> end
    of the transaction log
    at any instant, while the write location is the end of what has actually
    been written out from the server's internal buffers.  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 transaction log
    files.  The insertion point is 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_xlogfile_name_offset</> to extract the
    corresponding transaction log file name and byte offset from the results of any of the
    above functions.  For example:
<programlisting>
postgres=# select * from pg_xlogfile_name_offset(pg_stop_backup());
        file_name         | file_offset 
--------------------------+-------------
 00000001000000000000000D |     4039624
(1 row)
</programlisting>
    Similarly, <function>pg_xlogfile_name</> extracts just the transaction log file name.
    When the given transaction log location is exactly at a transaction log file boundary, both
    these functions return the name of the preceding transaction log file.
    This is usually the desired behavior for managing transaction log archiving
    behavior, since the preceding file is the last one that currently
    needs to be archived.
   </para>

   <para>
    For details about proper usage of these functions, see
    <xref linkend="continuous-archiving">.
   </para>

   <para>
    The functions shown in <xref linkend="functions-admin-dbsize"> calculate
    the actual disk space usage of database objects.
   </para>

   <indexterm>
    <primary>pg_column_size</primary>
   </indexterm>
   <indexterm>
    <primary>pg_database_size</primary>
   </indexterm>
   <indexterm>
    <primary>pg_relation_size</primary>
   </indexterm>
   <indexterm>
    <primary>pg_size_pretty</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</function>(<type>any</type>)</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</function>(<type>oid</type>)</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</function>(<type>name</type>)</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_relation_size</function>(<parameter>relation</parameter> <type>regclass</type>, <parameter>fork</parameter> <type>text</type>)</literal>
        </entry>
       <entry><type>bigint</type></entry>
       <entry>
        Disk space used by the specified fork, <literal>'main'</literal> or
        <literal>'fsm'</literal>, of a table or index with the specified OID
        or name. The table name can be qualified with a schema name
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_relation_size</function>(<parameter>relation</parameter> <type>regclass</type>)</literal>
        </entry>
       <entry><type>bigint</type></entry>
       <entry>
        Shorthand for <literal>pg_relation_size(..., 'main')</literal>
       </entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_size_pretty</function>(<type>bigint</type>)</literal>
        </entry>
       <entry><type>text</type></entry>
       <entry>Converts a size in bytes into a human-readable format with size units</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_tablespace_size</function>(<type>oid</type>)</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</function>(<type>name</type>)</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</function>(<type>regclass</type>)</literal>
        </entry>
       <entry><type>bigint</type></entry>
       <entry>
        Total disk space used by the table with the specified OID or name,
        including indexes and toasted data.  The table name can be
        qualified with a schema name
       </entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    <function>pg_column_size</> shows the space used to store any individual
    data value.
   </para>

   <para>
    <function>pg_database_size</function> and <function>pg_tablespace_size</>
    accept the OID or name of a database or tablespace, and return the total
    disk space used therein.
   </para>

   <para>
    <function>pg_relation_size</> accepts the OID or name of a table, index or
    toast table, and returns the size in bytes. Specifying
    <literal>'main'</literal> or leaving out the second argument returns the
    size of the main data fork of the relation. Specifying
    <literal>'fsm'</literal> returns the size of the
    Free Space Map (see <xref linkend="storage-fsm">) associated with the
    relation.
   </para>

   <para>
    <function>pg_size_pretty</> can be used to format the result of one of
    the other functions in a human-readable way, using kB, MB, GB or TB as
    appropriate.
   </para>

   <para>
    <function>pg_total_relation_size</> accepts the OID or name of a
    table or toast table, and returns the size in bytes of the data
    and all associated indexes and toast tables.
   </para>

   <para>
    The functions shown in <xref
    linkend="functions-admin-genfile"> provide native file access to
    files on the machine hosting the server. Only files within the
    database cluster directory and the <varname>log_directory</> can be
    accessed.  Use a relative path for files within the cluster directory,
    and a path matching the <varname>log_directory</> configuration setting
    for log files.  Use of these functions is restricted to superusers.
   </para>

   <table id="functions-admin-genfile">
    <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</function>(<parameter>dirname</> <type>text</>)</literal>
       </entry>
       <entry><type>setof text</type></entry>
       <entry>List the contents of a directory</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_read_file</function>(<parameter>filename</> <type>text</>, <parameter>offset</> <type>bigint</>, <parameter>length</> <type>bigint</>)</literal>
       </entry>
       <entry><type>text</type></entry>
       <entry>Return the contents of a text file</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_stat_file</function>(<parameter>filename</> <type>text</>)</literal>
       </entry>
       <entry><type>record</type></entry>
       <entry>Return information about a file</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <indexterm>
    <primary>pg_ls_dir</primary>
   </indexterm>
   <para>
    <function>pg_ls_dir</> returns all the names in the specified
    directory, except the special entries <quote><literal>.</></> and
    <quote><literal>..</></>.
   </para>

   <indexterm>
    <primary>pg_read_file</primary>
   </indexterm>
   <para>
    <function>pg_read_file</> returns part of a text file, starting
    at the given <parameter>offset</>, returning at most <parameter>length</>
    bytes (less if the end of file is reached first).  If <parameter>offset</>
    is negative, it is relative to the end of the file.
   </para>

   <indexterm>
    <primary>pg_stat_file</primary>
   </indexterm>
   <para>
    <function>pg_stat_file</> 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>

   <para>
    The functions shown in <xref linkend="functions-advisory-locks"> manage
    advisory locks.  For details about proper usage of these functions, see
    <xref linkend="advisory-locks">.
   </para>

   <table id="functions-advisory-locks">
    <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</function>(<parameter>key</> <type>bigint</>)</literal>
       </entry>
       <entry><type>void</type></entry>
       <entry>Obtain exclusive advisory lock</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_advisory_lock</function>(<parameter>key1</> <type>int</>, <parameter>key2</> <type>int</>)</literal>
       </entry>
       <entry><type>void</type></entry>
       <entry>Obtain exclusive advisory lock</entry>
      </row>

      <row>
       <entry>
        <literal><function>pg_advisory_lock_shared</function>(<parameter>key</> <type>bigint</>)</literal>
       </entry>
       <entry><type>void</type></entry>
       <entry>Obtain shared advisory lock</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_advisory_lock_shared</function>(<parameter>key1</> <type>int</>, <parameter>key2</> <type>int</>)</literal>
       </entry>
       <entry><type>void</type></entry>
       <entry>Obtain shared advisory lock</entry>
      </row>

      <row>
       <entry>
        <literal><function>pg_try_advisory_lock</function>(<parameter>key</> <type>bigint</>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Obtain exclusive advisory lock if available</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_try_advisory_lock</function>(<parameter>key1</> <type>int</>, <parameter>key2</> <type>int</>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Obtain exclusive advisory lock if available</entry>
      </row>

      <row>
       <entry>
        <literal><function>pg_try_advisory_lock_shared</function>(<parameter>key</> <type>bigint</>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Obtain shared advisory lock if available</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_try_advisory_lock_shared</function>(<parameter>key1</> <type>int</>, <parameter>key2</> <type>int</>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Obtain shared advisory lock if available</entry>
      </row>

      <row>
       <entry>
        <literal><function>pg_advisory_unlock</function>(<parameter>key</> <type>bigint</>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Release an exclusive advisory lock</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_advisory_unlock</function>(<parameter>key1</> <type>int</>, <parameter>key2</> <type>int</>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Release an exclusive advisory lock</entry>
      </row>

      <row>
       <entry>
        <literal><function>pg_advisory_unlock_shared</function>(<parameter>key</> <type>bigint</>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Release a shared advisory lock</entry>
      </row>
      <row>
       <entry>
        <literal><function>pg_advisory_unlock_shared</function>(<parameter>key1</> <type>int</>, <parameter>key2</> <type>int</>)</literal>
       </entry>
       <entry><type>boolean</type></entry>
       <entry>Release a shared advisory lock</entry>
      </row>

      <row>
       <entry>
        <literal><function>pg_advisory_unlock_all</function>()</literal>
       </entry>
       <entry><type>void</type></entry>
       <entry>Release all advisory locks held by the current session</entry>
      </row>

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

   <indexterm>
    <primary>pg_advisory_lock</primary>
   </indexterm>
   <para>
    <function>pg_advisory_lock</> 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). 
    The key type is specified in <literal>pg_locks.objid</>.  If
    another session already holds a lock on the same resource, the
    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 be also 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</> works the same as
    <function>pg_advisory_lock</>,
    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</> is similar to
    <function>pg_advisory_lock</>, except the function will not wait for the
    lock to become available.  It will either obtain the lock immediately and
    return <literal>true</>, or return <literal>false</> if the lock cannot be
    acquired now.
   </para>

   <indexterm>
    <primary>pg_try_advisory_lock_shared</primary>
   </indexterm>
   <para>
    <function>pg_try_advisory_lock_shared</> works the same as
    <function>pg_try_advisory_lock</>, except it attempts to acquire
    shared rather than exclusive lock.
   </para>

   <indexterm>
    <primary>pg_advisory_unlock</primary>
   </indexterm>
   <para>
    <function>pg_advisory_unlock</> will release a previously-acquired
    exclusive advisory lock.  It
    will return <literal>true</> if the lock is successfully released.
    If the lock was in fact not held, it will return <literal>false</>,
    and in addition, an SQL warning will be raised by the server.
   </para>

   <indexterm>
    <primary>pg_advisory_unlock_shared</primary>
   </indexterm>
   <para>
    <function>pg_advisory_unlock_shared</> works the same as
    <function>pg_advisory_unlock</>, 
    except to release a shared advisory lock.
   </para>

   <indexterm>
    <primary>pg_advisory_unlock_all</primary>
   </indexterm>
   <para>
    <function>pg_advisory_unlock_all</> will release all advisory locks
    held by the current session.  (This function is implicitly invoked
    at session end, even if the client disconnects ungracefully.)
   </para>

  </sect1>

  <sect1 id="functions-trigger">
   <title>Trigger Functions</title>

   <indexterm>
     <primary>suppress_redundant_updates_trigger</primary>
   </indexterm>

   <para>
      Currently <productname>PostgreSQL</> provides one built in trigger
      function, <function>suppress_redundant_updates_trigger</>, 
      which will prevent any update
      that does not actually change the data in the row from taking place, in
      contrast to the normal behaviour which always performs the update
      regardless of whether or not the data has changed. (This normal behaviour
      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</>, 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 can be 
      added to a table like this:
<programlisting>
CREATE TRIGGER z_min_update 
BEFORE UPDATE ON tablename
FOR EACH ROW EXECUTE PROCEDURE 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>
</chapter>
