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$PostgreSQL: pgsql/doc/src/sgml/datatype.sgml,v 1.143 2004/03/09 16:57:46 neilc Exp $
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 <chapter id="datatype">
  <title id="datatype-title">Data Types</title>

  <indexterm zone="datatype">
   <primary>data type</primary>
  </indexterm>

  <indexterm>
   <primary>type</primary>
   <see>data type</see>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname> has a rich set of native data 
   types available to users.
   Users may add new types to <productname>PostgreSQL</productname> using the
   <command>CREATE TYPE</command> command.
  </para>

  <para>
   <xref linkend="datatype-table"> shows all built-in general-purpose data types.
   Most of the alternative names
   listed in the 
   <quote>Aliases</quote> column are the names used internally by
   <productname>PostgreSQL</productname> for historical reasons.  In
   addition, some internally used or deprecated types are available,
   but they are not listed here.
  </para>

   <table id="datatype-table">
    <title>Data Types</title>
    <tgroup cols="3">
     <thead>
      <row>
       <entry>Name</entry>
       <entry>Aliases</entry>
       <entry>Description</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry><type>bigint</type></entry>
       <entry><type>int8</type></entry>
       <entry>signed eight-byte integer</entry>
      </row>

      <row>
       <entry><type>bigserial</type></entry>
       <entry><type>serial8</type></entry>
       <entry>autoincrementing eight-byte integer</entry>
      </row>

      <row>
       <entry><type>bit</type></entry>
       <entry></entry>
       <entry>fixed-length bit string</entry>
      </row>

      <row>
       <entry><type>bit varying(<replaceable>n</replaceable>)</type></entry>
       <entry><type>varbit(<replaceable>n</replaceable>)</type></entry>
       <entry>variable-length bit string</entry>
      </row>

      <row>
       <entry><type>boolean</type></entry>
       <entry><type>bool</type></entry>
       <entry>logical Boolean (true/false)</entry>
      </row>

      <row>
       <entry><type>box</type></entry>
       <entry></entry>
       <entry>rectangular box in the plane</entry>
      </row>

      <row>
       <entry><type>bytea</type></entry>
       <entry></entry>
       <entry>binary data</entry>
      </row>

      <row>
       <entry><type>character varying(<replaceable>n</replaceable>)</type></entry>
       <entry><type>varchar(<replaceable>n</replaceable>)</type></entry>
       <entry>variable-length character string</entry>
      </row>

      <row>
       <entry><type>character(<replaceable>n</replaceable>)</type></entry>
       <entry><type>char(<replaceable>n</replaceable>)</type></entry>
       <entry>fixed-length character string</entry>
      </row>

      <row>
       <entry><type>cidr</type></entry>
       <entry></entry>
       <entry>IPv4 or IPv6 network address</entry>
      </row>

      <row>
       <entry><type>circle</type></entry>
       <entry></entry>
       <entry>circle in the plane</entry>
      </row>

      <row>
       <entry><type>date</type></entry>
       <entry></entry>
       <entry>calendar date (year, month, day)</entry>
      </row>

      <row>
       <entry><type>double precision</type></entry>
       <entry><type>float8</type></entry>
       <entry>double precision floating-point number</entry>
      </row>

      <row>
       <entry><type>inet</type></entry>
       <entry></entry>
       <entry>IPv4 or IPv6 host address</entry>
      </row>

      <row>
       <entry><type>integer</type></entry>
       <entry><type>int</type>, <type>int4</type></entry>
       <entry>signed four-byte integer</entry>
      </row>

      <row>
       <entry><type>interval(<replaceable>p</replaceable>)</type></entry>
       <entry></entry>
       <entry>time span</entry>
      </row>

      <row>
       <entry><type>line</type></entry>
       <entry></entry>
       <entry>infinite line in the plane (not fully implemented)</entry>
      </row>

      <row>
       <entry><type>lseg</type></entry>
       <entry></entry>
       <entry>line segment in the plane</entry>
      </row>

      <row>
       <entry><type>macaddr</type></entry>
       <entry></entry>
       <entry>MAC address</entry>
      </row>

      <row>
       <entry><type>money</type></entry>
       <entry></entry>
       <entry>currency amount</entry>
      </row>

      <row>
       <entry><type>numeric [ (<replaceable>p</replaceable>,
         <replaceable>s</replaceable>) ]</type></entry>
       <entry><type>decimal [ (<replaceable>p</replaceable>,
         <replaceable>s</replaceable>) ]</type></entry>
       <entry>exact numeric with selectable precision</entry>
      </row>

      <row>
       <entry><type>path</type></entry>
       <entry></entry>
       <entry>open and closed geometric path in the plane</entry>
      </row>

      <row>
       <entry><type>point</type></entry>
       <entry></entry>
       <entry>geometric point in the plane</entry>
      </row>

      <row>
       <entry><type>polygon</type></entry>
       <entry></entry>
       <entry>closed geometric path in the plane</entry>
      </row>

      <row>
       <entry><type>real</type></entry>
       <entry><type>float4</type></entry>
       <entry>single precision floating-point number</entry>
      </row>

      <row>
       <entry><type>smallint</type></entry>
       <entry><type>int2</type></entry>
       <entry>signed two-byte integer</entry>
      </row>

      <row>
       <entry><type>serial</type></entry>
       <entry><type>serial4</type></entry>
       <entry>autoincrementing four-byte integer</entry>
      </row>

      <row>
       <entry><type>text</type></entry>
       <entry></entry>
       <entry>variable-length character string</entry>
      </row>

      <row>
       <entry><type>time [ (<replaceable>p</replaceable>) ] [ without time zone ]</type></entry>
       <entry></entry>
       <entry>time of day</entry>
      </row>

      <row>
       <entry><type>time [ (<replaceable>p</replaceable>) ] with time zone</type></entry>
       <entry><type>timetz</type></entry>
       <entry>time of day, including time zone</entry>
      </row>

      <row>
       <entry><type>timestamp [ (<replaceable>p</replaceable>) ] without time zone</type></entry>
       <entry><type>timestamp</type></entry>
       <entry>date and time</entry>
      </row>

      <row>
       <entry><type>timestamp [ (<replaceable>p</replaceable>) ] [ with time zone ]</type></entry>
       <entry><type>timestamptz</type></entry>
       <entry>date and time, including time zone</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <note>
   <title>Compatibility</title>
   <para>
    The following types (or spellings thereof) are specified by
    <acronym>SQL</acronym>: <type>bit</type>, <type>bit
    varying</type>, <type>boolean</type>, <type>char</type>,
    <type>character varying</type>, <type>character</type>,
    <type>varchar</type>, <type>date</type>, <type>double
    precision</type>, <type>integer</type>, <type>interval</type>,
    <type>numeric</type>, <type>decimal</type>, <type>real</type>,
    <type>smallint</type>, <type>time</type> (with or without time zone),
    <type>timestamp</type> (with or without time zone).
   </para>
  </note>

  <para>
   Each data type has an external representation determined by its input
   and output functions.  Many of the built-in types have
   obvious external formats.  However, several types are either unique
   to <productname>PostgreSQL</productname>, such as open and closed
   paths, or have several possibilities for formats, such as the date
   and time types.
   Some of the input and output functions are not invertible.  That is,
   the result of an output function may lose accuracy when compared to
   the original input.
  </para>

  <para>
   Some of the operators and functions (e.g.,
   addition and multiplication) do not perform run-time error-checking in the
   interests of improving execution speed.
   On some systems, for example, the numeric operators for some data types may
   silently cause underflow or overflow.
  </para>

  <sect1 id="datatype-numeric">
   <title>Numeric Types</title>

   <indexterm zone="datatype-numeric">
    <primary>data type</primary>
    <secondary>numeric</secondary>
   </indexterm>

   <para>
    Numeric types consist of two-, four-, and eight-byte integers,
    four- and eight-byte floating-point numbers, and fixed-precision
    decimals.  <xref linkend="datatype-numeric-table"> lists the
    available types.
   </para>

    <table id="datatype-numeric-table">
     <title>Numeric Types</title>
     <tgroup cols="4">
      <thead>
       <row>
        <entry>Name</entry>
        <entry>Storage Size</entry>
        <entry>Description</entry>
        <entry>Range</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><type>smallint</></entry>
        <entry>2 bytes</entry>
        <entry>small-range integer</entry>
        <entry>-32768 to +32767</entry>
       </row>
       <row>
        <entry><type>integer</></entry>
        <entry>4 bytes</entry>
        <entry>usual choice for integer</entry>
        <entry>-2147483648 to +2147483647</entry>
       </row>
       <row>
        <entry><type>bigint</></entry>
        <entry>8 bytes</entry>
        <entry>large-range integer</entry>
        <entry>-9223372036854775808 to 9223372036854775807</entry>
       </row>

       <row>
        <entry><type>decimal</></entry>
        <entry>variable</entry>
        <entry>user-specified precision, exact</entry>
        <entry>no limit</entry>
       </row>
       <row>
        <entry><type>numeric</></entry>
        <entry>variable</entry>
        <entry>user-specified precision, exact</entry>
        <entry>no limit</entry>
       </row>

       <row>
        <entry><type>real</></entry>
        <entry>4 bytes</entry>
        <entry>variable-precision, inexact</entry>
        <entry>6 decimal digits precision</entry>
       </row>
       <row>
        <entry><type>double precision</></entry>
        <entry>8 bytes</entry>
        <entry>variable-precision, inexact</entry>
        <entry>15 decimal digits precision</entry>
       </row>

       <row>
        <entry><type>serial</></entry>
        <entry>4 bytes</entry>
        <entry>autoincrementing integer</entry>
        <entry>1 to 2147483647</entry>
       </row>

       <row>
        <entry><type>bigserial</type></entry>
        <entry>8 bytes</entry>
        <entry>large autoincrementing integer</entry>
        <entry>1 to 9223372036854775807</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    The syntax of constants for the numeric types is described in
    <xref linkend="sql-syntax-constants">.  The numeric types have a
    full set of corresponding arithmetic operators and
    functions. Refer to <xref linkend="functions"> for more
    information.  The following sections describe the types in detail.
   </para>

   <sect2 id="datatype-int">
    <title>Integer Types</title>

    <indexterm zone="datatype-int">
     <primary>integer</primary>
    </indexterm>

    <indexterm zone="datatype-int">
     <primary>smallint</primary>
    </indexterm>

    <indexterm zone="datatype-int">
     <primary>bigint</primary>
    </indexterm>

    <indexterm>
     <primary>int4</primary>
     <see>integer</see>
    </indexterm>

    <indexterm>
     <primary>int2</primary>
     <see>smallint</see>
    </indexterm>

    <indexterm>
     <primary>int8</primary>
     <see>bigint</see>
    </indexterm>

    <para>
     The types <type>smallint</type>, <type>integer</type>, and
     <type>bigint</type> store whole numbers, that is, numbers without
     fractional components, of various ranges.  Attempts to store
     values outside of the allowed range will result in an error.
    </para>

    <para>
     The type <type>integer</type> is the usual choice, as it offers
     the best balance between range, storage size, and performance.
     The <type>smallint</type> type is generally only used if disk
     space is at a premium.  The <type>bigint</type> type should only
     be used if the <type>integer</type> range is not sufficient,
     because the latter is definitely faster.
    </para>

    <para>
     The <type>bigint</type> type may not function correctly on all
     platforms, since it relies on compiler support for eight-byte
     integers.  On a machine without such support, <type>bigint</type>
     acts the same as <type>integer</type> (but still takes up eight
     bytes of storage).  However, we are not aware of any reasonable
     platform where this is actually the case.
    </para>

    <para>
     <acronym>SQL</acronym> only specifies the integer types
     <type>integer</type> (or <type>int</type>) and
     <type>smallint</type>.  The type <type>bigint</type>, and the
     type names <type>int2</type>, <type>int4</type>, and
     <type>int8</type> are extensions, which are shared with various
     other <acronym>SQL</acronym> database systems.
    </para>

   </sect2>

   <sect2 id="datatype-numeric-decimal">
    <title>Arbitrary Precision Numbers</title>

    <indexterm zone="datatype-numeric-decimal">
     <primary>numeric (data type)</primary>
    </indexterm>

    <indexterm>
     <primary>decimal</primary>
     <see>numeric</see>
    </indexterm>

    <para>
     The type <type>numeric</type> can store numbers with up to 1000
     digits of precision and perform calculations exactly. It is
     especially recommended for storing monetary amounts and other
     quantities where exactness is required. However, the
     <type>numeric</type> type is very slow compared to the
     floating-point types described in the next section.
    </para>

    <para>
     In what follows we use these terms:  The
     <firstterm>scale</firstterm> of a <type>numeric</type> is the
     count of decimal digits in the fractional part, to the right of
     the decimal point.  The <firstterm>precision</firstterm> of a
     <type>numeric</type> is the total count of significant digits in
     the whole number, that is, the number of digits to both sides of
     the decimal point.  So the number 23.5141 has a precision of 6
     and a scale of 4.  Integers can be considered to have a scale of
     zero.
    </para>

    <para>
     Both the precision and the scale of the numeric type can be
     configured.  To declare a column of type <type>numeric</type> use
     the syntax
<programlisting>
NUMERIC(<replaceable>precision</replaceable>, <replaceable>scale</replaceable>)
</programlisting>
     The precision must be positive, the scale zero or positive.
     Alternatively,
<programlisting>
NUMERIC(<replaceable>precision</replaceable>)
</programlisting>
     selects a scale of 0.  Specifying
<programlisting>
NUMERIC
</programlisting>
     without any precision or scale creates a column in which numeric
     values of any precision and scale can be stored, up to the
     implementation limit on precision.  A column of this kind will
     not coerce input values to any particular scale, whereas
     <type>numeric</type> columns with a declared scale will coerce
     input values to that scale.  (The <acronym>SQL</acronym> standard
     requires a default scale of 0, i.e., coercion to integer
     precision.  We find this a bit useless.  If you're concerned
     about portability, always specify the precision and scale
     explicitly.)
    </para>

    <para>
     If the precision or scale of a value is greater than the declared
     precision or scale of a column, the system will attempt to round
     the value.  If the value cannot be rounded so as to satisfy the
     declared limits, an error is raised.
    </para>

    <para>
     The types <type>decimal</type> and <type>numeric</type> are
     equivalent.  Both types are part of the <acronym>SQL</acronym>
     standard.
    </para>
   </sect2>


   <sect2 id="datatype-float">
    <title>Floating-Point Types</title>

    <indexterm zone="datatype-float">
     <primary>real</primary>
    </indexterm>

    <indexterm zone="datatype-float">
     <primary>double precision</primary>
    </indexterm>

    <indexterm>
     <primary>float4</primary>
     <see>real</see>
    </indexterm>

    <indexterm>
     <primary>float8</primary>
     <see>double precision</see>
    </indexterm>

    <indexterm zone="datatype-float">
     <primary>floating point</primary>
    </indexterm>

    <para>
     The data types <type>real</type> and <type>double
     precision</type> are inexact, variable-precision numeric types.
     In practice, these types are usually implementations of
     <acronym>IEEE</acronym> Standard 754 for Binary Floating-Point
     Arithmetic (single and double precision, respectively), to the
     extent that the underlying processor, operating system, and
     compiler support it.
    </para>

    <para>
     Inexact means that some values cannot be converted exactly to the
     internal format and are stored as approximations, so that storing
     and printing back out a value may show slight discrepancies.
     Managing these errors and how they propagate through calculations
     is the subject of an entire branch of mathematics and computer
     science and will not be discussed further here, except for the
     following points:
     <itemizedlist>
      <listitem>
       <para>
        If you require exact storage and calculations (such as for
        monetary amounts), use the <type>numeric</type> type instead.
       </para>
      </listitem>

      <listitem>
       <para>
        If you want to do complicated calculations with these types
        for anything important, especially if you rely on certain
        behavior in boundary cases (infinity, underflow), you should
        evaluate the implementation carefully.
       </para>
      </listitem>

      <listitem>
       <para>
        Comparing two floating-point values for equality may or may
        not work as expected.
       </para>
      </listitem>
     </itemizedlist>
    </para>

    <para>
     On most platforms, the <type>real</type> type has a range of at least
     1E-37 to 1E+37 with a precision of at least 6 decimal digits.  The
     <type>double precision</type> type typically has a range of around
     1E-307 to 1E+308 with a precision of at least 15 digits.  Values that
     are too large or too small will cause an error.  Rounding may
     take place if the precision of an input number is too high.
     Numbers too close to zero that are not representable as distinct
     from zero will cause an underflow error.
    </para>

    <para>
     <productname>PostgreSQL</productname> also supports the SQL-standard
     notations <type>float</type> and
     <type>float(<replaceable>p</replaceable>)</type> for specifying
     inexact numeric types.  Here, <replaceable>p</replaceable> specifies
     the minimum acceptable precision in binary digits.
     <productname>PostgreSQL</productname> accepts 
     <type>float(1)</type> to <type>float(24)</type> as selecting the
     <type>real</type> type, while 
     <type>float(25)</type> to <type>float(53)</type> select
     <type>double precision</type>.  Values of <replaceable>p</replaceable>
     outside the allowed range draw an error.
     <type>float</type> with no precision specified is taken to mean
     <type>double precision</type>.
    </para>

    <note>
     <para>
      Prior to <productname>PostgreSQL</productname> 7.4, the precision in
      <type>float(<replaceable>p</replaceable>)</type> was taken to mean
      so many decimal digits.  This has been corrected to match the SQL
      standard, which specifies that the precision is measured in binary
      digits.  The assumption that <type>real</type> and
      <type>double precision</type> have exactly 24 and 53 bits in the
      mantissa respectively is correct for IEEE-standard floating point
      implementations.  On non-IEEE platforms it may be off a little, but
      for simplicity the same ranges of <replaceable>p</replaceable> are used
      on all platforms.
     </para>
    </note>

   </sect2>

   <sect2 id="datatype-serial">
    <title>Serial Types</title>

    <indexterm zone="datatype-serial">
     <primary>serial</primary>
    </indexterm>

    <indexterm zone="datatype-serial">
     <primary>bigserial</primary>
    </indexterm>

    <indexterm zone="datatype-serial">
     <primary>serial4</primary>
    </indexterm>

    <indexterm zone="datatype-serial">
     <primary>serial8</primary>
    </indexterm>

    <indexterm>
     <primary>auto-increment</primary>
     <see>serial</see>
    </indexterm>

    <indexterm>
     <primary>sequence</primary>
     <secondary>and serial type</secondary>
    </indexterm>

    <para>
     The data types <type>serial</type> and <type>bigserial</type>
     are not true types, but merely
     a notational convenience for setting up unique identifier columns
     (similar to the <literal>AUTO_INCREMENT</literal> property
     supported by some other databases). In the current
     implementation, specifying

<programlisting>
CREATE TABLE <replaceable class="parameter">tablename</replaceable> (
    <replaceable class="parameter">colname</replaceable> SERIAL
);
</programlisting>

     is equivalent to specifying:

<programlisting>
CREATE SEQUENCE <replaceable class="parameter">tablename</replaceable>_<replaceable class="parameter">colname</replaceable>_seq;
CREATE TABLE <replaceable class="parameter">tablename</replaceable> (
    <replaceable class="parameter">colname</replaceable> integer DEFAULT nextval('<replaceable class="parameter">tablename</replaceable>_<replaceable class="parameter">colname</replaceable>_seq') NOT NULL
);
</programlisting>

     Thus, we have created an integer column and arranged for its default
     values to be assigned from a sequence generator.  A <literal>NOT NULL</>
     constraint is applied to ensure that a null value cannot be explicitly
     inserted, either.  In most cases you would also want to attach a
     <literal>UNIQUE</> or <literal>PRIMARY KEY</> constraint to prevent
     duplicate values from being inserted by accident, but this is
     not automatic.
    </para>

    <note>
     <para>
      Prior to <productname>PostgreSQL</productname> 7.3, <type>serial</type>
      implied <literal>UNIQUE</literal>.  This is no longer automatic.  If
      you wish a serial column to be in a unique constraint or a 
      primary key, it must now be specified, same as with
      any other data type.
     </para>
    </note>

    <para>
     To insert the next value of the sequence into the <type>serial</type>
     column, specify that the <type>serial</type>
     column should be assigned its default value. This can be done
     either by excluding the column from the list of columns in
     the <command>INSERT</command> statement, or through the use of
     the <literal>DEFAULT</literal> key word.
    </para>

    <para>
     The type names <type>serial</type> and <type>serial4</type> are
     equivalent: both create <type>integer</type> columns.  The type
     names <type>bigserial</type> and <type>serial8</type> work just
     the same way, except that they create a <type>bigint</type>
     column.  <type>bigserial</type> should be used if you anticipate
     the use of more than 2<superscript>31</> identifiers over the
     lifetime of the table.
    </para>

    <para>
     The sequence created for a <type>serial</type> column is
     automatically dropped when the owning column is dropped, and
     cannot be dropped otherwise.  (This was not true in
     <productname>PostgreSQL</productname> releases before 7.3.  Note
     that this automatic drop linkage will not occur for a sequence
     created by reloading a dump from a pre-7.3 database; the dump
     file does not contain the information needed to establish the
     dependency link.) Furthermore, this dependency between sequence
     and column is made only for the <type>serial</> column itself; if
     any other columns reference the sequence (perhaps by manually
     calling the <function>nextval</> function), they will be broken
     if the sequence is removed. Using a <type>serial</> column's sequence
     in such a fashion is considered bad form; if you wish to feed several
     columns from the same sequence generator, create the sequence as an
     independent object.
    </para>
   </sect2>
  </sect1>

  <sect1 id="datatype-money">
   <title>Monetary Types</title>

   <note>
    <para>
     The <type>money</type> type is deprecated. Use
     <type>numeric</type> or <type>decimal</type> instead, in
     combination with the <function>to_char</function> function.
    </para>
   </note>

   <para>
    The <type>money</type> type stores a currency amount with a fixed
    fractional precision; see <xref
    linkend="datatype-money-table">.
    Input is accepted in a variety of formats, including integer and
    floating-point literals, as well as <quote>typical</quote>
    currency formatting, such as <literal>'$1,000.00'</literal>.
    Output is generally in the latter form but depends on the locale.
   </para>

    <table id="datatype-money-table">
     <title>Monetary Types</title>
     <tgroup cols="4">
      <thead>
       <row>
        <entry>Name</entry>
        <entry>Storage Size</entry>
        <entry>Description</entry>
        <entry>Range</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry>money</entry>
        <entry>4 bytes</entry>
        <entry>currency amount</entry>
        <entry>-21474836.48 to +21474836.47</entry>
       </row>
      </tbody>
     </tgroup>
    </table>
  </sect1>


  <sect1 id="datatype-character">
   <title>Character Types</title>

   <indexterm zone="datatype-character">
    <primary>character string</primary>
    <secondary>data types</secondary>
   </indexterm>

   <indexterm>
    <primary>string</primary>
    <see>character string</see>
   </indexterm>

   <indexterm zone="datatype-character">
    <primary>character</primary>
   </indexterm>

   <indexterm zone="datatype-character">
    <primary>character varying</primary>
   </indexterm>

   <indexterm zone="datatype-character">
    <primary>text</primary>
   </indexterm>

   <indexterm zone="datatype-character">
    <primary>char</primary>
   </indexterm>

   <indexterm zone="datatype-character">
    <primary>varchar</primary>
   </indexterm>

    <table id="datatype-character-table">
     <title>Character Types</title>
     <tgroup cols="2">
      <thead>
       <row>
        <entry>Name</entry>
        <entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><type>character varying(<replaceable>n</>)</type>, <type>varchar(<replaceable>n</>)</type></entry>
        <entry>variable-length with limit</entry>
       </row>
       <row>
        <entry><type>character(<replaceable>n</>)</type>, <type>char(<replaceable>n</>)</type></entry>
        <entry>fixed-length, blank padded</entry>
       </row>
       <row>
        <entry><type>text</type></entry>
        <entry>variable unlimited length</entry>
       </row>
     </tbody>
     </tgroup>
    </table>

   <para>
    <xref linkend="datatype-character-table"> shows the
    general-purpose character types available in
    <productname>PostgreSQL</productname>.
   </para>

   <para>
    <acronym>SQL</acronym> defines two primary character types:
    <type>character varying(<replaceable>n</>)</type> and
    <type>character(<replaceable>n</>)</type>, where <replaceable>n</>
    is a positive integer.  Both of these types can store strings up to
    <replaceable>n</> characters in length.  An attempt to store a
    longer string into a column of these types will result in an
    error, unless the excess characters are all spaces, in which case
    the string will be truncated to the maximum length. (This somewhat
    bizarre exception is required by the <acronym>SQL</acronym>
    standard.) If the string to be stored is shorter than the declared
    length, values of type <type>character</type> will be space-padded;
    values of type <type>character varying</type> will simply store the
    shorter
    string.
   </para>

    <para>
     If one explicitly casts a value to <type>character
     varying(<replaceable>n</>)</type> or
     <type>character(<replaceable>n</>)</type>, then an over-length
     value will be truncated to <replaceable>n</> characters without
     raising an error. (This too is required by the
     <acronym>SQL</acronym> standard.)
    </para>

   <note>
    <para>
     Prior to <productname>PostgreSQL</> 7.2, strings that were too long were
     always truncated without raising an error, in either explicit or
     implicit casting contexts.
    </para>
   </note>

   <para>
    The notations <type>varchar(<replaceable>n</>)</type> and
    <type>char(<replaceable>n</>)</type> are aliases for <type>character
    varying(<replaceable>n</>)</type> and
    <type>character(<replaceable>n</>)</type>, respectively.
    <type>character</type> without length specifier is equivalent to
    <type>character(1)</type>; if <type>character varying</type> is used
    without length specifier, the type accepts strings of any size. The
    latter is a <productname>PostgreSQL</> extension.
   </para>

   <para>
    In addition, <productname>PostgreSQL</productname> provides the
    <type>text</type> type, which stores strings of any length.
    Although the type <type>text</type> is not in the
    <acronym>SQL</acronym> standard, several other SQL database
    management systems have it as well.
   </para>

   <para>
    Values of type <type>character</type> are physically padded
    with spaces to the specified width <replaceable>n</>, and are
    stored and displayed that way.  However, the padding spaces are
    treated as semantically insignificant.  Trailing spaces are
    disregarded when comparing two values of type <type>character</type>,
    and they will be removed when converting a <type>character</type> value
    to one of the other string types.  Note that trailing spaces
    <emphasis>are</> semantically significant in
    <type>character varying</type> and <type>text</type> values.
   </para>

   <para>
    The storage requirement for data of these types is 4 bytes plus the
    actual string, and in case of <type>character</type> plus the
    padding. Long strings are compressed by the system automatically, so
    the physical requirement on disk may be less. Long values are also
    stored in background tables so they do not interfere with rapid
    access to the shorter column values. In any case, the longest
    possible character string that can be stored is about 1 GB. (The
    maximum value that will be allowed for <replaceable>n</> in the data
    type declaration is less than that. It wouldn't be very useful to
    change this because with multibyte character encodings the number of
    characters and bytes can be quite different anyway. If you desire to
    store long strings with no specific upper limit, use
    <type>text</type> or <type>character varying</type> without a length
    specifier, rather than making up an arbitrary length limit.)
   </para>

   <tip>
    <para>
     There are no performance differences between these three types,
     apart from the increased storage size when using the blank-padded
     type.  While <type>character(<replaceable>n</>)</type> has performance
     advantages in some other database systems, it has no such advantages in
     <productname>PostgreSQL</productname>.  In most situations
     <type>text</type> or <type>character varying</type> should be used
     instead.
    </para>
   </tip>

   <para>
    Refer to <xref linkend="sql-syntax-strings"> for information about
    the syntax of string literals, and to <xref linkend="functions">
    for information about available operators and functions.
   </para>

   <example>
    <title>Using the character types</title>

<programlisting>
CREATE TABLE test1 (a character(4));
INSERT INTO test1 VALUES ('ok');
SELECT a, char_length(a) FROM test1; -- <co id="co.datatype-char">
<computeroutput>
  a   | char_length
------+-------------
 ok   |           2
</computeroutput>

CREATE TABLE test2 (b varchar(5));
INSERT INTO test2 VALUES ('ok');
INSERT INTO test2 VALUES ('good      ');
INSERT INTO test2 VALUES ('too long');
<computeroutput>ERROR:  value too long for type character varying(5)</computeroutput>
INSERT INTO test2 VALUES ('too long'::varchar(5)); -- explicit truncation
SELECT b, char_length(b) FROM test2;
<computeroutput>
   b   | char_length
-------+-------------
 ok    |           2
 good  |           5
 too l |           5
</computeroutput>
</programlisting>
    <calloutlist>
     <callout arearefs="co.datatype-char">
      <para>
       The <function>char_length</function> function is discussed in
       <xref linkend="functions-string">.
      </para>
     </callout>
    </calloutlist>
   </example>

   <para>
    There are two other fixed-length character types in
    <productname>PostgreSQL</productname>, shown in <xref
    linkend="datatype-character-special-table">. The <type>name</type>
    type exists <emphasis>only</emphasis> for storage of identifiers
    in the internal system catalogs and is not intended for use by the general user. Its
    length is currently defined as 64 bytes (63 usable characters plus
    terminator) but should be referenced using the constant
    <symbol>NAMEDATALEN</symbol>. The length is set at compile time (and
    is therefore adjustable for special uses); the default maximum
    length may change in a future release. The type <type>"char"</type>
    (note the quotes) is different from <type>char(1)</type> in that it
    only uses one byte of storage. It is internally used in the system
    catalogs as a poor-man's enumeration type.
   </para>

    <table id="datatype-character-special-table">
     <title>Special Character Types</title>
     <tgroup cols="3">
      <thead>
       <row>
        <entry>Name</entry>
        <entry>Storage Size</entry>
        <entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><type>"char"</type></entry>
        <entry>1 byte</entry>
        <entry>single-character internal type</entry>
       </row>
       <row>
        <entry><type>name</type></entry>
        <entry>64 bytes</entry>
        <entry>internal type for object names</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

  </sect1>

 <sect1 id="datatype-binary">
  <title>Binary Data Types</title>

  <indexterm zone="datatype-binary">
   <primary>binary data</primary>
  </indexterm>

  <indexterm zone="datatype-binary">
   <primary>bytea</primary>
  </indexterm>

   <para>
    The <type>bytea</type> data type allows storage of binary strings;
    see <xref linkend="datatype-binary-table">.
   </para>

   <table id="datatype-binary-table">
    <title>Binary Data Types</title>
    <tgroup cols="3">
     <thead>
      <row>
       <entry>Name</entry>
       <entry>Storage Size</entry>
       <entry>Description</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry><type>bytea</type></entry>
       <entry>4 bytes plus the actual binary string</entry>
       <entry>variable-length binary string</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    A binary string is a sequence of octets (or bytes).  Binary
    strings are distinguished from characters strings by two
    characteristics: First, binary strings specifically allow storing
    octets of value zero and other <quote>non-printable</quote>
    octets (defined as octets outside the range 32 to 126).
    Second, operations on binary strings process the actual bytes,
    whereas the encoding and processing of character strings depends
    on locale settings.
   </para>

   <para>
    When entering <type>bytea</type> values, octets of certain values
    <emphasis>must</emphasis> be escaped (but all octet values
    <emphasis>may</emphasis> be escaped) when used as part of a string
    literal in an <acronym>SQL</acronym> statement. In general, to
    escape an octet, it is converted into the three-digit octal number
    equivalent of its decimal octet value, and preceded by two
    backslashes. <xref linkend="datatype-binary-sqlesc"> contains the
    characters which must be escaped, and gives the alternate escape
    sequences where applicable.
   </para>

   <table id="datatype-binary-sqlesc">
    <title><type>bytea</> Literal Escaped Octets</title>
    <tgroup cols="5">
     <thead>
      <row>
       <entry>Decimal Octet Value</entry>
       <entry>Description</entry>
       <entry>Escaped Input Representation</entry>
       <entry>Example</entry>
       <entry>Output Representation</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>0</entry>
       <entry>zero octet</entry>
       <entry><literal>'\\000'</literal></entry>
       <entry><literal>SELECT '\\000'::bytea;</literal></entry>
       <entry><literal>\000</literal></entry>
      </row>

      <row>
       <entry>39</entry>
       <entry>single quote</entry>
       <entry><literal>'\''</literal> or <literal>'\\047'</literal></entry>
       <entry><literal>SELECT '\''::bytea;</literal></entry>
       <entry><literal>'</literal></entry>
      </row>

      <row>
       <entry>92</entry>
       <entry>backslash</entry>
       <entry><literal>'\\\\'</literal> or <literal>'\\134'</literal></entry>
       <entry><literal>SELECT '\\\\'::bytea;</literal></entry>
       <entry><literal>\\</literal></entry>
      </row>

      <row>
       <entry>0 to 31 and 127 to 255</entry>
       <entry><quote>non-printable</quote> octets</entry>
       <entry><literal>'\\<replaceable>xxx'</></literal> (octal value)</entry>
       <entry><literal>SELECT '\\001'::bytea;</literal></entry>
       <entry><literal>\001</literal></entry>
      </row>

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

   <para>
    The requirement to escape <quote>non-printable</quote> octets actually
    varies depending on locale settings. In some instances you can get away
    with leaving them unescaped. Note that the result in each of the examples
    in <xref linkend="datatype-binary-sqlesc"> was exactly one octet in
    length, even though the output representation of the zero octet and
    backslash are more than one character.
   </para>

   <para>
    The reason that you have to write so many backslashes, as shown in
    <xref linkend="datatype-binary-sqlesc">, is that an input string
    written as a string literal must pass through two parse phases in
    the <productname>PostgreSQL</productname> server.  The first
    backslash of each pair is interpreted as an escape character by
    the string-literal parser and is therefore consumed, leaving the
    second backslash of the pair.  The remaining backslash is then
    recognized by the <type>bytea</type> input function as starting
    either a three digit octal value or escaping another backslash.
    For example, a string literal passed to the server as
    <literal>'\\001'</literal> becomes <literal>\001</literal> after
    passing through the string-literal parser. The
    <literal>\001</literal> is then sent to the <type>bytea</type>
    input function, where it is converted to a single octet with a
    decimal value of 1.  Note that the apostrophe character is not
    treated specially by <type>bytea</type>, so it follows the normal
    rules for string literals.  (See also <xref
    linkend="sql-syntax-strings">.)
   </para>

   <para>
    <type>Bytea</type> octets are also escaped in the output. In general, each
    <quote>non-printable</quote> octet is converted into
    its equivalent three-digit octal value and preceded by one backslash.
    Most <quote>printable</quote> octets are represented by their standard
    representation in the client character set. The octet with decimal
    value 92 (backslash) has a special alternative output representation.
    Details are in <xref linkend="datatype-binary-resesc">.
   </para>

   <table id="datatype-binary-resesc">
    <title><type>bytea</> Output Escaped Octets</title>
    <tgroup cols="5">
     <thead>
      <row>
       <entry>Decimal Octet Value</entry>
       <entry>Description</entry>
       <entry>Escaped Output Representation</entry>
       <entry>Example</entry>
       <entry>Output Result</entry>
      </row>
     </thead>

     <tbody>

      <row>
       <entry>92</entry>
       <entry>backslash</entry>
       <entry><literal>\\</literal></entry>
       <entry><literal>SELECT '\\134'::bytea;</literal></entry>
       <entry><literal>\\</literal></entry>
      </row>

      <row>
       <entry>0 to 31 and 127 to 255</entry>
       <entry><quote>non-printable</quote> octets</entry>
       <entry><literal>\<replaceable>xxx</></literal> (octal value)</entry>
       <entry><literal>SELECT '\\001'::bytea;</literal></entry>
       <entry><literal>\001</literal></entry>
      </row>

      <row>
       <entry>32 to 126</entry>
       <entry><quote>printable</quote> octets</entry>
       <entry>client character set representation</entry>
       <entry><literal>SELECT '\\176'::bytea;</literal></entry>
       <entry><literal>~</literal></entry>
      </row>

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

   <para>
    Depending on the front end to <productname>PostgreSQL</> you use,
    you may have additional work to do in terms of escaping and
    unescaping <type>bytea</type> strings. For example, you may also
    have to escape line feeds and carriage returns if your interface
    automatically translates these.
   </para>

   <para>
        The <acronym>SQL</acronym> standard defines a different binary
        string type, called <type>BLOB</type> or <type>BINARY LARGE
         OBJECT</type>.  The input format is different compared to
        <type>bytea</type>, but the provided functions and operators are
        mostly the same.
   </para>
 </sect1>


  <sect1 id="datatype-datetime">
   <title>Date/Time Types</title>

   <indexterm zone="datatype-datetime">
    <primary>date</primary>
   </indexterm>
   <indexterm zone="datatype-datetime">
    <primary>time</primary>
   </indexterm>
   <indexterm zone="datatype-datetime">
    <primary>time without time zone</primary>
   </indexterm>
   <indexterm zone="datatype-datetime">
    <primary>time with time zone</primary>
   </indexterm>
   <indexterm zone="datatype-datetime">
    <primary>timestamp</primary>
   </indexterm>
   <indexterm zone="datatype-datetime">
    <primary>timestamp with time zone</primary>
   </indexterm>
   <indexterm zone="datatype-datetime">
    <primary>timestamp without time zone</primary>
   </indexterm>
   <indexterm zone="datatype-datetime">
    <primary>interval</primary>
   </indexterm>
   <indexterm zone="datatype-datetime">
    <primary>time span</primary>
   </indexterm>

   <para>
    <productname>PostgreSQL</productname> supports the full set of
    <acronym>SQL</acronym> date and time types, shown in <xref
    linkend="datatype-datetime-table">.
   </para>

    <table id="datatype-datetime-table">
     <title>Date/Time Types</title>
     <tgroup cols="6">
      <thead>
       <row>
        <entry>Name</entry>
        <entry>Storage Size</entry>
        <entry>Description</entry>
        <entry>Low Value</entry>
        <entry>High Value</entry>
        <entry>Resolution</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><type>timestamp [ (<replaceable>p</replaceable>) ] [ without time zone ]</type></entry>
        <entry>8 bytes</entry>
        <entry>both date and time</entry>
        <entry>4713 BC</entry>
        <entry>5874897 AD</entry>
        <entry>1 microsecond / 14 digits</entry>
       </row>
       <row>
        <entry><type>timestamp [ (<replaceable>p</replaceable>) ] with time zone</type></entry>
        <entry>8 bytes</entry>
        <entry>both date and time, with time zone</entry>
        <entry>4713 BC</entry>
        <entry>5874897 AD</entry>
        <entry>1 microsecond / 14 digits</entry>
       </row>
       <row>
        <entry><type>interval [ (<replaceable>p</replaceable>) ]</type></entry>
        <entry>12 bytes</entry>
        <entry>time intervals</entry>
        <entry>-178000000 years</entry>
        <entry>178000000 years</entry>
        <entry>1 microsecond</entry>
       </row>
       <row>
        <entry><type>date</type></entry>
        <entry>4 bytes</entry>
        <entry>dates only</entry>
        <entry>4713 BC</entry>
        <entry>32767 AD</entry>
        <entry>1 day</entry>
       </row>
       <row>
        <entry><type>time [ (<replaceable>p</replaceable>) ] [ without time zone ]</type></entry>
        <entry>8 bytes</entry>
        <entry>times of day only</entry>
        <entry>00:00:00.00</entry>
        <entry>23:59:59.99</entry>
        <entry>1 microsecond</entry>
       </row>
       <row>
        <entry><type>time [ (<replaceable>p</replaceable>) ] with time zone</type></entry>
        <entry>12 bytes</entry>
        <entry>times of day only, with time zone</entry>
        <entry>00:00:00.00+12</entry>
        <entry>23:59:59.99-12</entry>
        <entry>1 microsecond</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <note>
    <para>
     Prior to <productname>PostgreSQL</productname> 7.3, writing just
     <type>timestamp</type> was equivalent to <type>timestamp with
     time zone</type>.  This was changed for SQL compliance.
    </para>
   </note>

   <para>
    <type>time</type>, <type>timestamp</type>, and
    <type>interval</type> accept an optional precision value
    <replaceable>p</replaceable> which specifies the number of
    fractional digits retained in the seconds field. By default, there
    is no explicit bound on precision.  The allowed range of
    <replaceable>p</replaceable> is from 0 to 6 for the
    <type>timestamp</type> and <type>interval</type> types.
   </para>

   <note>
   <para>
    When <type>timestamp</> values are stored as double precision floating-point
    numbers (currently the default), the effective limit of precision
    may be less than 6. <type>timestamp</type> values are stored as seconds
    before or after midnight 2000-01-01.  Microsecond precision is achieved for
    dates within a few years of 2000-01-01, but the precision degrades for
    dates further away.  When <type>timestamp</type> values are stored as
    eight-byte integers (a compile-time
    option), microsecond precision is available over the full range of
    values. However eight-byte integer timestamps have a more limited range of
    dates than shown above: from 4713 BC up to 294276 AD.
   </para>
   </note>

   <para>
    For the <type>time</type> types, the allowed range of
    <replaceable>p</replaceable> is from 0 to 6 when eight-byte integer
    storage is used, or from 0 to 10 when floating-point storage is used.
   </para>

   <para>
    The type <type>time with time zone</type> is defined by the SQL
    standard, but the definition exhibits properties which lead to
    questionable usefulness. In most cases, a combination of
    <type>date</type>, <type>time</type>, <type>timestamp without time
    zone</type>, and <type>timestamp with time zone</type> should
    provide a complete range of date/time functionality required by
    any application.
   </para>

   <para>
    The types <type>abstime</type>
    and <type>reltime</type> are lower precision types which are used internally.
    You are discouraged from using these types in new
    applications and are encouraged to move any old
    ones over when appropriate. Any or all of these internal types
    might disappear in a future release.
   </para>

   <sect2 id="datatype-datetime-input">
    <title>Date/Time Input</title>

    <para>
     Date and time input is accepted in almost any reasonable format, including
     ISO 8601, <acronym>SQL</acronym>-compatible, 
     traditional <productname>POSTGRES</productname>, and others.
     For some formats, ordering of month, day, and year in date input is
     ambiguous and there is support for specifying the expected
     ordering of these fields.  Set the <xref linkend="guc-datestyle"> parameter
     to <literal>MDY</> to select month-day-year interpretation,
     <literal>DMY</> to select day-month-year interpretation, or
     <literal>YMD</> to select year-month-day interpretation.
    </para>

    <para>
     <productname>PostgreSQL</productname> is more flexible in
     handling date/time input than the
     <acronym>SQL</acronym> standard requires.
     See <xref linkend="datetime-appendix">
     for the exact parsing rules of date/time input and for the
     recognized text fields including months, days of the week, and
     time zones. 
    </para>

    <para>
     Remember that any date or time literal input needs to be enclosed
     in single quotes, like text strings.  Refer to 
     <xref linkend="sql-syntax-constants-generic"> for more
     information.
     <acronym>SQL</acronym> requires the following syntax
<synopsis>
<replaceable>type</replaceable> [ (<replaceable>p</replaceable>) ] '<replaceable>value</replaceable>'
</synopsis>
     where <replaceable>p</replaceable> in the optional precision
     specification is an integer corresponding to the number of
     fractional digits in the seconds field. Precision can be
     specified for <type>time</type>, <type>timestamp</type>, and
     <type>interval</type> types.  The allowed values are mentioned
     above.  If no precision is specified in a constant specification,
     it defaults to the precision of the literal value.
    </para>

    <sect3>
    <title>Dates</title>

    <indexterm>
     <primary>date</primary>
    </indexterm>
 
    <para>
     <xref linkend="datatype-datetime-date-table"> shows some possible
     inputs for the <type>date</type> type.
    </para>

     <table id="datatype-datetime-date-table">
      <title>Date Input</title>
      <tgroup cols="2">
       <thead>
        <row>
         <entry>Example</entry>
         <entry>Description</entry>
        </row>
       </thead>
       <tbody>
        <row>
         <entry>January 8, 1999</entry>
         <entry>unambiguous in any <varname>datestyle</varname> input mode</entry>
        </row>
        <row>
         <entry>1999-01-08</entry>
         <entry>ISO 8601; January 8 in any mode
         (recommended format)</entry>
        </row>
        <row>
         <entry>1/8/1999</entry>
         <entry>January 8 in <literal>MDY</> mode;
          August 1 in <literal>DMY</> mode</entry>
        </row>
        <row>
         <entry>1/18/1999</entry>
         <entry>January 18 in <literal>MDY</> mode;
          rejected in other modes</entry>
        </row>
        <row>
         <entry>01/02/03</entry>
         <entry>January 2, 2003 in <literal>MDY</> mode;
          February 1, 2003 in <literal>DMY</> mode;
          February 3, 2001 in <literal>YMD</> mode
         </entry>
        </row>
        <row>
         <entry>1999-Jan-08</entry>
         <entry>January 8 in any mode</entry>
        </row>
        <row>
         <entry>Jan-08-1999</entry>
         <entry>January 8 in any mode</entry>
        </row>
        <row>
         <entry>08-Jan-1999</entry>
         <entry>January 8 in any mode</entry>
        </row>
        <row>
         <entry>99-Jan-08</entry>
         <entry>January 8 in <literal>YMD</> mode, else error</entry>
        </row>
        <row>
         <entry>08-Jan-99</entry>
         <entry>January 8, except error in <literal>YMD</> mode</entry>
        </row>
        <row>
         <entry>Jan-08-99</entry>
         <entry>January 8, except error in <literal>YMD</> mode</entry>
        </row>
        <row>
         <entry>19990108</entry>
         <entry>ISO 8601; January 8, 1999 in any mode</entry>
        </row>
        <row>
         <entry>990108</entry>
         <entry>ISO 8601; January 8, 1999 in any mode</entry>
        </row>
        <row>
         <entry>1999.008</entry>
         <entry>year and day of year</entry>
        </row>
        <row>
         <entry>J2451187</entry>
         <entry>Julian day</entry>
        </row>
        <row>
         <entry>January 8, 99 BC</entry>
         <entry>year 99 before the Common Era</entry>
        </row>
       </tbody>
      </tgroup>
     </table>
    </sect3>

    <sect3>
     <title>Times</title>

     <indexterm>
      <primary>time</primary>
     </indexterm>
     <indexterm>
      <primary>time without time zone</primary>
     </indexterm>
     <indexterm>
      <primary>time with time zone</primary>
     </indexterm>

     <para>
      The time-of-day types are <type>time [
      (<replaceable>p</replaceable>) ] without time zone</type> and
      <type>time [ (<replaceable>p</replaceable>) ] with time
      zone</type>.  Writing just <type>time</type> is equivalent to
      <type>time without time zone</type>.
     </para>

     <para>
      Valid input for these types consists of a time of day followed
      by an optional time zone. (See <xref
      linkend="datatype-datetime-time-table"> 
      and <xref linkend="datatype-timezone-table">.)  If a time zone is
      specified in the input for <type>time without time zone</type>,
      it is silently ignored.
     </para>

      <table id="datatype-datetime-time-table">
       <title>Time Input</title>
       <tgroup cols="2">
        <thead>
         <row>
          <entry>Example</entry>
          <entry>Description</entry>
         </row>
        </thead>
        <tbody>
         <row>
          <entry><literal>04:05:06.789</literal></entry>
          <entry>ISO 8601</entry>
         </row>
         <row>
          <entry><literal>04:05:06</literal></entry>
          <entry>ISO 8601</entry>
         </row>
         <row>
          <entry><literal>04:05</literal></entry>
          <entry>ISO 8601</entry>
         </row>
         <row>
          <entry><literal>040506</literal></entry>
          <entry>ISO 8601</entry>
         </row>
         <row>
          <entry><literal>04:05 AM</literal></entry>
          <entry>same as 04:05; AM does not affect value</entry>
         </row>
         <row>
          <entry><literal>04:05 PM</literal></entry>
          <entry>same as 16:05; input hour must be <= 12</entry>
         </row>
         <row>
          <entry><literal>04:05:06.789-8</literal></entry>
          <entry>ISO 8601</entry>
         </row>
         <row>
          <entry><literal>04:05:06-08:00</literal></entry>
          <entry>ISO 8601</entry>
         </row>
         <row>
          <entry><literal>04:05-08:00</literal></entry>
          <entry>ISO 8601</entry>
         </row>
         <row>
          <entry><literal>040506-08</literal></entry>
          <entry>ISO 8601</entry>
         </row>
         <row>
          <entry><literal>04:05:06 PST</literal></entry>
          <entry>time zone specified by name</entry>
         </row>
        </tbody>
       </tgroup>
      </table>

      <table tocentry="1" id="datatype-timezone-table">
       <title>Time Zone Input</title>
       <tgroup cols="2">
        <thead>
         <row>
          <entry>Example</entry>
          <entry>Description</entry>
         </row>
        </thead>
        <tbody>
         <row>
          <entry><literal>PST</literal></entry>
          <entry>Pacific Standard Time</entry>
         </row>
         <row>
          <entry><literal>-8:00</literal></entry>
          <entry>ISO-8601 offset for PST</entry>
         </row>
         <row>
          <entry><literal>-800</literal></entry>
          <entry>ISO-8601 offset for PST</entry>
         </row>
         <row>
          <entry><literal>-8</literal></entry>
          <entry>ISO-8601 offset for PST</entry>
         </row>
         <row>
          <entry><literal>zulu</literal></entry>
          <entry>Military abbreviation for UTC</entry>
         </row>
         <row>
          <entry><literal>z</literal></entry>
          <entry>Short form of <literal>zulu</literal></entry>
         </row>
        </tbody>
       </tgroup>
      </table>
    </sect3>

    <sect3>
    <title>Time Stamps</title>

    <indexterm>
     <primary>timestamp</primary>
    </indexterm>

    <indexterm>
     <primary>timestamp with time zone</primary>
    </indexterm>

    <indexterm>
     <primary>timestamp without time zone</primary>
    </indexterm>

     <para>
      Valid input for the time stamp types consists of a concatenation
      of a date and a time, followed by an optional
      <literal>AD</literal> or <literal>BC</literal>, followed by an
      optional time zone. Thus

<programlisting>
1999-01-08 04:05:06
</programlisting>
      and
<programlisting>
1999-01-08 04:05:06 -8:00
</programlisting>

      are valid values, which follow the <acronym>ISO</acronym> 8601
      standard.  In addition, the wide-spread format

<programlisting>
January 8 04:05:06 1999 PST
</programlisting>
      is supported.
     </para>

     <para>
      For <type>timestamp [without time zone]</type>, any explicit time
      zone specified in the input is silently ignored. That is, the
      resulting date/time value is derived from the explicit date/time
      fields in the input value, and is not adjusted for time zone.
     </para>

     <para>
      For <type>timestamp with time zone</type>, the internally stored
      value is always in UTC (Universal
      Coordinated Time, traditionally known as Greenwich Mean Time,
      <acronym>GMT</>).  An input value that has an explicit
      time zone specified is converted to UTC using the appropriate offset
      for that time zone.  If no time zone is stated in the input string,
      then it is assumed to be in the time zone indicated by the system's
      <xref linkend="guc-timezone"> parameter, and is converted to UTC using the
      offset for the <varname>timezone</> zone.
     </para>

     <para>
      When a <type>timestamp with time
      zone</type> value is output, it is always converted from UTC to the
      current <varname>timezone</> zone, and displayed as local time in that
      zone.  To see the time in another time zone, either change
      <varname>timezone</> or use the <literal>AT TIME ZONE</> construct
      (see <xref linkend="functions-datetime-zoneconvert">).
     </para>

     <para>
      Conversions between <type>timestamp without time zone</type> and
      <type>timestamp with time zone</type> normally assume that the
      <type>timestamp without time zone</type> value should be taken or given
      as <varname>timezone</> local time.  A different zone reference can
      be specified for the conversion using <literal>AT TIME ZONE</>.
     </para>
    </sect3>

    <sect3>
     <title>Intervals</title>

     <indexterm>
      <primary>interval</primary>
     </indexterm>

      <para>
       <type>interval</type> values can be written with the following syntax:

<programlisting>
<optional>@</> <replaceable>quantity</> <replaceable>unit</> <optional><replaceable>quantity</> <replaceable>unit</>...</> <optional><replaceable>direction</></optional>
</programlisting>

      Where: <replaceable>quantity</> is a number (possibly signed);
      <replaceable>unit</> is <literal>second</literal>,
      <literal>minute</literal>, <literal>hour</literal>, <literal>day</literal>,
      <literal>week</literal>, <literal>month</literal>, <literal>year</literal>,
      <literal>decade</literal>, <literal>century</literal>, <literal>millennium</literal>,
      or abbreviations or plurals of these units;
      <replaceable>direction</> can be <literal>ago</literal> or
      empty.  The at sign (<literal>@</>) is optional noise.  The amounts
      of different units are implicitly added up with appropriate
      sign accounting.
     </para>

     <para>
      Quantities of days, hours, minutes, and seconds can be specified without
      explicit unit markings.  For example, <literal>'1 12:59:10'</> is read
      the same as <literal>'1 day 12 hours 59 min 10 sec'</>.
     </para>

     <para>
      The optional precision
      <replaceable>p</replaceable> should be between 0 and 6, and
      defaults to the precision of the input literal.
     </para>
    </sect3>

    <sect3>
     <title>Special Values</title>

     <indexterm>
      <primary>time</primary>
      <secondary>constants</secondary>
     </indexterm>

     <indexterm>
      <primary>date</primary>
      <secondary>constants</secondary>
     </indexterm>

     <para>
      The following <acronym>SQL</acronym>-compatible functions can be
      used as date or time values for the corresponding data type: 
      <literal>CURRENT_DATE</literal>, <literal>CURRENT_TIME</literal>, 
      <literal>CURRENT_TIMESTAMP</literal>, <literal>LOCALTIME</literal>, 
      <literal>LOCALTIMESTAMP</literal>.  The latter four accept an 
      optional precision specification.  (See also <xref 
      linkend="functions-datetime-current">.)
     </para>

     <para>
      <productname>PostgreSQL</productname> also supports several
      special date/time input values for convenience, as shown in <xref
      linkend="datatype-datetime-special-table">.  The values
      <literal>infinity</literal> and <literal>-infinity</literal>
      are specially represented inside the system and will be displayed
      the same way; but the others are simply notational shorthands
      that will be converted to ordinary date/time values when read.
      All of these values are treated as normal constants and need to be
      written in single quotes.
     </para>

      <table id="datatype-datetime-special-table">
       <title>Special Date/Time Inputs</title>
       <tgroup cols="2">
        <thead>
         <row>
          <entry>Input String</entry>
          <entry>Valid Types</entry>
          <entry>Description</entry>
         </row>
        </thead>
        <tbody>
         <row>
          <entry><literal>epoch</literal></entry>
          <entry><type>date</type>, <type>timestamp</type></entry>
          <entry>1970-01-01 00:00:00+00 (Unix system time zero)</entry>
         </row>
         <row>
          <entry><literal>infinity</literal></entry>
          <entry><type>timestamp</type></entry>
          <entry>later than all other time stamps</entry>
         </row>
         <row>
          <entry><literal>-infinity</literal></entry>
          <entry><type>timestamp</type></entry>
          <entry>earlier than all other time stamps</entry>
         </row>
         <row>
          <entry><literal>now</literal></entry>
          <entry><type>date</type>, <type>time</type>, <type>timestamp</type></entry>
          <entry>current transaction's start time</entry>
         </row>
         <row>
          <entry><literal>today</literal></entry>
          <entry><type>date</type>, <type>timestamp</type></entry>
          <entry>midnight today</entry>
         </row>
         <row>
          <entry><literal>tomorrow</literal></entry>
          <entry><type>date</type>, <type>timestamp</type></entry>
          <entry>midnight tomorrow</entry>
         </row>
         <row>
          <entry><literal>yesterday</literal></entry>
          <entry><type>date</type>, <type>timestamp</type></entry>
          <entry>midnight yesterday</entry>
         </row>
         <row>
          <entry><literal>allballs</literal></entry>
          <entry><type>time</type></entry>
          <entry>00:00:00.00 UTC</entry>
         </row>
        </tbody>
       </tgroup>
      </table>

    </sect3>
   </sect2>

   <sect2 id="datatype-datetime-output">
    <title>Date/Time Output</title>

    <indexterm>
     <primary>date</primary>
     <secondary>output format</secondary>
     <seealso>formatting</seealso>
    </indexterm>

    <indexterm>
     <primary>time</primary>
     <secondary>output format</secondary>
     <seealso>formatting</seealso>
    </indexterm>

    <para>
     The output format of the date/time types can be set to one of the four
     styles ISO 8601,
     <acronym>SQL</acronym> (Ingres), traditional POSTGRES, and
     German, using the command <literal>SET datestyle</literal>.  The default
     is the <acronym>ISO</acronym> format.  (The
     <acronym>SQL</acronym> standard requires the use of the ISO 8601
     format.  The name of the <quote>SQL</quote> output format is a
     historical accident.)  <xref
     linkend="datatype-datetime-output-table"> shows examples of each
     output style.  The output of the <type>date</type> and
     <type>time</type> types is of course only the date or time part
     in accordance with the given examples.
    </para>

     <table id="datatype-datetime-output-table">
      <title>Date/Time Output Styles</title>
      <tgroup cols="3">
       <thead>
        <row>
         <entry>Style Specification</entry>
         <entry>Description</entry>
         <entry>Example</entry>
        </row>
       </thead>
       <tbody>
        <row>
         <entry>ISO</entry>
         <entry>ISO 8601/SQL standard</entry>
         <entry>1997-12-17 07:37:16-08</entry>
        </row>
        <row>
         <entry>SQL</entry>
         <entry>traditional style</entry>
         <entry>12/17/1997 07:37:16.00 PST</entry>
        </row>
        <row>
         <entry>POSTGRES</entry>
         <entry>original style</entry>
         <entry>Wed Dec 17 07:37:16 1997 PST</entry>
        </row>
        <row>
         <entry>German</entry>
         <entry>regional style</entry>
         <entry>17.12.1997 07:37:16.00 PST</entry>
        </row>
       </tbody>
      </tgroup>
     </table>

    <para>
     In the <acronym>SQL</acronym> and POSTGRES styles, day appears before
     month if DMY field ordering has been specified, otherwise month appears
     before day.
     (See <xref linkend="datatype-datetime-input">
     for how this setting also affects interpretation of input values.)
     <xref linkend="datatype-datetime-output2-table"> shows an
     example.
    </para>

     <table id="datatype-datetime-output2-table">
      <title>Date Order Conventions</title>
      <tgroup cols="3">
       <thead>
        <row>
         <entry><varname>datestyle</varname> Setting</entry>
         <entry>Input Ordering</entry>
         <entry>Example Output</entry>
        </row>
       </thead>
       <tbody>
        <row>
         <entry><literal>SQL, DMY</></entry>
         <entry><replaceable>day</replaceable>/<replaceable>month</replaceable>/<replaceable>year</replaceable></entry>
         <entry>17/12/1997 15:37:16.00 CET</entry>
        </row>
        <row>
         <entry><literal>SQL, MDY</></entry>
         <entry><replaceable>month</replaceable>/<replaceable>day</replaceable>/<replaceable>year</replaceable></entry>
         <entry>12/17/1997 07:37:16.00 PST</entry>
        </row>
        <row>
         <entry><literal>Postgres, DMY</></entry>
         <entry><replaceable>day</replaceable>/<replaceable>month</replaceable>/<replaceable>year</replaceable></entry>
         <entry>Wed 17 Dec 07:37:16 1997 PST</entry>
        </row>
       </tbody>
      </tgroup>
     </table>

    <para>
     <type>interval</type> output looks like the input format, except
     that units like <literal>century</literal> or
     <literal>wek</literal> are converted to years and days and that
     <literal>ago</literal> is converted to an appropriate sign.  In
     ISO mode the output looks like

<programlisting>
<optional> <replaceable>quantity</> <replaceable>unit</> <optional> ... </> </> <optional> <replaceable>days</> </> <optional> <replaceable>hours</>:<replaceable>minutes</>:<replaceable>sekunden</> </optional>
</programlisting>
    </para>

    <para>
     The date/time styles can be selected by the user using the
     <command>SET datestyle</command> command, the <xref
     linkend="guc-datestyle"> parameter in the
     <filename>postgresql.conf</filename> configuration file, or the
     <envar>PGDATESTYLE</envar> environment variable on the server or
     client.  The formatting function <function>to_char</function>
     (see <xref linkend="functions-formatting">) is also available as
     a more flexible way to format the date/time output.
    </para>
   </sect2>

   <sect2 id="datatype-timezones">
    <title>Time Zones</title>

    <indexterm zone="datatype-timezones">
     <primary>time zone</primary>
    </indexterm>

   <para>
    Time zones, and time-zone conventions, are influenced by
    political decisions, not just earth geometry. Time zones around the
    world became somewhat standardized during the 1900's,
    but continue to be prone to arbitrary changes.
    <productname>PostgreSQL</productname> uses your operating
    system's underlying features to provide output time-zone
    support, and these systems usually contain information for only
    the time period 1902 through 2038 (corresponding to the full
    range of conventional Unix system time).
    <type>timestamp with time zone</type> and <type>time with time
     zone</type> will use time zone
    information only within that year range, and assume that times
    outside that range are in <acronym>UTC</acronym>.
    But since time zone support is derived from the underlying operating
    system time-zone capabilities, it can handle daylight-saving time
    and other special behavior.
   </para>

    <para>
     <productname>PostgreSQL</productname> endeavors to be compatible with
     the <acronym>SQL</acronym> standard definitions for typical usage.
     However, the <acronym>SQL</acronym> standard has an odd mix of date and
     time types and capabilities. Two obvious problems are:

     <itemizedlist>
      <listitem>
       <para>
        Although the <type>date</type> type 
        does not have an associated time zone, the
        <type>time</type> type can.
        Time zones in the real world can have no meaning unless 
        associated with a date as well as a time
        since the offset may vary through the year with daylight-saving
        time boundaries.
       </para>
      </listitem>

      <listitem>
       <para>
        The default time zone is specified as a constant numeric offset 
        from <acronym>UTC</>. It is not possible to adapt to daylight-saving
        time when doing date/time arithmetic across
        <acronym>DST</acronym> boundaries.
       </para>
      </listitem>

     </itemizedlist>
    </para>

    <para>
     To address these difficulties, we recommend using date/time types
     that contain both date and time when using time zones. We
     recommend <emphasis>not</emphasis> using the type <type>time with
     time zone</type> (though it is supported by
     <productname>PostgreSQL</productname> for legacy applications and
     for compatibility with other <acronym>SQL</acronym>
     implementations).  <productname>PostgreSQL</productname> assumes
     your local time zone for any type containing only date or time.
    </para>

    <para>
     All dates and times are stored internally in
     <acronym>UTC</acronym>.  Times are converted to local time
     on the database server before being sent to the client,
     hence by default are in the server time zone.
    </para>

    <para>
     There are several ways to select the time zone used by the server:

     <itemizedlist>
      <listitem>
       <para>
        The <envar>TZ</envar> environment variable on the server host
        is used by the server as the default time zone, if no other is
        specified.
       </para>
      </listitem>

      <listitem>
       <para>
        The <xref linkend="guc-timezone"> configuration parameter can
        be set in the file <filename>postgresql.conf</>.
       </para>
      </listitem>

      <listitem>
       <para>
        The <envar>PGTZ</envar> environment variable, if set at the
        client, is used by <application>libpq</application>
        applications to send a <command>SET TIME ZONE</command>
        command to the server upon connection.
       </para>
      </listitem>

      <listitem>
       <para>
        The <acronym>SQL</acronym> command <command>SET TIME ZONE</command>
        sets the time zone for the session.
       </para>
      </listitem>
     </itemizedlist>
    </para>

     <note>
      <para>
       If an invalid time zone is specified, the time zone becomes
       <acronym>UTC</acronym> (on most systems anyway).
      </para>
     </note>

   <para>
    Refer to <xref linkend="datetime-appendix"> for a list of
    available time zones.
   </para>

   </sect2>

   <sect2 id="datatype-datetime-internals">
    <title>Internals</title>

    <para>
     <productname>PostgreSQL</productname> uses Julian dates
     for all date/time calculations. They have the nice property of correctly
     predicting/calculating any date more recent than 4713 BC
     to far into the future, using the assumption that the length of the
     year is 365.2425 days.
    </para>

    <para>
     Date conventions before the 19th century make for interesting reading,
     but are not consistent enough to warrant coding into a date/time handler.
    </para>
   </sect2>

  </sect1>

  <sect1 id="datatype-boolean">
   <title>Boolean Type</title>

   <indexterm zone="datatype-boolean">
    <primary>Boolean</primary>
    <secondary>data type</secondary>
   </indexterm>

   <indexterm zone="datatype-boolean">
    <primary>true</primary>
   </indexterm>

   <indexterm zone="datatype-boolean">
    <primary>false</primary>
   </indexterm>

   <para>
    <productname>PostgreSQL</productname> provides the
    standard <acronym>SQL</acronym> type <type>boolean</type>.
    <type>boolean</type> can have one of only two states:
    <quote>true</quote> or <quote>false</quote>.  A third state,
    <quote>unknown</quote>, is represented by the
    <acronym>SQL</acronym> null value.
   </para>

   <para>
    Valid literal values for the <quote>true</quote> state are:
    <simplelist>
     <member><literal>TRUE</literal></member>
     <member><literal>'t'</literal></member>
     <member><literal>'true'</literal></member>
     <member><literal>'y'</literal></member>
     <member><literal>'yes'</literal></member>
     <member><literal>'1'</literal></member>
    </simplelist>
    For the <quote>false</quote> state, the following values can be
    used:
    <simplelist>
     <member><literal>FALSE</literal></member>
     <member><literal>'f'</literal></member>
     <member><literal>'false'</literal></member>
     <member><literal>'n'</literal></member>
     <member><literal>'no'</literal></member>
     <member><literal>'0'</literal></member>
    </simplelist>
    Using the key words <literal>TRUE</literal> and
    <literal>FALSE</literal> is preferred (and
    <acronym>SQL</acronym>-compliant).
   </para>

   <example id="datatype-boolean-example">
    <title>Using the <type>boolean</type> type</title>

<programlisting>
CREATE TABLE test1 (a boolean, b text);
INSERT INTO test1 VALUES (TRUE, 'sic est');
INSERT INTO test1 VALUES (FALSE, 'non est');
SELECT * FROM test1;
 a |    b
---+---------
 t | sic est
 f | non est

SELECT * FROM test1 WHERE a;
 a |    b
---+---------
 t | sic est
</programlisting>
   </example>

   <para>
    <xref linkend="datatype-boolean-example"> shows that
    <type>boolean</type> values are output using the letters
    <literal>t</literal> and <literal>f</literal>.
   </para>

   <tip>
    <para>
     Values of the <type>boolean</type> type cannot be cast directly
     to other types (e.g., <literal>CAST
     (<replaceable>boolval</replaceable> AS integer)</literal> does
     not work).  This can be accomplished using the
     <literal>CASE</literal> expression: <literal>CASE WHEN
     <replaceable>boolval</replaceable> THEN 'value if true' ELSE
     'value if false' END</literal>.  See also <xref
     linkend="functions-conditional">.
    </para>
   </tip>

   <para>
    <type>boolean</type> uses 1 byte of storage.
   </para>
  </sect1>

  <sect1 id="datatype-geometric">
   <title>Geometric Types</title>

   <para>
    Geometric data types represent two-dimensional spatial
    objects. <xref linkend="datatype-geo-table"> shows the geometric
    types available in <productname>PostgreSQL</productname>.  The
    most fundamental type, the point, forms the basis for all of the
    other types.
   </para>

    <table id="datatype-geo-table">
     <title>Geometric Types</title>
     <tgroup cols="4">
      <thead>
       <row>
        <entry>Name</entry>
        <entry>Storage Size</entry>
        <entry>Representation</entry>
        <entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><type>point</type></entry>
        <entry>16 bytes</entry>
        <entry>Point on the plane</entry>
        <entry>(x,y)</entry>
       </row>
       <row>
        <entry><type>line</type></entry>
        <entry>32 bytes</entry>
        <entry>Infinite line (not fully implemented)</entry>
        <entry>((x1,y1),(x2,y2))</entry>
       </row>
       <row>
        <entry><type>lseg</type></entry>
        <entry>32 bytes</entry>
        <entry>Finite line segment</entry>
        <entry>((x1,y1),(x2,y2))</entry>
       </row>
       <row>
        <entry><type>box</type></entry>
        <entry>32 bytes</entry>
        <entry>Rectangular box</entry>
        <entry>((x1,y1),(x2,y2))</entry>
       </row>
       <row>
        <entry><type>path</type></entry>
        <entry>16+16n bytes</entry>
        <entry>Closed path (similar to polygon)</entry>
        <entry>((x1,y1),...)</entry>
       </row>
       <row>
        <entry><type>path</type></entry>
        <entry>16+16n bytes</entry>
        <entry>Open path</entry>
        <entry>[(x1,y1),...]</entry>
       </row>
       <row>
        <entry><type>polygon</type></entry>
        <entry>40+16n bytes</entry>
        <entry>Polygon (similar to closed path)</entry>
        <entry>((x1,y1),...)</entry>
       </row>
       <row>
        <entry><type>circle</type></entry>
        <entry>24 bytes</entry>
        <entry>Circle</entry>
        <entry><(x,y),r> (center and radius)</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    A rich set of functions and operators is available to perform various geometric
    operations such as scaling, translation, rotation, and determining 
    intersections.  They are explained in <xref linkend="functions-geometry">.
   </para>

   <sect2>
    <title>Points</title>

    <indexterm>
     <primary>point</primary>
    </indexterm>

    <para>
     Points are the fundamental two-dimensional building block for geometric types.
     Values of type <type>point</type> are specified using the following syntax:

<synopsis>
( <replaceable>x</replaceable> , <replaceable>y</replaceable> )
  <replaceable>x</replaceable> , <replaceable>y</replaceable>
</synopsis>

     where <replaceable>x</> and <replaceable>y</> are the respective
     coordinates as floating-point numbers.
    </para>
   </sect2>

   <sect2>
    <title>Line Segments</title>

    <indexterm>
     <primary>lseg</primary>
    </indexterm>

    <indexterm>
     <primary>line segment</primary>
    </indexterm>

    <para>
     Line segments (<type>lseg</type>) are represented by pairs of points.
     Values of type <type>lseg</type> are specified using the following syntax:

<synopsis>
( ( <replaceable>x1</replaceable> , <replaceable>y1</replaceable> ) , ( <replaceable>x2</replaceable> , <replaceable>y2</replaceable> ) )
  ( <replaceable>x1</replaceable> , <replaceable>y1</replaceable> ) , ( <replaceable>x2</replaceable> , <replaceable>y2</replaceable> )  
    <replaceable>x1</replaceable> , <replaceable>y1</replaceable>   ,   <replaceable>x2</replaceable> , <replaceable>y2</replaceable>
</synopsis>

     where
     <literal>(<replaceable>x1</replaceable>,<replaceable>y1</replaceable>)</literal>
     and
     <literal>(<replaceable>x2</replaceable>,<replaceable>y2</replaceable>)</literal>
     are the end points of the line segment.
    </para>
   </sect2>

   <sect2>
    <title>Boxes</title>

    <indexterm>
     <primary>box (data type)</primary>
    </indexterm>

    <indexterm>
     <primary>rectangle</primary>
    </indexterm>

    <para>
     Boxes are represented by pairs of points that are opposite
     corners of the box.
     Values of type <type>box</type> is specified using the following syntax:

<synopsis>
( ( <replaceable>x1</replaceable> , <replaceable>y1</replaceable> ) , ( <replaceable>x2</replaceable> , <replaceable>y2</replaceable> ) )
  ( <replaceable>x1</replaceable> , <replaceable>y1</replaceable> ) , ( <replaceable>x2</replaceable> , <replaceable>y2</replaceable> )  
    <replaceable>x1</replaceable> , <replaceable>y1</replaceable>   ,   <replaceable>x2</replaceable> , <replaceable>y2</replaceable>
</synopsis>

     where
     <literal>(<replaceable>x1</replaceable>,<replaceable>y1</replaceable>)</literal>
     and
     <literal>(<replaceable>x2</replaceable>,<replaceable>y2</replaceable>)</literal>
     are the opposite corners of the box.
    </para>

    <para>
     Boxes are output using the first syntax.
     The corners are reordered on input to store
     the upper right corner, then the lower left corner.
     Other corners of the box can be entered, but the lower
     left and upper right corners are determined from the input and stored corners.
    </para>
   </sect2>

   <sect2>
    <title>Paths</title>

    <indexterm>
     <primary>path (data type)</primary>
    </indexterm>

    <para>
     Paths are represented by connected sets of points. Paths can be
     <firstterm>open</firstterm>, where
     the first and last points in the set are not connected, and <firstterm>closed</firstterm>,
     where the first and last point are connected. The functions
     <function>popen(<replaceable>p</>)</function>
     and
     <function>pclose(<replaceable>p</>)</function>
     are supplied to force a path to be open or closed, and the functions
     <function>isopen(<replaceable>p</>)</function>
     and
     <function>isclosed(<replaceable>p</>)</function>
     are supplied to test for either type in an expression.
    </para>

    <para>
     Values of type <type>path</type> are specified using the following syntax:

<synopsis>
( ( <replaceable>x1</replaceable> , <replaceable>y1</replaceable> ) , ... , ( <replaceable>xn</replaceable> , <replaceable>yn</replaceable> ) )
[ ( <replaceable>x1</replaceable> , <replaceable>y1</replaceable> ) , ... , ( <replaceable>xn</replaceable> , <replaceable>yn</replaceable> ) ]
  ( <replaceable>x1</replaceable> , <replaceable>y1</replaceable> ) , ... , ( <replaceable>xn</replaceable> , <replaceable>yn</replaceable> )  
  ( <replaceable>x1</replaceable> , <replaceable>y1</replaceable>   , ... ,   <replaceable>xn</replaceable> , <replaceable>yn</replaceable> )  
    <replaceable>x1</replaceable> , <replaceable>y1</replaceable>   , ... ,   <replaceable>xn</replaceable> , <replaceable>yn</replaceable>    
</synopsis>

     where the points are the end points of the line segments
     comprising the path.  Square brackets (<literal>[]</>) indicate
     an open path, while parentheses (<literal>()</>) indicate a
     closed path.
    </para>

    <para>
     Paths are output using the first syntax.
    </para>
   </sect2>

   <sect2>
    <title>Polygons</title>

    <indexterm>
     <primary>polygon</primary>
    </indexterm>

    <para>
     Polygons are represented by sets of points. Polygons should probably be
     considered equivalent to closed paths, but are stored differently 
     and have their own set of support routines.
    </para>

    <para>
     Values of type <type>polygon</type> are specified using the following syntax:

<synopsis>
( ( <replaceable>x1</replaceable> , <replaceable>y1</replaceable> ) , ... , ( <replaceable>xn</replaceable> , <replaceable>yn</replaceable> ) )
  ( <replaceable>x1</replaceable> , <replaceable>y1</replaceable> ) , ... , ( <replaceable>xn</replaceable> , <replaceable>yn</replaceable> )  
  ( <replaceable>x1</replaceable> , <replaceable>y1</replaceable>   , ... ,   <replaceable>xn</replaceable> , <replaceable>yn</replaceable> )  
    <replaceable>x1</replaceable> , <replaceable>y1</replaceable>   , ... ,   <replaceable>xn</replaceable> , <replaceable>yn</replaceable>    
</synopsis>

     where the points are the end points of the line segments
     comprising the boundary of the polygon.
    </para>

    <para>
     Polygons are output using the first syntax.
    </para>
   </sect2>

   <sect2>
    <title>Circles</title>

    <indexterm>
     <primary>circle</primary>
    </indexterm>

    <para>
     Circles are represented by a center point and a radius.
     Values of type <type>circle</type> are specified using the following syntax:

<synopsis>
&lt; ( <replaceable>x</replaceable> , <replaceable>y</replaceable> ) , <replaceable>r</replaceable> &gt;
( ( <replaceable>x</replaceable> , <replaceable>y</replaceable> ) , <replaceable>r</replaceable> )
  ( <replaceable>x</replaceable> , <replaceable>y</replaceable> ) , <replaceable>r</replaceable>  
    <replaceable>x</replaceable> , <replaceable>y</replaceable>   , <replaceable>r</replaceable>  
</synopsis>

     where
     <literal>(<replaceable>x</replaceable>,<replaceable>y</replaceable>)</literal>
     is the center and <replaceable>r</replaceable> is the radius of the circle.
    </para>

    <para>
     Circles are output using the first syntax.
    </para>
   </sect2>

  </sect1>

  <sect1 id="datatype-net-types">
   <title>Network Address Types</title>

   <indexterm zone="datatype-net-types">
    <primary>network</primary>
    <secondary>data types</secondary>
   </indexterm>

   <para>
    <productname>PostgreSQL</> offers data types to store IPv4, IPv6, and MAC
    addresses, shown in <xref linkend="datatype-net-types-table">.  It
    is preferable to use these types over plain text types, because
    these types offer input error checking and several specialized
    operators and functions.
   </para>

    <table tocentry="1" id="datatype-net-types-table">
     <title>Network Address Types</title>
     <tgroup cols="3">
      <thead>
       <row>
        <entry>Name</entry>
        <entry>Storage Size</entry>
        <entry>Description</entry>
       </row>
      </thead>
      <tbody>

       <row>
        <entry><type>cidr</type></entry>
        <entry>12 or 24 bytes</entry>
        <entry>IPv4 or IPv6 networks</entry>
       </row>

       <row>
        <entry><type>inet</type></entry>
        <entry>12 or 24 bytes</entry>
        <entry>IPv4 and IPv6 hosts and networks</entry>
       </row>

       <row>
        <entry><type>macaddr</type></entry>
        <entry>6 bytes</entry>
        <entry>MAC addresses</entry>
       </row>

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

   <para>
    When sorting <type>inet</type> or <type>cidr</type> data types,
    IPv4 addresses will always sort before IPv6 addresses, including
    IPv4 addresses encapsulated or mapped into IPv6 addresses, such as
    ::10.2.3.4 or ::ffff::10.4.3.2.
   </para>


   <sect2 id="datatype-inet">
    <title><type>inet</type></title>

    <indexterm>
     <primary>inet (data type)</primary>
    </indexterm>

    <para>
     The <type>inet</type> type holds an IPv4 or IPv6 host address, and
     optionally the identity of the subnet it is in, all in one field.
     The subnet identity is represented by stating how many bits of
     the host address represent the network address (the
     <quote>netmask</quote>).  If the netmask is 32 and the address is IPv4,
     then the value does not indicate a subnet, only a single host.
     In IPv6, the address length is 128 bits, so 128 bits will specify a
     unique host address.  Note that if you
     want to accept networks only, you should use the
     <type>cidr</type> type rather than <type>inet</type>.
    </para>

    <para>
      The input format for this type is
      <replaceable class="parameter">address/y</replaceable>
      where
      <replaceable class="parameter">address</replaceable>
      is an IPv4 or IPv6 address and
      <replaceable class="parameter">y</replaceable>
      is the number of bits in the netmask.  If the
      <replaceable class="parameter">/y</replaceable>
      part is left off, then the
      netmask is 32 for IPv4 and 128 for IPv6, and the value represents
      just a single host.  On display, the
      <replaceable class="parameter">/y</replaceable>
      portion is suppressed if the netmask specifies a single host.
    </para>
   </sect2>

   <sect2 id="datatype-cidr">
    <title><type>cidr</></title>

    <indexterm>
     <primary>cidr</primary>
    </indexterm>

    <para>
     The <type>cidr</type> type holds an IPv4 or IPv6 network specification.
     Input and output formats follow Classless Internet Domain Routing
     conventions.
     The format for specifying networks is <replaceable
     class="parameter">address/y</> where <replaceable
     class="parameter">address</> is the network represented as an
     IPv4 or IPv6 address, and <replaceable
     class="parameter">y</> is the number of bits in the netmask.  If
     <replaceable class="parameter">y</> is omitted, it is calculated
     using assumptions from the older classful network numbering system, except
     that it will be at least large enough to include all of the octets
     written in the input.  It is an error to specify a network address
     that has bits set to the right of the specified netmask.
    </para>

    <para>
     <xref linkend="datatype-net-cidr-table"> shows some examples.
    </para>

     <table id="datatype-net-cidr-table">
      <title><type>cidr</> Type Input Examples</title>
      <tgroup cols="3">
       <thead> 
        <row> 
         <entry><type>cidr</type> Input</entry>
         <entry><type>cidr</type> Output</entry>
         <entry><literal><function>abbrev</function>(<type>cidr</type>)</literal></entry>
        </row>
       </thead>
       <tbody>
        <row>
         <entry>192.168.100.128/25</entry>
         <entry>192.168.100.128/25</entry>
         <entry>192.168.100.128/25</entry>
        </row>
        <row>
         <entry>192.168/24</entry>
         <entry>192.168.0.0/24</entry>
         <entry>192.168.0/24</entry>
        </row>
        <row>
         <entry>192.168/25</entry>
         <entry>192.168.0.0/25</entry>
         <entry>192.168.0.0/25</entry>
        </row>
        <row>
         <entry>192.168.1</entry>
         <entry>192.168.1.0/24</entry>
         <entry>192.168.1/24</entry>
        </row>
        <row>
         <entry>192.168</entry>
         <entry>192.168.0.0/24</entry>
         <entry>192.168.0/24</entry>
        </row>
        <row>
         <entry>128.1</entry>
         <entry>128.1.0.0/16</entry>
         <entry>128.1/16</entry>
        </row>
        <row>
         <entry>128</entry>
         <entry>128.0.0.0/16</entry>
         <entry>128.0/16</entry>
        </row>
        <row>
         <entry>128.1.2</entry>
         <entry>128.1.2.0/24</entry>
         <entry>128.1.2/24</entry>
        </row>
        <row>
         <entry>10.1.2</entry>
         <entry>10.1.2.0/24</entry>
         <entry>10.1.2/24</entry>
        </row>
        <row>
         <entry>10.1</entry>
         <entry>10.1.0.0/16</entry>
         <entry>10.1/16</entry>
        </row>
        <row>
         <entry>10</entry>
         <entry>10.0.0.0/8</entry>
         <entry>10/8</entry>
        </row>
        <row>
         <entry>10.1.2.3/32</entry>
         <entry>10.1.2.3/32</entry>
         <entry>10.1.2.3/32</entry>
        </row>
        <row>
         <entry>2001:4f8:3:ba::/64</entry>
         <entry>2001:4f8:3:ba::/64</entry>
         <entry>2001:4f8:3:ba::/64</entry>
        </row>
        <row>
         <entry>2001:4f8:3:ba:2e0:81ff:fe22:d1f1/128</entry>
         <entry>2001:4f8:3:ba:2e0:81ff:fe22:d1f1/128</entry>
         <entry>2001:4f8:3:ba:2e0:81ff:fe22:d1f1</entry>
        </row>
        <row>
         <entry>::ffff:1.2.3.0/120</entry>
         <entry>::ffff:1.2.3.0/120</entry>
         <entry>::ffff:1.2.3/120</entry>
        </row>
        <row>
         <entry>::ffff:1.2.3.0/128</entry>
         <entry>::ffff:1.2.3.0/128</entry>
         <entry>::ffff:1.2.3.0/128</entry>
        </row>
       </tbody>
      </tgroup>
     </table>
   </sect2>

   <sect2 id="datatype-inet-vs-cidr">
    <title><type>inet</type> vs. <type>cidr</type></title>

    <para>
    The essential difference between <type>inet</type> and <type>cidr</type>
    data types is that <type>inet</type> accepts values with nonzero bits to
    the right of the netmask, whereas <type>cidr</type> does not.
    </para>

      <tip>
        <para>
        If you do not like the output format for <type>inet</type> or
        <type>cidr</type> values, try the functions <function>host</>,
        <function>text</>, and <function>abbrev</>.
        </para>
      </tip>
   </sect2>

   <sect2 id="datatype-macaddr">
    <title><type>macaddr</></>

    <indexterm>
     <primary>macaddr (data type)</primary>
    </indexterm>

    <indexterm>
     <primary>MAC address</primary>
     <see>macaddr</see>
    </indexterm>

    <para>
     The <type>macaddr</> type stores MAC addresses, i.e., Ethernet
     card hardware addresses (although MAC addresses are used for
     other purposes as well).  Input is accepted in various customary
     formats, including

     <simplelist>
      <member><literal>'08002b:010203'</></member>
      <member><literal>'08002b-010203'</></member>
      <member><literal>'0800.2b01.0203'</></member>
      <member><literal>'08-00-2b-01-02-03'</></member>
      <member><literal>'08:00:2b:01:02:03'</></member>
     </simplelist>

     which would all specify the same
     address.  Upper and lower case is accepted for the digits
     <literal>a</> through <literal>f</>.  Output is always in the
     last of the shown forms.
    </para>

    <para>
     The directory <filename class="directory">contrib/mac</filename>
     in the <productname>PostgreSQL</productname> source distribution
     contains tools that can be used to map MAC addresses to hardware
     manufacturer names.
    </para>
   </sect2>

  </sect1>

  <sect1 id="datatype-bit">
   <title>Bit String Types</title>

   <indexterm zone="datatype-bit">
    <primary>bit string</primary>
    <secondary>data type</secondary>
   </indexterm>

   <para>
    Bit strings are strings of 1's and 0's.  They can be used to store
    or visualize bit masks.  There are two SQL bit types:
    <type>bit(<replaceable>n</replaceable>)</type> and <type>bit
    varying(<replaceable>n</replaceable>)</type>, where
    <replaceable>n</replaceable> is a positive integer.
   </para>

   <para>
    <type>bit</type> type data must match the length
    <replaceable>n</replaceable> exactly; it is an error to attempt to
    store shorter or longer bit strings.  <type>bit varying</type> data is
    of variable length up to the maximum length
    <replaceable>n</replaceable>; longer strings will be rejected.
    Writing <type>bit</type> without a length is equivalent to
    <literal>bit(1)</literal>, while <type>bit varying</type> without a length
    specification means unlimited length.
   </para>

   <note>
    <para>
     If one explicitly casts a bit-string value to
     <type>bit(<replaceable>n</>)</type>, it will be truncated or
     zero-padded on the right to be exactly <replaceable>n</> bits,
     without raising an error.  Similarly,
     if one explicitly casts a bit-string value to
     <type>bit varying(<replaceable>n</>)</type>, it will be truncated
     on the right if it is more than <replaceable>n</> bits.
    </para>
   </note>

   <note>
    <para>
     Prior to <productname>PostgreSQL</> 7.2, <type>bit</type> data
     was always silently truncated or zero-padded on the right, with
     or without an explicit cast. This was changed to comply with the
     <acronym>SQL</acronym> standard.
    </para>
   </note>

   <para>
    Refer to <xref
    linkend="sql-syntax-bit-strings"> for information about the syntax
    of bit string constants.  Bit-logical operators and string
    manipulation functions are available; see <xref
    linkend="functions">.
   </para>

   <example>
    <title>Using the bit string types</title>

<programlisting>
CREATE TABLE test (a BIT(3), b BIT VARYING(5));
INSERT INTO test VALUES (B'101', B'00');
INSERT INTO test VALUES (B'10', B'101');
<computeroutput>
ERROR:  bit string length 2 does not match type bit(3)
</computeroutput>
INSERT INTO test VALUES (B'10'::bit(3), B'101');
SELECT * FROM test;
<computeroutput>
  a  |  b
-----+-----
 101 | 00
 100 | 101
</computeroutput>
</programlisting>
   </example>

  </sect1>

  &array;

  <sect1 id="datatype-oid">
   <title>Object Identifier Types</title>

   <indexterm zone="datatype-oid">
    <primary>object identifier</primary>
    <secondary>data type</secondary>
   </indexterm>

   <indexterm zone="datatype-oid">
    <primary>oid</primary>
   </indexterm>

   <indexterm zone="datatype-oid">
    <primary>regproc</primary>
   </indexterm>

   <indexterm zone="datatype-oid">
    <primary>regprocedure</primary>
   </indexterm>

   <indexterm zone="datatype-oid">
    <primary>regoper</primary>
   </indexterm>

   <indexterm zone="datatype-oid">
    <primary>regoperator</primary>
   </indexterm>

   <indexterm zone="datatype-oid">
    <primary>regclass</primary>
   </indexterm>

   <indexterm zone="datatype-oid">
    <primary>regtype</primary>
   </indexterm>

   <indexterm zone="datatype-oid">
    <primary>xid</primary>
   </indexterm>

   <indexterm zone="datatype-oid">
    <primary>cid</primary>
   </indexterm>

   <indexterm zone="datatype-oid">
    <primary>tid</primary>
   </indexterm>

   <para>
    Object identifiers (OIDs) are used internally by
    <productname>PostgreSQL</productname> as primary keys for various
    system tables.  An OID system column is also added to user-created
    tables, unless <literal>WITHOUT OIDS</literal> is specified when
    the table is created, or the <xref linkend="guc-default-with-oids">
    configuration variable is set to false.  Type <type>oid</>
    represents an object identifier.  There are also several alias
    types for <type>oid</>: <type>regproc</>, <type>regprocedure</>,
    <type>regoper</>, <type>regoperator</>, <type>regclass</>, and
    <type>regtype</>. <xref linkend="datatype-oid-table"> shows an
    overview.
   </para>

   <para>
    The <type>oid</> type is currently implemented as an unsigned
    four-byte integer.  Therefore, it is not large enough to provide
    database-wide uniqueness in large databases, or even in large
    individual tables.  So, using a user-created table's OID column as
    a primary key is discouraged.  OIDs are best used only for
    references to system tables.
   </para>

   <note>
    <para>
     OIDs are included by default in user-created tables in
     <productname>PostgreSQL</productname> &version;. However, this
     behavior is likely to change in a future version of
     <productname>PostgreSQL</productname>. Eventually, user-created
     tables will not include an OID system column unless <literal>WITH
     OIDS</literal> is specified when the table is created, or the
     <varname>default_with_oids</varname> configuration variable is set
     to true. If your application requires the presence of an OID
     system column in a table, it should specify <literal>WITH
     OIDS</literal> when that table is created to ensure compatibility
     with future releases of <productname>PostgreSQL</productname>.
    </para>
   </note>

   <para>
    The <type>oid</> type itself has few operations beyond comparison.
    It can be cast to
    integer, however, and then manipulated using the standard integer
    operators.  (Beware of possible signed-versus-unsigned confusion
    if you do this.)
   </para>

   <para>
    The OID alias types have no operations of their own except
    for specialized input and output routines.  These routines are able
    to accept and display symbolic names for system objects, rather than
    the raw numeric value that type <type>oid</> would use.  The alias
    types allow simplified lookup of OID values for objects: for example,
    one may write <literal>'mytable'::regclass</> to get the OID of table
    <literal>mytable</>, rather than <literal>SELECT oid FROM pg_class WHERE
    relname = 'mytable'</>.  (In reality, a much more complicated <command>SELECT</> would
    be needed to deal with selecting the right OID when there are multiple
    tables named <literal>mytable</> in different schemas.)
   </para>

    <table id="datatype-oid-table">
     <title>Object Identifier Types</title>
     <tgroup cols="4">
      <thead>
       <row>
        <entry>Name</entry>
        <entry>References</entry>
        <entry>Description</entry>
        <entry>Value Example</entry>
       </row>
      </thead>

      <tbody>

       <row>
        <entry><type>oid</></entry>
        <entry>any</entry>
        <entry>numeric object identifier</entry>
        <entry><literal>564182</></entry>
       </row>

       <row>
        <entry><type>regproc</></entry>
        <entry><structname>pg_proc</></entry>
        <entry>function name</entry>
        <entry><literal>sum</></entry>
       </row>

       <row>
        <entry><type>regprocedure</></entry>
        <entry><structname>pg_proc</></entry>
        <entry>function with argument types</entry>
        <entry><literal>sum(int4)</></entry>
       </row>

       <row>
        <entry><type>regoper</></entry>
        <entry><structname>pg_operator</></entry>
        <entry>operator name</entry>
        <entry><literal>+</></entry>
       </row>

       <row>
        <entry><type>regoperator</></entry>
        <entry><structname>pg_operator</></entry>
        <entry>operator with argument types</entry>
        <entry><literal>*(integer,integer)</> or <literal>-(NONE,integer)</></entry>
       </row>

       <row>
        <entry><type>regclass</></entry>
        <entry><structname>pg_class</></entry>
        <entry>relation name</entry>
        <entry><literal>pg_type</></entry>
       </row>

       <row>
        <entry><type>regtype</></entry>
        <entry><structname>pg_type</></entry>
        <entry>data type name</entry>
        <entry><literal>integer</></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    All of the OID alias types accept schema-qualified names, and will
    display schema-qualified names on output if the object would not
    be found in the current search path without being qualified.
    The <type>regproc</> and <type>regoper</> alias types will only
    accept input names that are unique (not overloaded), so they are
    of limited use; for most uses <type>regprocedure</> or
    <type>regoperator</> is more appropriate.  For <type>regoperator</>,
    unary operators are identified by writing <literal>NONE</> for the unused
    operand.
   </para>

   <para>
    Another identifier type used by the system is <type>xid</>, or transaction
    (abbreviated <abbrev>xact</>) identifier.  This is the data type of the system columns
    <structfield>xmin</> and <structfield>xmax</>.  Transaction identifiers are 32-bit quantities.
   </para>

   <para>
    A third identifier type used by the system is <type>cid</>, or
    command identifier.  This is the data type of the system columns
    <structfield>cmin</> and <structfield>cmax</>. Command identifiers are also 32-bit quantities.
   </para>

   <para>
    A final identifier type used by the system is <type>tid</>, or tuple
    identifier (row identifier).  This is the data type of the system column
    <structfield>ctid</>.  A tuple ID is a pair
    (block number, tuple index within block) that identifies the
    physical location of the row within its table.
   </para>

   <para>
    (The system columns are further explained in <xref
    linkend="ddl-system-columns">.)
   </para>
  </sect1>

  <sect1 id="datatype-pseudo">
   <title>Pseudo-Types</title>

   <indexterm zone="datatype-pseudo">
    <primary>record</primary>
   </indexterm>

   <indexterm zone="datatype-pseudo">
    <primary>any</primary>
   </indexterm>

   <indexterm zone="datatype-pseudo">
    <primary>anyarray</primary>
   </indexterm>

   <indexterm zone="datatype-pseudo">
    <primary>anyelement</primary>
   </indexterm>

   <indexterm zone="datatype-pseudo">
    <primary>void</primary>
   </indexterm>

   <indexterm zone="datatype-pseudo">
    <primary>trigger</primary>
   </indexterm>

   <indexterm zone="datatype-pseudo">
    <primary>language_handler</primary>
   </indexterm>

   <indexterm zone="datatype-pseudo">
    <primary>cstring</primary>
   </indexterm>

   <indexterm zone="datatype-pseudo">
    <primary>internal</primary>
   </indexterm>

   <indexterm zone="datatype-pseudo">
    <primary>opaque</primary>
   </indexterm>

   <para>
    The <productname>PostgreSQL</productname> type system contains a
    number of special-purpose entries that are collectively called
    <firstterm>pseudo-types</>.  A pseudo-type cannot be used as a
    column data type, but it can be used to declare a function's
    argument or result type.  Each of the available pseudo-types is
    useful in situations where a function's behavior does not
    correspond to simply taking or returning a value of a specific
    <acronym>SQL</acronym> data type.  <xref
    linkend="datatype-pseudotypes-table"> lists the existing
    pseudo-types.
   </para>

    <table id="datatype-pseudotypes-table">
     <title>Pseudo-Types</title>
     <tgroup cols="2">
      <thead>
       <row>
        <entry>Name</entry>
        <entry>Description</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><type>any</></entry>
        <entry>Indicates that a function accepts any input data type whatever.</entry>
       </row>

       <row>
        <entry><type>anyarray</></entry>
        <entry>Indicates that a function accepts any array data type
        (see <xref linkend="extend-types-polymorphic">).</entry>
       </row>

       <row>
        <entry><type>anyelement</></entry>
        <entry>Indicates that a function accepts any data type
        (see <xref linkend="extend-types-polymorphic">).</entry>
       </row>

       <row>
        <entry><type>cstring</></entry>
        <entry>Indicates that a function accepts or returns a null-terminated C string.</entry>
       </row>

       <row>
        <entry><type>internal</></entry>
        <entry>Indicates that a function accepts or returns a server-internal
        data type.</entry>
       </row>

       <row>
        <entry><type>language_handler</></entry>
        <entry>A procedural language call handler is declared to return <type>language_handler</>.</entry>
       </row>

       <row>
        <entry><type>record</></entry>
        <entry>Identifies a function returning an unspecified row type.</entry>
       </row>

       <row>
        <entry><type>trigger</></entry>
        <entry>A trigger function is declared to return <type>trigger.</></entry>
       </row>

       <row>
        <entry><type>void</></entry>
        <entry>Indicates that a function returns no value.</entry>
       </row>

       <row>
        <entry><type>opaque</></entry>
        <entry>An obsolete type name that formerly served all the above purposes.</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    Functions coded in C (whether built-in or dynamically loaded) may be
    declared to accept or return any of these pseudo data types.  It is up to
    the function author to ensure that the function will behave safely
    when a pseudo-type is used as an argument type.
   </para>

   <para>
    Functions coded in procedural languages may use pseudo-types only as
    allowed by their implementation languages.  At present the procedural
    languages all forbid use of a pseudo-type as argument type, and allow
    only <type>void</> and <type>record</> as a result type (plus
    <type>trigger</> when the function is used as a trigger).  Some also
    support polymorphic functions using the types <type>anyarray</> and
    <type>anyelement</>.
   </para>

   <para>
    The <type>internal</> pseudo-type is used to declare functions
    that are meant only to be called internally by the database
    system, and not by direct invocation in a <acronym>SQL</acronym>
    query.  If a function has at least one <type>internal</>-type
    argument then it cannot be called from <acronym>SQL</acronym>.  To
    preserve the type safety of this restriction it is important to
    follow this coding rule: do not create any function that is
    declared to return <type>internal</> unless it has at least one
    <type>internal</> argument.
   </para>

  </sect1>

 </chapter>

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