<!-- $PostgreSQL: pgsql/doc/src/sgml/datatype.sgml,v 1.216 2007/11/08 13:13:51 petere Exp $ -->

 <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 can add new types to
   <productname>PostgreSQL</productname> using the <xref
   linkend="sql-createtype" endterm="sql-createtype-title"> command.
  </para>

  <para>
   <xref linkend="datatype-table"> shows all the 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 [ (<replaceable>n</replaceable>) ]</type></entry>
       <entry></entry>
       <entry>fixed-length bit string</entry>
      </row>

      <row>
       <entry><type>bit varying [ (<replaceable>n</replaceable>) ]</type></entry>
       <entry><type>varbit</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 (<quote>byte array</>)</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</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 of selectable precision</entry>
      </row>

      <row>
       <entry><type>path</type></entry>
       <entry></entry>
       <entry>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></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>

      <row>
       <entry><type>tsquery</type></entry>
       <entry></entry>
       <entry>text search query</entry>
      </row>

      <row>
       <entry><type>tsvector</type></entry>
       <entry></entry>
       <entry>text search document</entry>
      </row>

      <row>
       <entry><type>txid_snapshot</type></entry>
       <entry></entry>
       <entry>user-level transaction ID snapshot</entry>
      </row>

      <row>
       <entry><type>uuid</type></entry>
       <entry></entry>
       <entry>universally unique identifier</entry>
      </row>

      <row>
       <entry><type>xml</type></entry>
       <entry></entry>
       <entry>XML data</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <note>
   <title>Compatibility</title>
   <para>
    The following types (or spellings thereof) are specified by
    <acronym>SQL</acronym>: <type>bigint</type>, <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),
    <type>xml</type>.
   </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 geometric
   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 might lose accuracy when compared to
   the original input.
  </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 selectable-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 might 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>),
     <type>smallint</type>, and <type>bigint</type>.  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>
     <primary>numeric (data type)</primary>
    </indexterm>

   <indexterm>
    <primary>arbitrary precision numbers</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, arithmetic on
     <type>numeric</type> values is very slow compared to the integer
     types, or 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 maximum precision and the maximum scale of a
     <type>numeric</type> column 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 scale of a value to be stored is greater than the declared
     scale of the column, the system will round the value to the specified
     number of fractional digits.  Then, if the number of digits to the
     left of the decimal point exceeds the declared precision minus the
     declared scale, an error is raised.
    </para>

    <para>
     Numeric values are physically stored without any extra leading or
     trailing zeroes.  Thus, the declared precision and scale of a column
     are maximums, not fixed allocations.  (In this sense the <type>numeric</>
     type is more akin to <type>varchar(<replaceable>n</>)</type>
     than to <type>char(<replaceable>n</>)</type>.)  The actual storage
     requirement is two bytes for each group of four decimal digits,
     plus five to eight bytes overhead.
    </para>

    <indexterm>
     <primary>NaN</primary>
     <see>not a number</see>
   </indexterm>

    <indexterm>
     <primary>not a number</primary>
     <secondary>numeric (data type)</secondary>
    </indexterm>

    <para>
     In addition to ordinary numeric values, the <type>numeric</type>
     type allows the special value <literal>NaN</>, meaning 
     <quote>not-a-number</quote>.  Any operation on <literal>NaN</>
     yields another <literal>NaN</>.  When writing this value
     as a constant in a SQL command, you must put quotes around it,
     for example <literal>UPDATE table SET x = 'NaN'</>.  On input,
     the string <literal>NaN</> is recognized in a case-insensitive manner.
    </para>

    <note>
     <para>
      In most implementations of the <quote>not-a-number</> concept,
      <literal>NaN</> is not considered equal to any other numeric
      value (including <literal>NaN</>).  In order to allow
      <type>numeric</> values to be sorted and used in tree-based
      indexes, <productname>PostgreSQL</> treats <literal>NaN</>
      values as equal, and greater than all non-<literal>NaN</>
      values.
     </para>
    </note>

    <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 might 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 might or might
        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 might
     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>

    <indexterm>
     <primary>not a number</primary>
     <secondary>double precision</secondary>
    </indexterm>

    <para>
     In addition to ordinary numeric values, the floating-point types
     have several special values:
<literallayout>
<literal>Infinity</literal>
<literal>-Infinity</literal>
<literal>NaN</literal>
</literallayout>
     These represent the IEEE 754 special values
     <quote>infinity</quote>, <quote>negative infinity</quote>, and
     <quote>not-a-number</quote>, respectively.  (On a machine whose
     floating-point arithmetic does not follow IEEE 754, these values
     will probably not work as expected.)  When writing these values
     as constants in a SQL command, you must put quotes around them,
     for example <literal>UPDATE table SET x = 'Infinity'</>.  On input,
     these strings are recognized in a case-insensitive manner.
    </para>

    <note>
     <para>
      IEEE754 specifies that <literal>NaN</> should not compare equal
      to any other floating-point value (including <literal>NaN</>).
      In order to allow floating-point values to be sorted and used
      in tree-based indexes, <productname>PostgreSQL</> treats
      <literal>NaN</> values as equal, and greater than all
      non-<literal>NaN</> values.
     </para>
    </note>

    <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 might 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 NOT NULL DEFAULT nextval('<replaceable class="parameter">tablename</replaceable>_<replaceable class="parameter">colname</replaceable>_seq')
);
ALTER SEQUENCE <replaceable class="parameter">tablename</replaceable>_<replaceable class="parameter">colname</replaceable>_seq OWNED BY <replaceable class="parameter">tablename</replaceable>.<replaceable class="parameter">colname</replaceable>;
</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.)  Lastly, the sequence is marked as <quote>owned by</>
     the column, so that it will be dropped if the column or table is dropped.
    </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.
     You can drop the sequence without dropping the column, but this
     will force removal of the column default expression.
    </para>
   </sect2>
  </sect1>

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

   <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>

   <para>
    Since the output of this data type is locale-sensitive, it may not
    work to load <type>money</> data into a database that has a different
    setting of <varname>lc_monetary</>.  To avoid problems, before
    restoring a dump make sure <varname>lc_monetary</> has the same or
    equivalent value as in the database that was dumped.
   </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>8 bytes</entry>
        <entry>currency amount</entry>
        <entry>-92233720368547758.08 to +92233720368547758.07</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>

   <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 a short string (up to 126 bytes) is 1 byte
    plus the actual string, which includes the space padding in the case of
    <type>character</type>.  Longer strings have 4 bytes overhead instead
    of 1.  Long strings are compressed by the system automatically, so
    the physical requirement on disk might be less. Very long values are also
    stored in background tables so that they do not interfere with rapid
    access to 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 increased storage size when using the blank-padded
     type, and a few extra cycles to check the length when storing into
     a length-constrained column.  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. The
    database character set determines the character set used to store
    textual values; for more information on character set support,
    refer to <xref linkend="multibyte">.
   </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 might 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-byte 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>1 or 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 character strings by two
    characteristics: First, binary strings specifically allow storing
    octets of value zero and other <quote>non-printable</quote>
    octets (usually, octets outside the range 32 to 126).
    Character strings disallow zero octets, and also disallow any
    other octet values and sequences of octet values that are invalid
    according to the database's selected character set encoding.
    Second, operations on binary strings process the actual bytes,
    whereas the processing of character strings depends on locale settings.
    In short, binary strings are appropriate for storing data that the
    programmer thinks of as <quote>raw bytes</>, whereas character
    strings are appropriate for storing text.
   </para>

   <para>
    When entering <type>bytea</type> values, octets of certain
    values <emphasis>must</emphasis> be escaped (but all octet
    values <emphasis>can</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">
    shows the characters that must be escaped, and gives the alternative
    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>E'\\000'</literal></entry>
       <entry><literal>SELECT E'\\000'::bytea;</literal></entry>
       <entry><literal>\000</literal></entry>
      </row>

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

      <row>
       <entry>92</entry>
       <entry>backslash</entry>
       <entry><literal>E'\\\\'</literal> or <literal>E'\\134'</literal></entry>
       <entry><literal>SELECT E'\\\\'::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>E'\\<replaceable>xxx'</></literal> (octal value)</entry>
       <entry><literal>SELECT E'\\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 (assuming escape string
    syntax is used) and is therefore consumed, leaving the second backslash of the
    pair.  (Dollar-quoted strings can be used to avoid this level
    of escaping.)  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>E'\\001'</literal>
    becomes <literal>\001</literal> after passing through the
    escape string 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
    single-quote 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 E'\\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 E'\\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 E'\\176'::bytea;</literal></entry>
       <entry><literal>~</literal></entry>
      </row>

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

   <para>
    Depending on the front end to <productname>PostgreSQL</> you use,
    you might have additional work to do in terms of escaping and
    unescaping <type>bytea</type> strings. For example, you might 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 from
        <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">.  The operations available
    on these data types are described in
    <xref linkend="functions-datetime">.
   </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 / 14 digits</entry>
       </row>
       <row>
        <entry><type>date</type></entry>
        <entry>4 bytes</entry>
        <entry>dates only</entry>
        <entry>4713 BC</entry>
        <entry>5874897 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</entry>
        <entry>24:00:00</entry>
        <entry>1 microsecond / 14 digits</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+1459</entry>
        <entry>24:00:00-1459</entry>
        <entry>1 microsecond / 14 digits</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
    might 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.  The same
    compile-time option also determines whether <type>time</type> and
    <type>interval</type> values are stored as floating-point or eight-byte
    integers.  In the floating-point case, large <type>interval</type> values
    degrade in precision as the size of the interval increases.
   </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. You can also specify a date but it will
      be ignored, except when you use a time zone name that involves a
      daylight-savings rule, such as
      <literal>America/New_York</literal>. In this case specifying the date
      is required in order to determine whether standard or daylight-savings
      time applies.  The appropriate time zone offset is recorded in the
      <type>time with time zone</type> value.
     </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 &lt;= 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 abbreviation</entry>
         </row>
         <row>
          <entry><literal>2003-04-12 04:05:06 America/New_York</literal></entry>
          <entry>time zone specified by full 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>Abbreviation (for Pacific Standard Time)</entry>
         </row>
         <row>
          <entry><literal>America/New_York</literal></entry>
          <entry>Full time zone name</entry>
         </row>
         <row>
          <entry><literal>PST8PDT</literal></entry>
          <entry>POSIX-style time zone specification</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>

     <para>
     Refer to <xref linkend="datatype-timezones"> for more information on how
     to specify time zones.
    </para>
    </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 time zone,
      followed by an optional <literal>AD</literal> or <literal>BC</literal>.
      (Alternatively, <literal>AD</literal>/<literal>BC</literal> can appear
      before the time zone, but this is not the preferred ordering.)
      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>
      The <acronym>SQL</acronym> standard differentiates <type>timestamp without time zone</type> 
      and <type>timestamp with time zone</type> literals by the presence of a 
      <quote>+</quote> or <quote>-</quote>. Hence, according to the standard, 
      <programlisting>TIMESTAMP '2004-10-19 10:23:54'</programlisting>
      is a <type>timestamp without time zone</type>, while
      <programlisting>TIMESTAMP '2004-10-19 10:23:54+02'</programlisting>
      is a <type>timestamp with time zone</type>.
      <productname>PostgreSQL</productname> never examines the content of a
      literal string before determining its type, and therefore will treat
      both of the above as <type>timestamp without time zone</type>.  To
      ensure that a literal is treated as <type>timestamp with time
      zone</type>, give it the correct explicit type:
      <programlisting>TIMESTAMP WITH TIME ZONE '2004-10-19 10:23:54+02'</programlisting>
      In a literal that has been decided to be <type>timestamp without time
      zone</type>, <productname>PostgreSQL</productname> will silently ignore
      any time zone indication.
      That is, the resulting value is derived from the 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>microsecond</literal>,
      <literal>millisecond</literal>, <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 subsecond precision <replaceable>p</replaceable> should 
      be between 0 and 6, and defaults to the precision of the input literal.
     </para>

     <para>
      Internally <type>interval</> values are stored as months, days,
      and seconds. This is done because the number of days in a month
      varies, and a day can have 23 or 25 hours if a daylight savings
      time adjustment is involved. Because intervals are usually created
      from constant strings or <type>timestamp</> subtraction, this
      storage method works well in most cases. Functions
      <function>justify_days</> and <function>justify_hours</> are
      available for adjusting days and hours that overflow their normal
      periods.
     </para>
    </sect3>

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

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

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

     <para>
      <productname>PostgreSQL</productname> 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.
      (In particular, <literal>now</> and related strings are converted
      to a specific time value as soon as they are read.)
      All of these values need to be written in single quotes when used
      as constants in SQL commands.
     </para>

      <table id="datatype-datetime-special-table">
       <title>Special Date/Time Inputs</title>
       <tgroup cols="3">
        <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>

     <para>
      The following <acronym>SQL</acronym>-compatible functions can also
      be used to obtain the current time value 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 subsecond precision specification.  (See <xref 
      linkend="functions-datetime-current">.)  Note however that these are
      SQL functions and are <emphasis>not</> recognized as data input strings.
     </para>

    </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>week</literal> are converted to years and days and
     <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>seconds</> </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, particularly with
    respect to daylight-savings rules.
    <productname>PostgreSQL</productname> currently supports daylight-savings
    rules over the time period 1902 through 2038 (corresponding to the full
    range of conventional Unix system time).  Times outside that range are
    taken to be in <quote>standard time</> for the selected time zone, no
    matter what part of the year they fall in.
   </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 have little meaning unless 
        associated with a date as well as a time,
        since the offset can 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 therefore 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 compliance with the <acronym>SQL</acronym> standard).
     <productname>PostgreSQL</productname> assumes
     your local time zone for any type containing only date or time.
    </para>

    <para>
     All timezone-aware dates and times are stored internally in
     <acronym>UTC</acronym>.  They are converted to local time
     in the zone specified by the <xref linkend="guc-timezone"> configuration
     parameter before being displayed to the client.
    </para>

    <para>
     <productname>PostgreSQL</productname> allows you to specify time zones in
     three different forms:
     <itemizedlist>
      <listitem>
       <para>
        A full time zone name, for example <literal>America/New_York</>.
        The recognized time zone names are listed in the
        <literal>pg_timezone_names</literal> view (see <xref
        linkend="view-pg-timezone-names">).
        <productname>PostgreSQL</productname> uses the widely-used
        <literal>zic</> time zone data for this purpose, so the same
        names are also recognized by much other software.
       </para>
      </listitem>
      <listitem>
       <para>
        A time zone abbreviation, for example <literal>PST</>.  Such a
        specification merely defines a particular offset from UTC, in
        contrast to full time zone names which might imply a set of daylight
        savings transition-date rules as well.  The recognized abbreviations
        are listed in the <literal>pg_timezone_abbrevs</> view (see <xref
        linkend="view-pg-timezone-abbrevs">).  You cannot set the
        configuration parameters <xref linkend="guc-timezone"> or
        <xref linkend="guc-log-timezone"> using a time
        zone abbreviation, but you can use abbreviations in
        date/time input values and with the <literal>AT TIME ZONE</>
        operator.
       </para>
      </listitem>
      <listitem>
       <para>
        In addition to the timezone names and abbreviations,
        <productname>PostgreSQL</productname> will accept POSIX-style time zone
        specifications of the form <replaceable>STD</><replaceable>offset</> or
        <replaceable>STD</><replaceable>offset</><replaceable>DST</>, where
        <replaceable>STD</> is a zone abbreviation, <replaceable>offset</> is a
        numeric offset in hours west from UTC, and <replaceable>DST</> is an
        optional daylight-savings zone abbreviation, assumed to stand for one
        hour ahead of the given offset. For example, if <literal>EST5EDT</>
        were not already a recognized zone name, it would be accepted and would
        be functionally equivalent to USA East Coast time.  When a
        daylight-savings zone name is present, it is assumed to be used
        according to the same daylight-savings transition rules used in the
        <literal>zic</> time zone database's <filename>posixrules</> entry.
        In a standard <productname>PostgreSQL</productname> installation,
        <filename>posixrules</> is the same as <literal>US/Eastern</>, so
        that POSIX-style time zone specifications follow USA daylight-savings
        rules.  If needed, you can adjust this behavior by replacing the
        <filename>posixrules</> file.
       </para>
      </listitem>
     </itemizedlist>

     There is a conceptual and practical difference between the abbreviations
     and the full names: abbreviations always represent a fixed offset from
     UTC, whereas most of the full names imply a local daylight-savings time
     rule and so have two possible UTC offsets.
    </para>

    <para>
     One should be wary that the POSIX-style time zone feature can
     lead to silently accepting bogus input, since there is no check on the
     reasonableness of the zone abbreviations.  For example, <literal>SET
     TIMEZONE TO FOOBAR0</> will work, leaving the system effectively using
     a rather peculiar abbreviation for UTC.
     Another issue to keep in mind is that in POSIX time zone names,
     positive offsets are used for locations <emphasis>west</> of Greenwich.
     Everywhere else, <productname>PostgreSQL</productname> follows the
     ISO-8601 convention that positive timezone offsets are <emphasis>east</>
     of Greenwich.
    </para>

    <para>
     In all cases, timezone names are recognized case-insensitively.
     (This is a change from <productname>PostgreSQL</productname> versions
     prior to 8.2, which were case-sensitive in some contexts and not others.)
    </para>

    <para>
     Neither full names nor abbreviations are hard-wired into the server;
     they are obtained from configuration files stored under
     <filename>.../share/timezone/</> and <filename>.../share/timezonesets/</>
     of the installation directory
     (see <xref linkend="datetime-config-files">).
    </para>

    <para>
     The <xref linkend="guc-timezone"> configuration parameter can
     be set in the file <filename>postgresql.conf</>, or in any of the
     other standard ways described in <xref linkend="runtime-config">.
     There are also several special ways to set it:

     <itemizedlist>
      <listitem>
       <para>
        If <varname>timezone</> is not specified in
        <filename>postgresql.conf</> nor as a server command-line option,
        the server attempts to use the value of the <envar>TZ</envar>
        environment variable as the default time zone.  If <envar>TZ</envar>
        is not defined or is not any of the time zone names known to
        <productname>PostgreSQL</productname>, the server attempts to
        determine the operating system's default time zone by checking the
        behavior of the C library function <literal>localtime()</>.  The
        default time zone is selected as the closest match among
        <productname>PostgreSQL</productname>'s known time zones.
        (These rules are also used to choose the default value of
        <xref linkend="guc-log-timezone">, if it is not specified.)
       </para>
      </listitem>

      <listitem>
       <para>
        The <acronym>SQL</acronym> command <command>SET TIME ZONE</command>
        sets the time zone for the session.  This is an alternative spelling
        of <command>SET TIMEZONE TO</> with a more SQL-spec-compatible syntax.
       </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>
     </itemizedlist>
    </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>
    Leading and trailing whitespace is ignored. 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>

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

  <sect1 id="datatype-enum">
   <title>Enumerated Types</title>

   <indexterm zone="datatype-enum">
    <primary>data type</primary>
    <secondary>enumerated (enum)</secondary>
   </indexterm>

   <para>
    Enumerated (enum) types are data types that
    are comprised of a static, predefined set of values with a
    specific order. They are equivalent to the <type>enum</type>
    types in a number of programming languages. An example of an enum
    type might be the days of the week, or a set of status values for
    a piece of data.
   </para>

   <sect2>
    <title>Declaration of Enumerated Types</title>

    <para>
     Enum types are created using the <xref
     linkend="sql-createtype" endterm="sql-createtype-title"> command,
     for example:

<programlisting>
CREATE TYPE mood AS ENUM ('sad', 'ok', 'happy');
</programlisting>

     Once created, the enum type can be used in table and function
     definitions much like any other type:
    </para>

    <example>
     <title>Basic Enum Usage</title>
<programlisting>
CREATE TYPE mood AS ENUM ('sad', 'ok', 'happy');
CREATE TABLE person (
    name text,
    current_mood mood
);
INSERT INTO person VALUES ('Moe', 'happy');
SELECT * FROM person WHERE current_mood = 'happy';
 name | current_mood 
------+--------------
 Moe  | happy
(1 row)
</programlisting>
     </example>
    </sect2>

    <sect2>
     <title>Ordering</title>

     <para>
      The ordering of the values in an enum type is the
      order in which the values were listed when the type was declared.
      All standard comparison operators and related
      aggregate functions are supported for enums.  For example:
     </para>

     <example>
      <title>Enum Ordering</title>
<programlisting>
INSERT INTO person VALUES ('Larry', 'sad');
INSERT INTO person VALUES ('Curly', 'ok');
SELECT * FROM person WHERE current_mood > 'sad';
 name  | current_mood 
-------+--------------
 Moe   | happy
 Curly | ok
(2 rows)

SELECT * FROM person WHERE current_mood > 'sad' ORDER BY current_mood;
 name  | current_mood 
-------+--------------
 Curly | ok
 Moe   | happy
(2 rows)

SELECT name FROM person
  WHERE current_mood = (SELECT MIN(current_mood) FROM person);
 name  
-------
 Larry
(1 row)
</programlisting>
    </example>
   </sect2>

   <sect2>
    <title>Type Safety</title>

    <para>
     Enumerated types are completely separate data types and may not
     be compared with each other.
    </para>

    <example>
     <title>Lack of Casting</title>
<programlisting>
CREATE TYPE happiness AS ENUM ('happy', 'very happy', 'ecstatic');
CREATE TABLE holidays (                                           
    num_weeks int,
    happiness happiness
);
INSERT INTO holidays(num_weeks,happiness) VALUES (4, 'happy');
INSERT INTO holidays(num_weeks,happiness) VALUES (6, 'very happy');
INSERT INTO holidays(num_weeks,happiness) VALUES (8, 'ecstatic');
INSERT INTO holidays(num_weeks,happiness) VALUES (2, 'sad');
ERROR:  invalid input value for enum happiness: "sad"
SELECT person.name, holidays.num_weeks FROM person, holidays
  WHERE person.current_mood = holidays.happiness;
ERROR:  operator does not exist: mood = happiness
</programlisting>
    </example>

    <para>
     If you really need to do something like that, you can either
     write a custom operator or add explicit casts to your query:
    </para>

    <example>
     <title>Comparing Different Enums by Casting to Text</title>
<programlisting>
SELECT person.name, holidays.num_weeks FROM person, holidays
  WHERE person.current_mood::text = holidays.happiness::text;
 name | num_weeks 
------+-----------
 Moe  |         4
(1 row)

</programlisting>
    </example>
   </sect2>

   <sect2>
    <title>Implementation Details</title>
    
    <para>
     An enum value occupies four bytes on disk.  The length of an enum
     value's textual label is limited by the <symbol>NAMEDATALEN</symbol>
     setting compiled into <productname>PostgreSQL</productname>; in standard
     builds this means at most 63 bytes.
    </para>

    <para>
     Enum labels are case sensitive, so
     <type>'happy'</type> is not the same as <type>'HAPPY'</type>.
     Spaces in the labels are significant, too.
    </para>

   </sect2>
  </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>&lt;(x,y),r&gt; (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> 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 any two 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.
    </para>
   </sect2>

   <sect2>
    <title>Paths</title>

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

    <para>
     Paths are represented by lists of connected points. Paths can be
     <firstterm>open</firstterm>, where
     the first and last points in the list are not considered connected, or
     <firstterm>closed</firstterm>,
     where the first and last points are considered connected.
    </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 lists of points (the vertexes of the
     polygon). 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, as shown in <xref linkend="datatype-net-types-table">.  It
    is preferable to use these types instead of plain text types to store
    network addresses, because
    these types offer input error checking and several specialized
    operators and functions (see <xref linkend="functions-net">).
   </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>7 or 19 bytes</entry>
        <entry>IPv4 and IPv6 networks</entry>
       </row>

       <row>
        <entry><type>inet</type></entry>
        <entry>7 or 19 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 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, so 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 forms shown.
    </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>

   <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-bitstring">.
   </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>

   <para>
    A bit string value requires 1 byte for each group of 8 bits, plus
    5 or 8 bytes overhead depending on the length of the string
    (but long values may be compressed or moved out-of-line, as explained
    in <xref linkend="datatype-character"> for character strings).
   </para>
  </sect1>

  <sect1 id="datatype-textsearch">
   <title>Text Search Types</title>

   <indexterm zone="datatype-textsearch">
    <primary>full text search</primary>
    <secondary>data types</secondary>
   </indexterm>

   <indexterm zone="datatype-textsearch">
    <primary>text search</primary>
    <secondary>data types</secondary>
   </indexterm>

   <para>
    <productname>PostgreSQL</productname> provides two data types that
    are designed to support full text search, which is the activity of
    searching through a collection of natural-language <firstterm>documents</>
    to locate those that best match a <firstterm>query</>.
    The <type>tsvector</type> type represents a document in a form suited
    for text search, while the <type>tsquery</type> type similarly represents
    a query.
    <xref linkend="textsearch"> provides a detailed explanation of this
    facility, and <xref linkend="functions-textsearch"> summarizes the
    related functions and operators.
   </para>

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

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

    <para>
     A <type>tsvector</type> value is a sorted list of distinct
     <firstterm>lexemes</>, which are words that have been
     <firstterm>normalized</> to make different variants of the same word look
     alike (see <xref linkend="textsearch"> for details).  Sorting and
     duplicate-elimination are done automatically during input, as shown in
     this example:

<programlisting>
SELECT 'a fat cat sat on a mat and ate a fat rat'::tsvector;
                      tsvector
----------------------------------------------------
 'a' 'on' 'and' 'ate' 'cat' 'fat' 'mat' 'rat' 'sat'
</programlisting>

     (As the example shows, the sorting is first by length and then
     alphabetically, but that detail is seldom important.)  To represent
     lexemes containing whitespace, surround them with quotes:

<programlisting>
SELECT $$the lexeme '    ' contains spaces$$::tsvector;
                 tsvector                  
-------------------------------------------
 'the' '    ' 'lexeme' 'spaces' 'contains'
</programlisting>

     (We use dollar-quoted string literals in this example and the next one,
     to avoid confusing matters by having to double quote marks within the
     literals.)  Embedded quotes can be handled by doubling them:

<programlisting>
SELECT $$the lexeme 'Joe''s' contains a quote$$::tsvector;
                    tsvector                    
------------------------------------------------
 'a' 'the' 'Joe''s' 'quote' 'lexeme' 'contains'
</programlisting>

     Optionally, integer <firstterm>position(s)</>
     can be attached to any or all of the lexemes:

<programlisting>
SELECT 'a:1 fat:2 cat:3 sat:4 on:5 a:6 mat:7 and:8 ate:9 a:10 fat:11 rat:12'::tsvector;
                                  tsvector
-------------------------------------------------------------------------------
 'a':1,6,10 'on':5 'and':8 'ate':9 'cat':3 'fat':2,11 'mat':7 'rat':12 'sat':4
</programlisting>

     A position normally indicates the source word's location in the
     document.  Positional information can be used for
     <firstterm>proximity ranking</firstterm>.  Position values can
     range from 1 to 16383; larger numbers are silently clamped to 16383.
     Duplicate position entries are discarded.
    </para>

    <para>
     Lexemes that have positions can further be labeled with a
     <firstterm>weight</>, which can be <literal>A</literal>,
     <literal>B</literal>, <literal>C</literal>, or <literal>D</literal>.
     <literal>D</literal> is the default and hence is not shown on output:

<programlisting>
SELECT 'a:1A fat:2B,4C cat:5D'::tsvector;
          tsvector          
----------------------------
 'a':1A 'cat':5 'fat':2B,4C
</programlisting>

     Weights are typically used to reflect document structure, for example
     by marking title words differently from body words.  Text search
     ranking functions can assign different priorities to the different
     weight markers.
    </para>

    <para>
     It is important to understand that the
     <type>tsvector</type> type itself does not perform any normalization;
     it assumes that the words it is given are normalized appropriately
     for the application.  For example,

<programlisting>
select 'The Fat Rats'::tsvector;
      tsvector      
--------------------
 'Fat' 'The' 'Rats'
</programlisting>

     For most English-text-searching applications the above words would
     be considered non-normalized, but <type>tsvector</type> doesn't care.
     Raw document text should usually be passed through
     <function>to_tsvector</> to normalize the words appropriately
     for searching:

<programlisting>
SELECT to_tsvector('english', 'The Fat Rats');         
   to_tsvector   
-----------------
 'fat':2 'rat':3
</programlisting>

     Again, see <xref linkend="textsearch"> for more detail.
    </para>

   </sect2>

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

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

    <para>
     A <type>tsquery</type> value stores lexemes that are to be
     searched for, and combines them using the boolean operators
     <literal>&amp;</literal> (AND), <literal>|</literal> (OR), and
     <literal>!</> (NOT).  Parentheses can be used to enforce grouping
     of the operators:

<programlisting>
 SELECT 'fat &amp; rat'::tsquery;
    tsquery    
---------------
 'fat' &amp; 'rat'

SELECT 'fat &amp; (rat | cat)'::tsquery;
          tsquery          
---------------------------
 'fat' &amp; ( 'rat' | 'cat' )

SELECT 'fat &amp; rat &amp; ! cat'::tsquery;
        tsquery         
------------------------
 'fat' &amp; 'rat' &amp; !'cat'
</programlisting>

     In the absence of parentheses, <literal>!</> (NOT) binds most tightly,
     and <literal>&amp;</literal> (AND) binds more tightly than
     <literal>|</literal> (OR).
    </para>

    <para>
     Optionally, lexemes in a <type>tsquery</type> can be labeled with
     one or more weight letters, which restricts them to match only
     <type>tsvector</> lexemes with one of those weights:

<programlisting>
SELECT 'fat:ab &amp; cat'::tsquery;
    tsquery
------------------
 'fat':AB &amp; 'cat'
</programlisting>
    </para>

    <para>
     Quoting rules for lexemes are the same as described above for
     lexemes in <type>tsvector</>; and, as with <type>tsvector</>,
     any required normalization of words must be done before putting
     them into the <type>tsquery</> type.  The <function>to_tsquery</>
     function is convenient for performing such normalization:

<programlisting>
SELECT to_tsquery('Fat:ab & Cats');
    to_tsquery    
------------------
 'fat':AB & 'cat'
</programlisting>
    </para>

   </sect2>

  </sect1>

  <sect1 id="datatype-uuid">
   <title><acronym>UUID</acronym> Type</title>

   <indexterm zone="datatype-uuid">
    <primary>UUID</primary>
   </indexterm>

   <para>
    The data type <type>uuid</type> stores Universally Unique Identifiers
    (UUID) as defined by RFC 4122, ISO/IEC 9834-8:2005, and related standards.
    (Some systems refer to this data type as globally unique identifier, or
    GUID,<indexterm><primary>GUID</primary></indexterm> instead.)  Such an
    identifier is a 128-bit quantity that is generated by an algorithm chosen
    to make it very unlikely that the same identifier will be generated by
    anyone else in the known universe using the same algorithm.  Therefore,
    for distributed systems, these identifiers provide a better uniqueness
    guarantee than that which can be achieved using sequence generators, which
    are only unique within a single database.
   </para>

   <para>
    A UUID is written as a sequence of lower-case hexadecimal digits,
    in several groups separated by hyphens, specifically a group of 8
    digits followed by three groups of 4 digits followed by a group of
    12 digits, for a total of 32 digits representing the 128 bits.  An
    example of a UUID in this standard form is:
<programlisting>
a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a11
</programlisting>
    <productname>PostgreSQL</productname> also accepts the following
    alternative forms for input:
    use of upper-case digits, the standard format surrounded by
    braces, and omitting the hyphens.  Examples are:
<programlisting>
A0EEBC99-9C0B-4EF8-BB6D-6BB9BD380A11
{a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a11}
a0eebc999c0b4ef8bb6d6bb9bd380a11
</programlisting>
    Output is always in the standard form.
   </para>

   <para>
    <productname>PostgreSQL</productname> provides storage and comparison
    functions for UUIDs, but the core database does not include any
    function for generating UUIDs, because no single algorithm is well
    suited for every application.  The contrib module
    <filename>contrib/uuid-ossp</filename> provides functions that implement 
    several standard algorithms.
    Alternatively, UUIDs could be generated by client applications or
    other libraries invoked through a server-side function.
   </para>
  </sect1>

  <sect1 id="datatype-xml">
   <title><acronym>XML</> Type</title>

   <indexterm zone="datatype-xml">
    <primary>XML</primary>
   </indexterm>

   <para>
    The data type <type>xml</type> can be used to store XML data.  Its
    advantage over storing XML data in a <type>text</type> field is that it
    checks the input values for well-formedness, and there are support
    functions to perform type-safe operations on it; see <xref
    linkend="functions-xml">.  Use of this data type requires the
    installation to have been built with <command>configure 
    --with-libxml</>.
   </para>

   <para>
    The <type>xml</type> type can store well-formed
    <quote>documents</quote>, as defined by the XML standard, as well
    as <quote>content</quote> fragments, which are defined by the
    production <literal>XMLDecl? content</literal> in the XML
    standard.  Roughly, this means that content fragments can have
    more than one top-level element or character node.  The expression
    <literal><replaceable>xmlvalue</replaceable> IS DOCUMENT</literal>
    can be used to evaluate whether a particular <type>xml</type>
    value is a full document or only a content fragment.
   </para>

   <sect2>
    <title>Creating XML Values</title>
   <para>
    To produce a value of type <type>xml</type> from character data,
    use the function
    <function>xmlparse</function>:<indexterm><primary>xmlparse</primary></indexterm>
<synopsis>
XMLPARSE ( { DOCUMENT | CONTENT } <replaceable>value</replaceable>)
</synopsis>
    Examples:
<programlisting><![CDATA[
XMLPARSE (DOCUMENT '<?xml version="1.0"?><book><title>Manual</title><chapter>...</chapter><book>')
XMLPARSE (CONTENT 'abc<foo>bar</foo><bar>foo</bar>')
]]></programlisting>
    While this is the only way to convert character strings into XML
    values according to the SQL standard, the PostgreSQL-specific
    syntaxes:
<programlisting><![CDATA[
xml '<foo>bar</foo>'
'<foo>bar</foo>'::xml
]]></programlisting>
    can also be used.
   </para>

   <para>
    The <type>xml</type> type does not validate its input values
    against a possibly included document type declaration
    (DTD).<indexterm><primary>DTD</primary></indexterm>
   </para>

   <para>
    The inverse operation, producing character string type values from
    <type>xml</type>, uses the function
    <function>xmlserialize</function>:<indexterm><primary>xmlserialize</primary></indexterm>
<synopsis>
XMLSERIALIZE ( { DOCUMENT | CONTENT } <replaceable>value</replaceable> AS <replaceable>type</replaceable> )
</synopsis>
    <replaceable>type</replaceable> can be one of
    <type>character</type>, <type>character varying</type>, or
    <type>text</type> (or an alias name for those).  Again, according
    to the SQL standard, this is the only way to convert between type
    <type>xml</type> and character types, but PostgreSQL also allows
    you to simply cast the value.
   </para>

   <para>
    When character string values are cast to or from type
    <type>xml</type> without going through <type>XMLPARSE</type> or
    <type>XMLSERIALIZE</type>, respectively, the choice of
    <literal>DOCUMENT</literal> versus <literal>CONTENT</literal> is
    determined by the <quote>XML option</quote>
    <indexterm><primary>XML option</primary></indexterm>
    session configuration parameter, which can be set using the
    standard command
<synopsis>
SET XML OPTION { DOCUMENT | CONTENT };
</synopsis>
    or the more PostgreSQL-like syntax
<synopsis>
SET xmloption TO { DOCUMENT | CONTENT };
</synopsis>
    The default is <literal>CONTENT</literal>, so all forms of XML
    data are allowed.
   </para>
   </sect2>

   <sect2>
    <title>Encoding Handling</title>
   <para>
    Care must be taken when dealing with multiple character encodings
    on the client, server, and in the XML data passed through them.
    When using the text mode to pass queries to the server and query
    results to the client (which is the normal mode), PostgreSQL
    converts all character data passed between the client and the
    server and vice versa to the character encoding of the respective
    end; see <xref linkend="multibyte">.  This includes string
    representations of XML values, such as in the above examples.
    This would ordinarily mean that encoding declarations contained in
    XML data might become invalid as the character data is converted
    to other encodings while travelling between client and server,
    while the embedded encoding declaration is not changed.  To cope
    with this behavior, an encoding declaration contained in a
    character string presented for input to the <type>xml</type> type
    is <emphasis>ignored</emphasis>, and the content is always assumed
    to be in the current server encoding.  Consequently, for correct
    processing, such character strings of XML data must be sent off
    from the client in the current client encoding.  It is the
    responsibility of the client to either convert the document to the
    current client encoding before sending it off to the server or to
    adjust the client encoding appropriately.  On output, values of
    type <type>xml</type> will not have an encoding declaration, and
    clients must assume that the data is in the current client
    encoding.
   </para>

   <para>
    When using the binary mode to pass query parameters to the server
    and query results back to the client, no character set conversion
    is performed, so the situation is different.  In this case, an
    encoding declaration in the XML data will be observed, and if it
    is absent, the data will be assumed to be in UTF-8 (as required by
    the XML standard; note that PostgreSQL does not support UTF-16 at
    all).  On output, data will have an encoding declaration
    specifying the client encoding, unless the client encoding is
    UTF-8, in which case it will be omitted.
   </para>

   <para>
    Needless to say, processing XML data with PostgreSQL will be less
    error-prone and more efficient if data encoding, client encoding,
    and server encoding are the same.  Since XML data is internally
    processed in UTF-8, computations will be most efficient if the
    server encoding is also UTF-8.
   </para>
   </sect2>

   <sect2>
   <title>Accessing XML Values</title>

   <para>
    The <type>xml</type> data type is unusual in that it does not
    provide any comparison operators.  This is because there is no
    well-defined and universally useful comparison algorithm for XML
    data.  One consequence of this is that you cannot retrieve rows by
    comparing an <type>xml</type> column against a search value.  XML
    values should therefore typically be accompanied by a separate key
    field such as an ID.  An alternative solution for comparing XML
    values is to convert them to character strings first, but note
    that character string comparison has little to do with a useful
    XML comparison method.
   </para>

   <para>
    Since there are no comparison operators for the <type>xml</type>
    data type, it is not possible to create an index directly on a
    column of this type.  If speedy searches in XML data are desired,
    possible workarounds would be casting the expression to a
    character string type and indexing that, or indexing an XPath
    expression.  The actual query would of course have to be adjusted
    to search by the indexed expression.
   </para>

   <para>
    The text-search functionality in PostgreSQL could also be used to speed
    up full-document searches in XML data.  The necessary
    preprocessing support is, however, not available in the PostgreSQL
    distribution in this release.
   </para>
   </sect2>
  </sect1>

  &array;

  &rowtypes;

  <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>regconfig</primary>
   </indexterm>

   <indexterm zone="datatype-oid">
    <primary>regdictionary</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.  OIDs are not added to user-created tables, unless
    <literal>WITH OIDS</literal> is specified when the table is
    created, or the <xref linkend="guc-default-with-oids">
    configuration variable is enabled.  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</>,
    <type>regtype</>, <type>regconfig</>, and <type>regdictionary</>.
    <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>

   <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,
    to examine the <structname>pg_attribute</> rows related to a table
    <literal>mytable</>, one could write:
<programlisting>
SELECT * FROM pg_attribute WHERE attrelid = 'mytable'::regclass;
</programlisting>
    rather than:
<programlisting>
SELECT * FROM pg_attribute
  WHERE attrelid = (SELECT oid FROM pg_class WHERE relname = 'mytable');
</programlisting>
    While that doesn't look all that bad by itself, it's still oversimplified.
    A far more complicated sub-select would be needed to
    select the right OID if there are multiple tables named
    <literal>mytable</> in different schemas.
    The <type>regclass</> input converter handles the table lookup according
    to the schema path setting, and so it does the <quote>right thing</>
    automatically.  Similarly, casting a table's OID to
    <type>regclass</> is handy for symbolic display of a numeric OID.
   </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>

       <row>
        <entry><type>regconfig</></entry>
        <entry><structname>pg_ts_config</></entry>
        <entry>text search configuration</entry>
        <entry><literal>english</></entry>
       </row>

       <row>
        <entry><type>regdictionary</></entry>
        <entry><structname>pg_ts_dict</></entry>
        <entry>text search dictionary</entry>
        <entry><literal>simple</></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>
    An additional property of the OID alias types is that if a
    constant of one of these types appears in a stored expression
    (such as a column default expression or view), it creates a dependency
    on the referenced object.  For example, if a column has a default
    expression <literal>nextval('my_seq'::regclass)</>,
    <productname>PostgreSQL</productname>
    understands that the default expression depends on the sequence
    <literal>my_seq</>; the system will not let the sequence be dropped
    without first removing the default expression.
   </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>anyelement</primary>
   </indexterm>

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

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

   <indexterm zone="datatype-pseudo">
    <primary>anyenum</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>anyenum</></entry>
        <entry>Indicates that a function accepts any enum data type
        (see <xref linkend="extend-types-polymorphic"> and
        <xref linkend="datatype-enum">).</entry>
       </row>

       <row>
        <entry><type>anynonarray</></entry>
        <entry>Indicates that a function accepts any non-array 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) can 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 can 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</>,
    <type>anyelement</>, <type>anyenum</>, and <type>anynonarray</>.
   </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>
