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 <chapter id="datatype">
  <title id="datatype-title">Data Types</title>

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

  <indexterm>
   <primary>types</primary>
   <see>data types</see>
  </indexterm>

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

  <para>
   <xref linkend="datatype-table"> shows all general-purpose data types
   available to users.  Most of the alternative names listed in the
   <quote>Aliases</quote> column are the names used internally by
   <productname>Postgres</productname> for historical reasons.  In
   addition, some internally used or deprecated types are available,
   but they are not documented here.  Many of the built-in types have
   obvious external formats. However, several types are either unique
   to <productname>Postgres</productname>, such as open and closed
   paths, or have several possibilities for formats, such as the date
   and time types.
  </para>

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

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

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

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

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

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

      <row>
       <entry><type>box</type></entry>
       <entry></entry>
       <entry>rectangular box in 2D plane</entry>
      </row>

      <row>
       <entry><type>bytea</type></entry>
       <entry></entry>
       <entry>binary data</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>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>cidr</type></entry>
       <entry></entry>
       <entry>IP network address</entry>
      </row>

      <row>
       <entry><type>circle</type></entry>
       <entry></entry>
       <entry>circle in 2D 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>IP 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</type></entry>
       <entry></entry>
       <entry>general-use time span</entry>
      </row>

      <row>
       <entry><type>line</type></entry>
       <entry></entry>
       <entry>infinite line in 2D plane</entry>
      </row>

      <row>
       <entry><type>lseg</type></entry>
       <entry></entry>
       <entry>line segment in 2D 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>US-style currency</entry>
      </row>

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

      <row>
       <entry><type>oid</type></entry>
       <entry></entry>
       <entry>object identifier</entry>
      </row>

      <row>
       <entry><type>path</type></entry>
       <entry></entry>
       <entry>open and closed geometric path in 2D plane</entry>
      </row>

      <row>
       <entry><type>point</type></entry>
       <entry></entry>
       <entry>geometric point in 2D plane</entry>
      </row>

      <row>
       <entry><type>polygon</type></entry>
       <entry></entry>
       <entry>closed geometric path in 2D 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 [ without time zone ]</type></entry>
       <entry></entry>
       <entry>time of day</entry>
      </row>

      <row>
       <entry><type>time with time zone</type></entry>
       <entry>timetz</entry>
       <entry>time of day, including time zone</entry>
      </row>

      <row>
       <entry><type>timestamp without time zone</type></entry>
       <entry>timestamp</entry>
       <entry>date and time</entry>
      </row>

      <row>
       <entry><type>timestamp [ with time zone ]</type></entry>
       <entry>timestamptz</entry>
       <entry>date and time</entry>
      </row>
     </tbody>
    </tgroup>
   </table>
  </para>

  <note>
   <title>Compatibility</title>
   <para>
    The following types (or spellings thereof) are specified by SQL:
    <type>bit</type>, <type>bit varying</type>, <type>boolean</type>,
    <type>char</type>, <type>character</type>, <type>character
    varying</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>,
    <type>timestamp</type> (both with or without time zone).
   </para>
  </note>

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

  <para>
   Some of the input and output functions are not invertible.  That is,
   the result of an output function may lose precision when compared to
   the original input.
  </para>

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

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

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

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

   <indexterm zone="datatype-numeric">
    <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>

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

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

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

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

   <para>
    Numeric types consist of two-, four-, and eight-byte integers,
    four- and eight-byte
    floating point numbers and fixed-precision decimals.
   </para>

   <para>
    <table tocentry="1">
     <title>Numeric Types</title>
     <tgroup cols="4">
      <thead>
       <row>
	<entry>Type 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>Fixed-precision</entry>
	<entry>-32768 to +32767</entry>
       </row>
       <row>
	<entry><type>integer</></entry>
	<entry>4 bytes</entry>
	<entry>Usual choice for fixed-precision</entry>
	<entry>-2147483648 to +2147483647</entry>
       </row>
       <row>
	<entry><type>bigint</></entry>
	<entry>8 bytes</entry>
	<entry>Very large range fixed-precision</entry>
	<entry>about 18 decimal digits</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>autoincrementing integer</entry>
	<entry>1 to 9223372036854775807</entry>
       </row>
      </tbody>
     </tgroup>
    </table>
   </para>

   <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>The Integer Types</title>

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

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

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

    <note>
     <para>
      If you have a column of type <type>smallint</type> or
      <type>bigint</type> with an index, you may encounter problems
      getting the system to use that index.  For instance, a clause of
      the form
<programlisting>
... WHERE smallint_column = 42
</programlisting>
      will not use an index, because the system assigns type
      <type>integer</type> to the 42, and PostgreSQL currently cannot
      use an index when two different data types are involved.  A
      workaround is to single-quote the constant, thus:
<programlisting>
... WHERE smallint_column = '42'
</programlisting>
      This will cause the system to delay the type resolution and will
      assign the right type to the constant.
     </para>
    </note>

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

   </sect2>

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

    <para>
     The type <type>numeric</type> can store numbers of practically
     unlimited size and precision, while being able to store all
     numbers and carry out all calculations exactly.  It is especially
     recommended for storing monetary amounts and other quantities
     where exactness is required.  However, the <type>numeric</type>
     type is very slow compared to the floating point types described
     in the next section.
    </para>

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

    <para>
     Both the precision and the scale of the numeric type can be
     configured.  To declare a column of type <type>numeric</type> use
     the syntax
<programlisting>
NUMERIC(<replaceable>precision</replaceable>, <replaceable>scale</replaceable>)
</programlisting>
     The precision must be positive, the scale zero or positive.
     Alternatively,
<programlisting>
NUMERIC(<replaceable>precision</replaceable>)
</programlisting>
     selects a scale of 0.  Merely specifying
<programlisting>
NUMERIC
</programlisting>
     uses a default precision and scale, which is currently (30,6).
     (The SQL standard requires a default scale of 0.  We find this a
     bit useless.  If you're concerned about portability, always
     specify the precision and scale explicitly.)
    </para>

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

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


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

    <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> 754
     binary floating point (single and double precision,
     respectively), to the extent that the underlying processor,
     operating system, and compiler support it.
    </para>

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

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

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

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

   </sect2>

   <sect2 id="datatype-serial">
    <title>The 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>sequences</primary>
     <secondary>and serial type</secondary>
    </indexterm>

    <para>
     The <type>serial</type> data types are not truly types, but are a
     notational convenience for setting up unique identifier columns
     in tables.
     In the current implementation, specifying

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

     is equivalent to specifying:

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

     Thus, we have created an integer column and arranged for its default
     values to be assigned from a sequence generator.  UNIQUE and NOT NULL
     constraints are applied to ensure that explicitly-inserted values
     will never be duplicates, either.
    </para>

     <important>
      <para>
       The implicit sequence created for the <type>serial</type> type will
       <emphasis>not</emphasis> be automatically removed when the
       table is dropped.
      </para>
     </important>

    <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
     use of more than 2^31 identifiers over the lifetime of the table.
    </para>

    <para>
     Implicit sequences supporting the <type>serial</type> types are
     not automatically dropped when a table containing a serial type
     is dropped. So, the following commands executed in order will likely fail:

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

     The sequence will remain in the database until explicitly dropped using
     <command>DROP SEQUENCE</command>.  (This annoyance will probably be
     changed in some future release.)
    </para>
   </sect2>
  </sect1>

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

   <note>
    <title>Deprecated</title>
    <para>
     The <type>money</type> type is deprecated. Use
     <type>numeric</type> or <type>decimal</type> instead, in
     combination with the <function>to_char</function> function.  The
     money type may become a locale-aware layer over the
     <type>numeric</type> type in a future release.
    </para>
   </note>

   <para>
    The <type>money</type> type stores U.S.-style currency with fixed
    decimal point representation.  If
    <productname>Postgres</productname> is compiled with locale
    support then the <type>money</type> type uses locale-specific
    output formatting.
   </para>

   <para>
    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 in the latter form.
   </para>

   <para>
    <table tocentry="1">
     <title>Monetary Types</title>
     <tgroup cols="4">
      <thead>
       <row>
	<entry>Type Name</entry>
	<entry>Storage</entry>
	<entry>Description</entry>
	<entry>Range</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry>money</entry>
	<entry>4 bytes</entry>
	<entry>Fixed-precision</entry>
	<entry>-21474836.48 to +21474836.47</entry>
       </row>
      </tbody>
     </tgroup>
    </table>
   </para>
  </sect1>


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

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

   <indexterm>
    <primary>strings</primary>
    <see>character strings</see>
   </indexterm>

   <indexterm>
    <primary>text</primary>
    <see>character strings</see>
   </indexterm>

    <table tocentry="1">
     <title>Character Types</title>
     <tgroup cols="2">
      <thead>
       <row>
	<entry>Type name</entry>
	<entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry><type>character(<replaceable>n</>)</type>, <type>char(<replaceable>n</>)</type></entry>
	<entry>Fixed-length blank padded</entry>
       </row>
       <row>
	<entry><type>character varying(<replaceable>n</>)</type>, <type>varchar(<replaceable>n</>)</type></entry>
	<entry>Variable-length with limit</entry>
       </row>
       <row>
	<entry><type>text</type></entry>
	<entry>Variable unlimited length</entry>
       </row>
     </tbody>
     </tgroup>
    </table>

   <para>
    <acronym>SQL</acronym> defines two primary character types:
    <type>character(<replaceable>n</>)</type> and <type>character
    varying(<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 SQL 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>

   <note>
    <para>
     Prior to PostgreSQL 7.2, strings that were too long were silently
     truncated, no error was raised.
    </para>
   </note>

   <para>
    The notations <type>char(<replaceable>n</>)</type> and
    <type>varchar(<replaceable>n</>)</type> are aliases for
    <type>character(<replaceable>n</>)</type> and <type>character
    varying(<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 PostgreSQL extension.
   </para>

   <para>
    In addition, <productname>PostgreSQL</productname> supports the
    more general <type>text</type> type, which stores strings of any
    length.  Unlike <type>character varying</type>, <type>text</type>
    does not require an explicit declared upper limit on the size of
    the string.  Although the type <type>text</type> is not in the SQL
    standard, many other RDBMS packages have it as well.
   </para>

   <para>
    The storage requirement for data of these types is 4 bytes plus
    the actual string, and in case of <type>character</type> plus the
    padding.  Long strings will actually be compressed by the system
    automatically.  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
    actually larger 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.)
   </para>

   <tip>
    <para>
     There are no performance differences between these three types,
     apart from the increased storage size when using the blank-padded
     type.
    </para>
   </tip>

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

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

<programlisting>
CREATE TABLE test1 (a character(4));
INSERT INTO test1 VALUES ('ok');
SELECT a, char_length(a) FROM test1; -- <co id="co.datatype-char">
<computeroutput>
  a   | char_length
------+-------------
 ok   |           4
</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>
SELECT b, char_length(b) FROM test2;
<computeroutput>
   b   | char_length
-------+-------------
 ok    |           2
 good  |           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>Postgres</productname>.  The <type>name</type> type
    exists <emphasis>only</emphasis> for storage of internal catalog
    names and is not intended for use by the general user.  Its length
    is currently defined as 32 bytes (31 characters plus terminator)
    but should be referenced using the macro
    <symbol>NAMEDATALEN</symbol>.  The length is set at compile time
    (and is therefore adjustable for special uses); the default
    maximum length may change in a future release.  The type
    <type>"char"</type> (note the quotes) is different from
    <type>char(1)</type> in that it only uses one byte of storage.  It
    is internally used in the system catalogs as a poor-man's
    enumeration type.
   </para>

    <table tocentry="1">
     <title>Specialty Character Type</title>
     <tgroup cols="3">
      <thead>
       <row>
	<entry>Type Name</entry>
	<entry>Storage</entry>
	<entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry>"char"</entry>
	<entry>1 byte</entry>
	<entry>Single character internal type</entry>
       </row>
       <row>
	<entry>name</entry>
	<entry>32 bytes</entry>
	<entry>Thirty-one character internal type</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

  </sect1>

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

   <para>
    The <type>bytea</type> data type allows storage of binary data,
    specifically allowing storage of NULLs which are entered as
    <literal>'\\000'</>. The first backslash is interpreted by the
    single quotes, and the second is recognized by <type>bytea</> and
    precedes a three digit octal value. For a similar reason, a
    backslash must be entered into a field as <literal>'\\\\'</> or
    <literal>'\\134'</>. You may also have to escape line feeds and
    carriage return if your interface automatically translates these. It
    can store values of any length. <type>Bytea</> is a non-standard
    data type.
   </para>
  </sect1>

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

   <para>
    <productname>Postgres</productname> supports the full set of
    <acronym>SQL</acronym> date and time types.
   </para>

   <para>
    <table tocentry="1">
     <title>Date/Time Types</title>
     <tgroup cols="6">
      <thead>
       <row>
	<entry>Type</entry>
        <entry>Description</entry>
	<entry>Storage</entry>
        <entry>Earliest</entry>
        <entry>Latest</entry>
        <entry>Resolution</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><type>timestamp without time zone</type></entry>
        <entry>both date and time</entry>
        <entry>8 bytes</entry>
        <entry>4713 BC</entry>
        <entry>AD 1465001</entry>
        <entry>1 microsecond / 14 digits</entry>
       </row>
       <row>
        <entry><type>timestamp [ with time zone ]</type></entry>
        <entry>both date and time</entry>
        <entry>8 bytes</entry>
        <entry>4713 BC</entry>
        <entry>AD 1465001</entry>
        <entry>1 microsecond / 14 digits</entry>
       </row>
       <row>
        <entry><type>interval</type></entry>
        <entry>for time intervals</entry>
        <entry>12 bytes</entry>
        <entry>-178000000 years</entry>
        <entry>178000000 years</entry>
        <entry>1 microsecond</entry>
       </row>
       <row>
        <entry><type>date</type></entry>
        <entry>dates only</entry>
        <entry>4 bytes</entry>
        <entry>4713 BC</entry>
        <entry>32767 AD</entry>
        <entry>1 day</entry>
       </row>
       <row>
        <entry><type>time [ without time zone ]</type></entry>
        <entry>times of day only</entry>
        <entry>8 bytes</entry>
        <entry>00:00:00.00</entry>
        <entry>23:59:59.99</entry>
        <entry>1 microsecond</entry>
       </row>
       <row>
        <entry><type>time with time zone</type></entry>
        <entry>times of day only</entry>
        <entry>12 bytes</entry>
        <entry>00:00:00.00+12</entry>
        <entry>23:59:59.99-12</entry>
        <entry>1 microsecond</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <note>
     <para>
      Time zones, and time zone conventions, are influenced by
      political conventions, not just physical effects. Time zones have
      become somewhat standardized during the 1900's, but continue to
      be prone to arbitrary changes with time.
      <productname>PostgreSQL</productname> uses your operating
      system's underlying features to provide time zone
      support, and these systems usually contain information for only
      the time period 1902 through 2038 (corresponding to the full
      range of conventional Unix system time).
      <type>timestamp with time zone</type> will use time zone
      information only within that year range, and assumes that times
      are in UTC outside that range.
     </para>
    </note>
   </para>

   <para>
    To ensure compatibility to earlier versions of <productname>Postgres</productname>
    we also continue to provide <type>datetime</type>
    (equivalent to <type>timestamp</type>) and
    <type>timespan</type> (equivalent to <type>interval</type>),
    however support for these is now restricted to having an
    implicit translation to <type>timestamp</type> and
    <type>interval</type>.
    The types <type>abstime</type>
    and <type>reltime</type> are lower precision types which are used internally.
    You are discouraged from using any of 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
     <acronym>ISO-8601</acronym>, <acronym>SQL</acronym>-compatible, 
     traditional <productname>Postgres</productname>, and others.
     The ordering of month and day in date input can be ambiguous, therefore a setting
     exists to specify how it should be interpreted in ambiguous cases. The command
     <literal>SET DateStyle TO 'US'</literal> or <literal>SET DateStyle TO 'NonEuropean'</literal>
     specifies the variant <quote>month before day</quote>, the command
     <literal>SET DateStyle TO 'European'</literal> sets the variant
     <quote>day before month</quote>. The <literal>ISO</literal> style
     is the default but this default can be changed at compile time or at run time.
    </para>

    <para>
     See <xref linkend="datetime-appendix">
     for the exact parsing rules of date/time input and for the recognized time zones.
    </para>

    <para>
     Remember that any date or time input needs to be enclosed into
     single quotes, like text strings.  Refer to <xref
     linkend="sql-syntax-constants-generic"> for more information.
     SQL requires the following syntax
<synopsis>
<replaceable>type</replaceable> '<replaceable>value</replaceable>'
</synopsis>
     but <productname>Postgres</productname> is more flexible.
    </para>

    <sect3>
    <title><type>date</type></title>

    <indexterm>
     <primary>date</primary>
     <secondary>data type</secondary>
    </indexterm>
 
    <para>
     The following are possible inputs for the <type>date</type> type.

     <table tocentry="1">
      <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</entry>
	</row>
	<row>
	 <entry>1999-01-08</entry>
	 <entry>ISO-8601 format, preferred</entry>
	</row>
	<row>
	 <entry>1/8/1999</entry>
	 <entry>US; read as August 1 in European mode</entry>
	</row>
	<row>
	 <entry>8/1/1999</entry>
	 <entry>European; read as August 1 in US mode</entry>
	</row>
	<row>
	 <entry>1/18/1999</entry>
	 <entry>US; read as January 18 in any mode</entry>
	</row>
	<row>
	 <entry>19990108</entry>
	 <entry>ISO-8601 year, month, day</entry>
	</row>
	<row>
	 <entry>990108</entry>
	 <entry>ISO-8601 year, month, day</entry>
	</row>
	<row>
	 <entry>1999.008</entry>
	 <entry>Year and day of year</entry>
	</row>
	<row>
	 <entry>99008</entry>
	 <entry>Year and day of year</entry>
	</row>
	<row>
	 <entry>January 8, 99 BC</entry>
	 <entry>Year 99 before the Common Era</entry>
	</row>
       </tbody>
      </tgroup>
     </table>
    </para>

    <para>
     <table tocentry="1">
      <title>Month Abbreviations</title>
      <tgroup cols="2">
       <thead>
	<row>
	 <entry>Month</entry>
	 <entry>Abbreviations</entry>
	</row>
       </thead>
       <tbody>
	<row>
	 <entry>April</entry>
	 <entry>Apr</entry>
	</row>
	<row>
	 <entry>August</entry>
	 <entry>Aug</entry>
	</row>
	<row>
	 <entry>December</entry>
	 <entry>Dec</entry>
	</row>
	<row>
	 <entry>February</entry>
	 <entry>Feb</entry>
	</row>
	<row>
	 <entry>January</entry>
	 <entry>Jan</entry>
	</row>
	<row>
	 <entry>July</entry>
	 <entry>Jul</entry>
	</row>
	<row>
	 <entry>June</entry>
	 <entry>Jun</entry>
	</row>
	<row>
	 <entry>March</entry>
	 <entry>Mar</entry>
	</row>
	<row>
	 <entry>November</entry>
	 <entry>Nov</entry>
	</row>
	<row>
	 <entry>October</entry>
	 <entry>Oct</entry>
	</row>
	<row>
	 <entry>September</entry>
	 <entry>Sep, Sept</entry>
	</row>
       </tbody>
      </tgroup>
     </table>

     <note>
      <para>
       The month <literal>May</literal> has no explicit abbreviation, for obvious reasons.
      </para>
     </note>
    </para>

    <para>
     <table tocentry="1">
      <title>Day of the Week Abbreviations</title>
      <tgroup cols="2">
       <thead>
	<row>
	 <entry>Day</entry>
	 <entry>Abbreviation</entry>
	</row>
       </thead>
       <tbody>
	<row>
	 <entry>Sunday</entry>
	 <entry>Sun</entry>
	</row>
	<row>
	 <entry>Monday</entry>
	 <entry>Mon</entry>
	</row>
	<row>
	 <entry>Tuesday</entry>
	 <entry>Tue, Tues</entry>
	</row>
	<row>
	 <entry>Wednesday</entry>
	 <entry>Wed, Weds</entry>
	</row>
	<row>
	 <entry>Thursday</entry>
	 <entry>Thu, Thur, Thurs</entry>
	</row>
	<row>
	 <entry>Friday</entry>
	 <entry>Fri</entry>
	</row>
	<row>
	 <entry>Saturday</entry>
	 <entry>Sat</entry>
	</row>
       </tbody>
      </tgroup>
     </table>
    </para>
    </sect3>

    <sect3>
     <title><type>time [ without time zone ]</type></title>

     <indexterm>
      <primary>time</primary>
      <secondary>data type</secondary>
     </indexterm>
     <indexterm>
      <primary>time without time zone</primary>
      <secondary>time</secondary>
     </indexterm>

     <para>
      Per SQL99, this type can be referenced as <type>time</type> and
      as <type>time without time zone</type>.
     </para>

     <para>
      The following are valid <type>time</type> inputs.

      <table tocentry="1">
       <title>Time Input</title>
       <tgroup cols="2">
	<thead>
	 <row>
	  <entry>Example</entry>
	  <entry>Description</entry>
	 </row>
	</thead>
	<tbody>
	 <row>
	  <entry>04:05:06.789</entry>
	  <entry>ISO-8601</entry>
	 </row>
	 <row>
	  <entry>04:05:06</entry>
	  <entry>ISO-8601</entry>
	 </row>
	 <row>
	  <entry>04:05</entry>
	  <entry>ISO-8601</entry>
	 </row>
	 <row>
	  <entry>040506</entry>
	  <entry>ISO-8601</entry>
	 </row>
	 <row>
	  <entry>04:05 AM</entry>
	  <entry>Same as 04:05; AM does not affect value</entry>
	 </row>
	 <row>
	  <entry>04:05 PM</entry>
	  <entry>Same as 16:05; input hour must be <= 12</entry>
	 </row>
	 <row>
	  <entry>z</entry>
	  <entry>Same as 00:00:00</entry>
	 </row>
	 <row>
	  <entry>zulu</entry>
	  <entry>Same as 00:00:00</entry>
	 </row>
	 <row>
	  <entry>allballs</entry>
	  <entry>Same as 00:00:00</entry>
	 </row>
	</tbody>
       </tgroup>
      </table>
     </para>
    </sect3>

    <sect3>
     <title><type>time with time zone</type></title>

     <indexterm>
      <primary>time with time zone</primary>
      <secondary>data type</secondary>
     </indexterm>
     <indexterm>
      <primary>time</primary>
      <secondary>data type</secondary>
     </indexterm>

     <para>
      This type is defined by SQL92, 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>
      <type>time with time zone</type> accepts all input also legal
      for the <type>time</type> type, appended with a legal time zone,
      as follows:

      <table tocentry="1">
       <title>Time With Time Zone Input</title>
       <tgroup cols="2">
       <thead>
	<row>
	 <entry>Example</entry>
	 <entry>Description</entry>
	</row>
       </thead>
       <tbody>
	<row>
	 <entry>04:05:06.789-8</entry>
	 <entry>ISO-8601</entry>
	</row>
	<row>
	 <entry>04:05:06-08:00</entry>
	 <entry>ISO-8601</entry>
	</row>
	<row>
	 <entry>04:05-08:00</entry>
	 <entry>ISO-8601</entry>
	</row>
	<row>
	 <entry>040506-08</entry>
	 <entry>ISO-8601</entry>
	</row>
       </tbody>
      </tgroup>
     </table>
    </para>

     <para>
      Refer to <xref linkend="datatype-timezone-table"> for
      more examples of time zones.
     </para>
    </sect3>

    <sect3>
    <title><type>timestamp without time zone</type></title>

    <indexterm>
     <primary>timestamp without time zone</primary>
     <secondary>data type</secondary>
    </indexterm>

     <para>
      Valid input for the <type>timestamp without time zone</type>
      type consists of a concatenation
      of a date and a time, followed by an optional <literal>AD</literal> or
      <literal>BC</literal>, followed by an optional time zone. (See below.)
      Thus

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

      is a valid <type>timestamp without time zone</type> value that
      is <acronym>ISO</acronym>-compliant.
      In addition, the wide-spread format

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

     <para>
      For <type>timestamp without time zone</type>, any explicit time
      zone specified in the input is silently swallowed. That is, the
      resulting date/time value is derived from the explicit date/time
      fields in the input value, and is not adjusted for time zone.
     </para>
    </sect3>

    <sect3>
    <title><type>timestamp with time zone</type></title>

    <indexterm>
     <primary>timestamp</primary>
     <secondary>data type</secondary>
    </indexterm>

     <para>
      Valid input for the <type>timestamp</type> type consists of a concatenation
      of a date and a time, followed by an optional <literal>AD</literal> or
      <literal>BC</literal>, followed by an optional time zone. (See below.)
      Thus

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

      is a valid <type>timestamp</type> value that is <acronym>ISO</acronym>-compliant.
      In addition, the wide-spread format

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

     <para>
      <table tocentry="1" id="datatype-timezone-table">
       <title>Time Zone Input</title>
       <tgroup cols="2">
	<thead>
	 <row>
	  <entry>Time Zone</entry>
	  <entry>Description</entry>
	 </row>
	</thead>
	<tbody>
	 <row>
	  <entry>PST</entry>
	  <entry>Pacific Standard Time</entry>
	 </row>
	 <row>
	  <entry>-8:00</entry>
	  <entry>ISO-8601 offset for PST</entry>
	 </row>
	 <row>
	  <entry>-800</entry>
	  <entry>ISO-8601 offset for PST</entry>
	 </row>
	 <row>
	  <entry>-8</entry>
	  <entry>ISO-8601 offset for PST</entry>
	 </row>
	</tbody>
       </tgroup>
      </table>
     </para>
    </sect3>

    <sect3>
     <title><type>interval</type></title>

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

      <para>
       <type>interval</type>s can be specified with the following syntax:

      <programlisting>
  Quantity Unit [Quantity Unit...] [Direction]
@ Quantity Unit [Direction]
      </programlisting>

      where: <literal>Quantity</literal> is ..., <literal>-1</literal>,
      <literal>0</literal>, <literal>1</literal>, <literal>2</literal>, ...;
      <literal>Unit</literal> is <literal>second</literal>,
      <literal>minute</literal>, <literal>hour</literal>, <literal>day</literal>,
      <literal>week</literal>, <literal>month</literal>, <literal>year</literal>,
      <literal>decade</literal>, <literal>century</literal>, <literal>millennium</literal>,
      or abbreviations or plurals of these units;
      <literal>Direction</literal> can be <literal>ago</literal> or
      empty.
     </para>
    </sect3>

    <sect3>
     <title>Special values</title>

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

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

     <para>
      The following <acronym>SQL</acronym>-compatible functions can be used as date or time
      input for the corresponding data type: <literal>CURRENT_DATE</literal>,
      <literal>CURRENT_TIME</literal>, <literal>CURRENT_TIMESTAMP</literal>.
     </para>
     <para>
      <productname>Postgres</productname> also supports several special constants for
      convenience.

     <table tocentry="1">
      <title>Special Date/Time Constants</title>
      <tgroup cols="2">
       <thead>
	<row>
	 <entry>Constant</entry>
	 <entry>Description</entry>
	</row>
       </thead>
       <tbody>
<!--
	<row>
	 <entry>current</entry>
	 <entry>Current transaction time, deferred</entry>
	</row>
-->
	<row>
	 <entry>epoch</entry>
	 <entry>1970-01-01 00:00:00+00 (Unix system time zero)</entry>
	</row>
	<row>
	 <entry>infinity</entry>
	 <entry>Later than other valid times</entry>
	</row>
	<row>
	 <entry>-infinity</entry>
	 <entry>Earlier than other valid times</entry>
	</row>
	<row>
	 <entry>invalid</entry>
	 <entry>Illegal entry</entry>
	</row>
	<row>
	 <entry>now</entry>
	 <entry>Current transaction time</entry>
	</row>
	<row>
	 <entry>today</entry>
	 <entry>Midnight today</entry>
	</row>
	<row>
	 <entry>tomorrow</entry>
	 <entry>Midnight tomorrow</entry>
        </row>
	<row>
	 <entry>yesterday</entry>
	 <entry>Midnight yesterday</entry>
	</row>
       </tbody>
      </tgroup>
     </table>
     <literal>'now'</literal> is resolved when the value is inserted, <literal>'current'</literal>
     is resolved every time the value is retrieved. So you probably want to use <literal>'now'</literal>
     in most applications. (Of course you <emphasis>really</emphasis> want to use
     <literal>CURRENT_TIMESTAMP</literal>, which is equivalent to <literal>'now'</literal>.)
    </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>
     Output formats can be set to one of the four styles
     ISO-8601, <acronym>SQL</acronym> (Ingres), traditional
     Postgres, and German, using the <command>SET DateStyle</command>.
     The default is the <acronym>ISO</acronym> format.

     <table tocentry="1">
      <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 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>

    <para>
     The output of the <type>date</type> and <type>time</type> styles is of course
     only the date or time part in accordance with the above examples.
    </para>

    <para>
     The <acronym>SQL</acronym> style has European and non-European (US) variants, 
     which determines whether month follows day or vice versa. (See also above
     at Date/Time Input, how this setting affects interpretation of input values.)

     <table tocentry="1">
      <title>Date Order Conventions</title>
      <tgroup cols="3">
       <thead>
	<row>
	 <entry>Style Specification</entry>
	 <entry>Description</entry>
	 <entry>Example</entry>
	</row>
       </thead>
       <tbody>
	<row>
	 <entry>European</entry>
	 <entry><replaceable>day</replaceable>/<replaceable>month</replaceable>/<replaceable>year</replaceable></entry>
	 <entry>17/12/1997 15:37:16.00 MET</entry>
	</row>
	<row>
	 <entry>US</entry>
	 <entry><replaceable>month</replaceable>/<replaceable>day</replaceable>/<replaceable>year</replaceable></entry>
	 <entry>12/17/1997 07:37:16.00 PST</entry>
	</row>
       </tbody>
      </tgroup>
     </table>
    </para>

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

<programlisting>
[ Quantity Units [ ... ] ] [ Days ] Hours:Minutes [ ago ]
</programlisting>
    </para>

    <para>
     There are several ways to affect the appearance of date/time types:

     <itemizedlist spacing="compact" mark="bullet">
      <listitem>
       <para>
	The <envar>PGDATESTYLE</envar> environment variable used by the backend directly 
	on postmaster start-up.
       </para>
      </listitem>
      <listitem>
       <para>
	The <envar>PGDATESTYLE</envar> environment variable used by the frontend <application>libpq</application>
	on session start-up.
       </para>
      </listitem>
      <listitem>
       <para>
	<command>SET DATESTYLE</command> <acronym>SQL</acronym> command.
       </para>
      </listitem>
     </itemizedlist>
    </para>

   </sect2>

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

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

    <para>
     <productname>Postgres</productname> endeavors to be compatible with
     <acronym>SQL92</acronym> definitions for typical usage.
     However, the <acronym>SQL92</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 or does.
	Time zones in the real world can have no meaning unless 
	associated with a date as well as a time
	since the offset may vary through the year with daylight savings
	time boundaries.
       </para>
      </listitem>

      <listitem>
       <para>
	The default time zone is specified as a constant integer offset 
	from GMT/UTC. It is not possible to adapt to daylight savings
	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 SQL92 type TIME
     WITH TIME ZONE (though it is supported by
     <productname>Postgres</productname> for legacy applications and
     for compatibility with other RDBMS implementations).
     <productname>Postgres</productname> 
     assumes local time for any type containing only
     date or time. Further, time zone support is derived from 
     the underlying operating system
     time zone capabilities, and hence can handle daylight savings time 
     and other expected behavior.
    </para>

    <para>
     <productname>Postgres</productname> obtains time zone support 
     from the underlying operating system for dates between 1902 and
     2038 (near the typical date limits for Unix-style
     systems). Outside of this range, all dates are assumed to be
     specified and used in Universal Coordinated Time (UTC).
    </para>

    <para>
     All dates and times are stored internally in UTC,
     traditionally known as Greenwich Mean Time (GMT). 
     Times are converted to local time on the database server before being
     sent to the client frontend, hence by default are in the server
     time zone.
    </para>

    <para>
     There are several ways to affect the time zone behavior:

     <itemizedlist spacing="compact" mark="bullet">
      <listitem>
       <para>
	The <envar>TZ</envar> environment variable is used by the backend directly
	on postmaster start-up as the default time zone.
       </para>
      </listitem>
      <listitem>
       <para>
	The <envar>PGTZ</envar> environment variable, if set at the client, is used by libpq 
	to send a <command>SET TIME ZONE</command> command to the backend upon
	connection.
       </para>
      </listitem>
      <listitem>
       <para>
	The <acronym>SQL</acronym> command <command>SET TIME ZONE</command>
	sets the time zone for the session.
       </para>
      </listitem>
      <listitem>
       <para>
	The <acronym>SQL92</acronym> qualifier on
	<programlisting>
<replaceable>timestamp</replaceable> AT TIME ZONE '<replaceable>zone</replaceable>'
	</programlisting>
	where <replaceable>zone</replaceable> can be specified as a
	text time zone (e.g. <literal>'PST'</literal>) or as an
	interval (e.g. <literal>INTERVAL '-08:00'</literal>).
       </para>
      </listitem>
     </itemizedlist>
    </para>

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

     <note>
      <para>
       If the runtime option <literal>AUSTRALIAN_TIMEZONES</literal> is set 
       then <literal>CST</literal> and <literal>EST</literal> refer to
       Australian time zones, not American ones.
      </para>
     </note>
    </para>

   </sect2>

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

    <para>
     <productname>Postgres</productname> uses Julian dates
     for all date/time calculations. They have the nice property of correctly
     predicting/calculating any date more recent than 4713BC
     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>Postgres</productname> provides the
    <acronym>SQL99</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 state.
   </para>

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

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

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

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

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

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

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

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

   <para>
    Geometric types represent two-dimensional spatial objects. 
    The most fundamental type,
    the point, forms the basis for all of the other types.
   </para>

   <para>
    <table tocentry="1">
     <title>Geometric Types</title>
     <tgroup cols="4">
      <thead>
       <row>
	<entry>Geometric Type</entry>
	<entry>Storage</entry>
	<entry>Representation</entry>
	<entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry><type>point</type></entry>
	<entry>16 bytes</entry>
	<entry>(x,y)</entry>
	<entry>Point in space</entry>
       </row>
       <row>
	<entry><type>line</type></entry>
	<entry>32 bytes</entry>
	<entry>((x1,y1),(x2,y2))</entry>
	<entry>Infinite line</entry>
       </row>
       <row>
	<entry><type>lseg</type></entry>
	<entry>32 bytes</entry>
	<entry>((x1,y1),(x2,y2))</entry>
	<entry>Finite line segment</entry>
       </row>
       <row>
	<entry><type>box</type></entry>
	<entry>32 bytes</entry>
	<entry>((x1,y1),(x2,y2))</entry>
	<entry>Rectangular box</entry>
       </row>
       <row>
	<entry><type>path</type></entry>
	<entry>4+32n bytes</entry>
	<entry>((x1,y1),...)</entry>
	<entry>Closed path (similar to polygon)</entry>
       </row>
       <row>
	<entry><type>path</type></entry>
	<entry>4+32n bytes</entry>
	<entry>[(x1,y1),...]</entry>
	<entry>Open path</entry>
       </row>
       <row>
	<entry><type>polygon</type></entry>
	<entry>4+32n bytes</entry>
	<entry>((x1,y1),...)</entry>
	<entry>Polygon (similar to closed path)</entry>
       </row>
       <row>
	<entry><type>circle</type></entry>
	<entry>24 bytes</entry>
	<entry><(x,y),r></entry>
	<entry>Circle (center and radius)</entry>
       </row>
      </tbody>
     </tgroup>
    </table>
   </para>

   <para>
    A rich set of functions and operators is available to perform various geometric
    operations such as scaling, translation, rotation, and determining 
    intersections.
   </para>

   <sect2>
    <title>Point</title>

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

    <para>
     Points are the fundamental two-dimensional building block for geometric types.
    </para>

    <para>
     <type>point</type> is specified using the following syntax:

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

     where the arguments are

     <variablelist>
      <varlistentry>
       <term><replaceable>x</replaceable></term>
       <listitem>
	<para>
	 The x-axis coordinate as a floating point number.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><replaceable>y</replaceable></term>
       <listitem>
	<para>
	 The y-axis coordinate as a floating point number.
	</para>
       </listitem>
      </varlistentry>
     </variablelist>
    </para>
   </sect2>

   <sect2>
    <title>Line Segment</title>

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

    <para>
     Line segments (<type>lseg</type>) are represented by pairs of points.
    </para>

    <para>
     <type>lseg</type> is specified using the following syntax:

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

     where the arguments are

     <variablelist>
      <varlistentry>
       <term>(<replaceable>x1</replaceable>,<replaceable>y1</replaceable>)</term>
       <term>(<replaceable>x2</replaceable>,<replaceable>y2</replaceable>)</term>
       <listitem>
	<para>
	 The end points of the line segment.
	</para>
       </listitem>
      </varlistentry>
     </variablelist>
    </para>
   </sect2>

   <sect2>
    <title>Box</title>

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

    <para>
     Boxes are represented by pairs of points that are opposite
     corners of the box.
    </para>

    <para>
     <type>box</type> is specified using the following syntax:

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

     where the arguments are

     <variablelist>
      <varlistentry>
       <term>(<replaceable>x1</replaceable>,<replaceable>y1</replaceable>)</term>
       <term>(<replaceable>x2</replaceable>,<replaceable>y2</replaceable>)</term>
       <listitem>
	<para>
	 Opposite corners of the box.
	</para>
       </listitem>
      </varlistentry>
     </variablelist>
    </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>Path</title>

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

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

    <para>
     <type>path</type> is 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 arguments are

     <variablelist>
      <varlistentry>
       <term>(<replaceable>x</replaceable>,<replaceable>y</replaceable>)</term>
       <listitem>
	<para>
	 End points of the line segments comprising the path.
	 A leading square bracket ("[") indicates an open path, while
	 a leading parenthesis ("(") indicates a closed path.
	</para>
       </listitem>
      </varlistentry>
     </variablelist>
    </para>

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

   <sect2>
    <title>Polygon</title>

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

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

    <para>
     <type>polygon</type> is 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 arguments are

     <variablelist>
      <varlistentry>
       <term>(<replaceable>x</replaceable>,<replaceable>y</replaceable>)</term>
       <listitem>
	<para>
	 End points of the line segments comprising the boundary of the
	 polygon.
	</para>
       </listitem>
      </varlistentry>
     </variablelist>
    </para>

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

   <sect2>
    <title>Circle</title>

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

    <para>
     Circles are represented by a center point and a radius.
    </para>

    <para>
     <type>circle</type> is 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 the arguments are

     <variablelist>
      <varlistentry>
       <term>(<replaceable>x</replaceable>,<replaceable>y</replaceable>)</term>
       <listitem>
	<para>
	 Center of the circle.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><replaceable>r</replaceable></term>
       <listitem>
	<para>
	 Radius of the circle.
	</para>
       </listitem>
      </varlistentry>
     </variablelist>
    </para>

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

  </sect1>

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

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

   <para>
    <productname>Postgres</> offers data types to store IP and MAC
    addresses.  It is preferable to use these types over plain text
    types, because these types offer input error checking and several
    specialized operators and functions.

    <table tocentry="1" id="datatype-net-types-table">
     <title>Network Address Data Types</title>
     <tgroup cols="4">
      <thead>
       <row>
	<entry>Name</entry>
	<entry>Storage</entry>
	<entry>Description</entry>
	<entry>Range</entry>
       </row>
      </thead>
      <tbody>

       <row>
	<entry><type>cidr</type></entry>
	<entry>12 bytes</entry>
	<entry>IP networks</entry>
	<entry>valid IPv4 networks</entry>
       </row>

       <row>
	<entry><type>inet</type></entry>
	<entry>12 bytes</entry>
	<entry>IP hosts and networks</entry>
	<entry>valid IPv4 hosts or networks</entry>
       </row>

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

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

   <para>
    IP v6 is not supported, yet.
   </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 IP host address, and
     optionally the identity of the subnet it is in, all in one field.
     The subnet identity is represented by the number of bits in the
     network part of the address (the <quote>netmask</quote>).  If the netmask is 32,
     then the value does not indicate a subnet, only a single host.
     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">x.x.x.x/y</replaceable> where <replaceable
     class="parameter">x.x.x.x</replaceable> is an IP 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, and the value represents just a single host.
     On display, the <replaceable class="parameter">/y</replaceable>
     portion is suppressed if the netmask is 32.
    </para>
   </sect2>

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

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

    <para>
     The <type>cidr</type> type holds an IP network specification.
     Input and output formats follow Classless Internet Domain Routing
     conventions.
     The format for
     specifying classless networks is <replaceable
     class="parameter">x.x.x.x/y</> where <replaceable
     class="parameter">x.x.x.x</> is the network 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 numbering system, except
     that it will be at least large enough to include all of the octets
     written in the input.
    </para>

    <para>
     Here are some examples:

     <table tocentry="1">
      <title><type>cidr</> Type Input Examples</title>
      <tgroup cols="3">
       <thead> 
	<row> 
	 <entry><type>CIDR</type> Input</entry>
	 <entry><type>CIDR</type> Displayed</entry>
	 <entry><function>abbrev</function>(<type>CIDR</type>)</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>
       </tbody>
      </tgroup>
     </table>
    </para>
   </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.

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

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

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

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

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

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

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

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

  </sect1>

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

   <indexterm zone="datatype-bit">
    <primary>bit strings</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>x</replaceable>)</type> and <type>BIT
    VARYING(<replaceable>x</replaceable>)</type>; where
    <replaceable>x</replaceable> is a positive integer.
   </para>

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

   <note>
    <para>
     Prior to PostgreSQL 7.2, <type>BIT</type> type data was
     zero-padded on the right.  This was changed to comply with the
     SQL standard.  To implement zero-padded bit strings, a
     combination of the concatenation operator and the
     <function>substring</function> function can be used.
    </para>
   </note>

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

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

<programlisting>
CREATE TABLE test (a BIT(3), b BIT VARYING(5));
INSERT INTO test VALUES (B'101', B'00');
INSERT INTO test VALUES (B'10', B'101');
<computeroutput>
ERROR:  bit string length does not match type bit(3)
</computeroutput>
SELECT SUBSTRING(b FROM 1 FOR 2) FROM test;
</programlisting>
   </example>

  </sect1>

 </chapter>

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