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

  <abstract>
   <para>
    Describes the built-in data types available in 
    <productname>Postgres</productname>.
   </para>
  </abstract>

  <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>DEFINE TYPE</command>
   command described elsewhere.
  </para>

  <para>
   In the context of data types, the following sections will discuss 
   <acronym>SQL</acronym> standards compliance, porting issues, and usage.

   Some <productname>Postgres</productname> types correspond directly to 
   <acronym>SQL92</acronym>-compatible types. In other
   cases, data types defined by <acronym>SQL92</acronym> syntax are mapped directly
   into native <productname>Postgres</productname> types. 

   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 tocentry="1">
    <title><productname>Postgres</productname> Data Types</title>
    <titleabbrev>Data Types</titleabbrev>
    <tgroup cols="3">
     <thead>
      <row>
       <entry><productname>Postgres</productname> Type</entry>
       <entry><acronym>SQL92</acronym> or <acronym>SQL3</acronym> Type</entry>
       <entry>Description</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>bool</entry>
       <entry>boolean</entry>
       <entry>logical boolean (true/false)</entry>
      </row>
      <row>
       <entry>box</entry>
       <entry></entry>
       <entry>rectangular box in 2D plane</entry>
      </row>
      <row>
       <entry>char(n)</entry>
       <entry>character(n)</entry>
       <entry>fixed-length character string</entry>
      </row>
      <row>
       <entry>cidr</entry>
       <entry></entry>
       <entry>IP version 4 network or host address</entry>
      </row>
      <row>
       <entry>circle</entry>
       <entry></entry>
       <entry>circle in 2D plane</entry>
      </row>
      <row>
       <entry>date</entry>
       <entry>date</entry>
       <entry>calendar date without time of day</entry>
      </row>
      <row>
       <entry>float4/8</entry>
       <entry>float(p)</entry>
       <entry>floating-point number with precision p</entry>
      </row>
      <row>
       <entry>float8</entry>
       <entry>real, double precision</entry>
       <entry>double-precision floating-point number</entry>
      </row>
      <row>
       <entry>inet</entry>
       <entry></entry>
       <entry>IP version 4 network or host address</entry>
      </row>
      <row>
       <entry>int2</entry>
       <entry>smallint</entry>
       <entry>signed two-byte integer</entry>
      </row>
      <row>
       <entry>int4</entry>
       <entry>int, integer</entry>
       <entry>signed 4-byte integer</entry>
      </row>
      <row>
       <entry>int4</entry>
       <entry>decimal(p,s)</entry>
       <entry>exact numeric for p <= 9, s = 0</entry>
      </row>
      <row>
       <entry>int4</entry>
       <entry>numeric(p,s)</entry>
       <entry>exact numeric for p == 9, s = 0</entry>
      </row>
      <row>
       <entry>int8</entry>
       <entry></entry>
       <entry>signed 8-byte integer</entry>
      </row>
      <row>
       <entry>line</entry>
       <entry></entry>
       <entry>infinite line in 2D plane</entry>
      </row>
      <row>
       <entry>lseg</entry>
       <entry></entry>
       <entry>line segment in 2D plane</entry>
      </row>
      <row>
       <entry>money</entry>
       <entry>decimal(9,2)</entry>
       <entry>US-style currency</entry>
      </row>
      <row>
       <entry>path</entry>
       <entry></entry>
       <entry>open and closed geometric path in 2D plane</entry>
      </row>
      <row>
       <entry>point</entry>
       <entry></entry>
       <entry>geometric point in 2D plane</entry>
      </row>
      <row>
       <entry>polygon</entry>
       <entry></entry>
       <entry>closed geometric path in 2D plane</entry>
      </row>
      <row>
       <entry>serial</entry>
       <entry></entry>
       <entry>unique id for indexing and cross-reference</entry>
      </row>
      <row>
       <entry>time</entry>
       <entry>time</entry>
       <entry>time of day</entry>
      </row>
      <row>
       <entry>timespan</entry>
       <entry>interval</entry>
       <entry>general-use time span</entry>
      </row>
      <row>
       <entry>timestamp</entry>
       <entry>timestamp with time zone</entry>
       <entry>date/time</entry>
      </row>
      <row>
       <entry>varchar(n)</entry>
       <entry>character varying(n)</entry>
       <entry>variable-length character string</entry>
      </row>
     </tbody>
    </tgroup>
   </table>
  </para>

  <para>
   <note>
    <para>
     The <type>cidr</type> and <type>inet</type> types are designed to handle any IP type 
     but only ipv4 is handled in the current implementation.  
     Everything here that talks about ipv4 will apply to ipv6 in a future release.</para>
   </note></para>

  <para>
   <table tocentry="1">
    <title><productname>Postgres</productname> Function Constants</title>
    <titleabbrev>Constants</titleabbrev>
    <tgroup cols="3">
     <thead>
      <row>
       <entry><productname>Postgres</productname> Function</entry>
       <entry><acronym>SQL92</acronym> Constant</entry>
       <entry>Description</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>getpgusername()</entry>
       <entry>current_user</entry>
       <entry>user name in current session</entry>
      </row>
      <row>
       <entry>date('now')</entry>
       <entry>current_date</entry>
       <entry>date of current transaction</entry>
      </row>
      <row>
       <entry>time('now')</entry>
       <entry>current_time</entry>
       <entry>time of current transaction</entry>
      </row>
      <row>
       <entry>timestamp('now')</entry>
       <entry>current_timestamp</entry>
       <entry>date and time of current transaction</entry>
      </row>
     </tbody>
    </tgroup>
   </table>
  </para>

  <para>
   <productname>Postgres</productname> has features at the forefront of 
   <acronym>ORDBMS</acronym> development. In addition to
   <acronym>SQL3</acronym> conformance, substantial portions 
   of <acronym>SQL92</acronym> are also supported.
   Although we strive for <acronym>SQL92</acronym> compliance, 
   there are some aspects of the standard
   which are ill considered and which should not live through subsequent standards.
   <productname>Postgres</productname> will not make great efforts to 
   conform to these features; however, these tend to apply in little-used 
   or obsure cases, and a typical user is not likely to run into them.</para>

  <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>
   Note that 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.

   <note>
    <para>
     The original <productname>Postgres</productname> v4.2 code received from
     Berkeley rounded all double precision floating point results to six digits for
     output. Starting with v6.1, floating point numbers are allowed to retain
     most of the intrinsic precision of the type (typically 15 digits for doubles, 
     6 digits for 4-byte floats). 
     Other types with underlying floating point fields (e.g. geometric
     types) carry similar precision.</para>
   </note>
  </para>

  <sect1>
   <title>Numeric Types</title>

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

<para>
<table tocentry="1">
<title><productname>Postgres</productname> Numeric Types</title>
<titleabbrev>Numerics</titleabbrev>
<tgroup cols="4">
<thead>
  <row>
    <entry>Numeric Type</entry>
    <entry>Storage</entry>
    <entry>Description</entry>
    <entry>Range</entry>
  </row>
</thead>
<tbody>
  <row>
    <entry>float4</entry>
    <entry>4 bytes</entry>
    <entry>Variable-precision</entry>
    <entry>6 decimal places</entry>
  </row>
  <row>
    <entry>float8</entry>
    <entry>8 bytes</entry>
    <entry>Variable-precision</entry>
    <entry>15 decimal places</entry>
  </row>
  <row>
    <entry>int2</entry>
    <entry>2 bytes</entry>
    <entry>Fixed-precision</entry>
    <entry>-32768 to +32767</entry>
  </row>
  <row>
    <entry>int4</entry>
    <entry>4 bytes</entry>
    <entry>Usual choice for fixed-precision</entry>
    <entry>-2147483648 to +2147483647</entry>
  </row>
  <row>
    <entry>int8</entry>
    <entry>8 bytes</entry>
    <entry>Very large range fixed-precision</entry>
    <entry>+/- &gt; 18 decimal places</entry>
  </row>
  <row>
    <entry>serial</entry>
    <entry>4 bytes</entry>
    <entry>Identifer or cross-reference</entry>
    <entry>0 to +2147483647</entry>
  </row>
</tbody>
</tgroup>
</table>
</para>

<para>
The numeric types have a full set of corresponding arithmetic operators and
functions. Refer to <xref endterm="math-opers" linkend="math-opers"> 
and <xref endterm="math-funcs" linkend="math-funcs"> for more information.
   </para>

<para>
The <type>serial</type> type is a special-case type constructed by
<productname>Postgres</productname> from other existing components.
It is typically used to create unique identifiers for table entries.
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> INT4 DEFAULT nextval('<replaceable class="parameter">tablename</replaceable>_<replaceable class="parameter">colname</replaceable>_seq');
CREATE UNIQUE INDEX <replaceable class="parameter">tablename</replaceable>_<replaceable class="parameter">colname</replaceable>_key on <replaceable class="parameter">tablename</replaceable> (<replaceable class="parameter">colname</replaceable>);
</programlisting>

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

<para>
The <firstterm>exact numerics</firstterm> <type>decimal</type> and 
<type>numeric</type>
have fully implemented syntax but currently 
(<productname>Postgres</productname> v6.4)
 support only a small range of precision and/or range values.
The <type>int8</type> type may not be available on all platforms since
it relies on compiler support for this.
</para>

</sect1>

<sect1>
<title>Monetary Type</title>

<para>
The <type>money</type> type supports US-style currency with 
fixed decimal point representation.
If <productname>Postgres</productname> is compiled with USE_LOCALE 
then the money type should use the monetary conventions defined for
<citetitle>locale(7)</citetitle>.
   </para>

<para>
<table tocentry="1">
<title><productname>Postgres</productname> Monetary Types</title>
<titleabbrev>Money</titleabbrev>
<tgroup cols="4">
<thead>
  <row>
    <entry>Monetary Type</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>

<para>
<type>numeric</type>
should eventually replace the money type. It has a
fully implemented syntax but currently 
(<productname>Postgres</productname> v6.4)
 support only a small range of precision and/or range values 
and cannot adequately substitute for the money type.
</para>

</sect1>

<sect1>
<title>Character Types</title>

<para>
<acronym>SQL92</acronym> defines two primary character types:
 <type>char</type> and <type>varchar</type>. 
<productname>Postgres</productname> supports these types, in
addition to the more general <type>text</type> type, 
which unlike <type>varchar</type>
does not require an upper
limit to be declared on the size of the field.
</para>

<para>
<table tocentry="1">
<title><productname>Postgres</productname> Character Types</title>
<titleabbrev>Characters</titleabbrev>
<tgroup cols="4">
<thead>
  <row>
    <entry>Character Type</entry>
    <entry>Storage</entry>
    <entry>Recommendation</entry>
    <entry>Description</entry>
  </row>
</thead>
<tbody>
  <row>
    <entry>char</entry>
    <entry>1 byte</entry>
    <entry><acronym>SQL92</acronym>-compatible</entry>
    <entry>Single character</entry>
  </row>
  <row>
    <entry>char(n)</entry>
    <entry>(4+n) bytes</entry>
    <entry><acronym>SQL92</acronym>-compatible</entry>
    <entry>Fixed-length blank padded</entry>
  </row>
  <row>
    <entry>text</entry>
    <entry>(4+x) bytes</entry>
    <entry>Best choice</entry>
    <entry>Variable-length</entry>
  </row>
  <row>
    <entry>varchar(n)</entry>
    <entry>(4+n) bytes</entry>
    <entry><acronym>SQL92</acronym>-compatible</entry>
    <entry>Variable-length with limit</entry>
  </row>
</tbody>
</tgroup>
</table>
</para>

<para>
There is one other fixed-length character type. 
The <type>name</type> type
only has one purpose and that is to provide 
<productname>Postgres</productname> with a
special type to use for internal names.  
It is not intended for use by the general user.
It's length is currently defined as 32 chars 
but should be reference using NAMEDATALEN.
This is set at compile time and may change in a future release.
</para>

<para>
<table tocentry="1">
<title><productname>Postgres</productname> Specialty Character Type</title>
<titleabbrev>Specialty Characters</titleabbrev>
<tgroup cols="3">
<thead>
  <row>
    <entry>Character Type</entry>
    <entry>Storage</entry>
    <entry>Description</entry>
  </row>
</thead>
<tbody>
  <row>
    <entry>name</entry>
    <entry>32 bytes</entry>
    <entry>Thirty-two character internal type</entry>
  </row>
</tbody>
</tgroup>
</table>
</para>

</sect1>

  <sect1>
   <title>Date/Time Types</title>

   <para>
    There are two fundamental kinds of date and time measurements
    provided by <productname>Postgres</productname>:
    absolute clock times and relative time intervals.
    Both kinds of time measurements should demonstrate both
    continuity and smoothness.
   </para>

   <para>
    <productname>Postgres</productname> supplies two primary user-oriented 
    date and time types,
    <type>datetime</type> and <type>timespan</type>, as well as 
    the related <acronym>SQL92</acronym> types <type>timestamp</type>,
    <type>interval</type>,
    <type>date</type> and <type>time</type>.
   </para>

   <para>
    In a future release, <type>datetime</type> and <type>timespan</type> are likely
    to merge with the <acronym>SQL92</acronym> types <type>timestamp</type>,
    <type>interval</type>.
    Other date and time types are also available, mostly
    for historical reasons.
   </para>

   <para>
    <table tocentry="1">
     <title><productname>Postgres</productname> Date/Time Types</title>
     <titleabbrev>Date/Time</titleabbrev>
     <tgroup cols="4">
      <thead>
       <row>
	<entry>Date/Time Type</entry>
	<entry>Storage</entry>
	<entry>Recommendation</entry>
	<entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry>abstime</entry>
	<entry>4 bytes</entry>
	<entry>original date and time</entry>
	<entry>limited range</entry>
       </row>
       <row>
	<entry>date</entry>
	<entry>4 bytes</entry>
	<entry><acronym>SQL92</acronym> type</entry>
	<entry>wide range</entry>
       </row>
       <row>
	<entry>datetime</entry>
	<entry>8 bytes</entry>
	<entry>best general date and time</entry>
	<entry>wide range, high precision</entry>
       </row>
       <row>
	<entry>interval</entry>
	<entry>12 bytes</entry>
	<entry><acronym>SQL92</acronym> type</entry>
	<entry>equivalent to timespan</entry>
       </row>
       <row>
	<entry>reltime</entry>
	<entry>4 bytes</entry>
	<entry>original time interval</entry>
	<entry>limited range, low precision</entry>
       </row>
       <row>
	<entry>time</entry>
	<entry>4 bytes</entry>
	<entry><acronym>SQL92</acronym> type</entry>
	<entry>wide range</entry>
       </row>
       <row>
	<entry>timespan</entry>
	<entry>12 bytes</entry>
	<entry>best general time interval</entry>
	<entry>wide range, high precision</entry>
       </row>
       <row>
	<entry>timestamp</entry>
	<entry>4 bytes</entry>
	<entry><acronym>SQL92</acronym> type</entry>
	<entry>limited range</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <type>timestamp</type> is currently implemented separately from
    <type>datetime</type>, although they share input and output routines.
   </para>

   <para>
    <table tocentry="1">
     <title><productname>Postgres</productname> Date/Time Ranges</title>
     <titleabbrev>Ranges</titleabbrev>
     <tgroup cols="4">
      <thead>
       <row>
	<entry>Date/Time Type</entry>
	<entry>Earliest</entry>
	<entry>Latest</entry>
	<entry>Resolution</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry>abstime</entry>
	<entry>1901-12-14</entry>
	<entry>2038-01-19</entry>
	<entry>1 sec</entry>
       </row>
       <row>
	<entry>date</entry>
	<entry>4713 BC</entry>
	<entry>32767 AD</entry>
	<entry>1 day</entry>
       </row>
       <row>
	<entry>datetime</entry>
	<entry>4713 BC</entry>
	<entry>1465001 AD</entry>
	<entry>1 microsec to 14 digits</entry>
       </row>
       <row>
	<entry>interval</entry>
	<entry>-178000000 years</entry>
	<entry>178000000 years</entry>
	<entry>1 microsec</entry>
       </row>
       <row>
	<entry>reltime</entry>
	<entry>-68 years</entry>
	<entry>+68 years</entry>
	<entry>1 sec</entry>
       </row>
       <row>
	<entry>time</entry>
	<entry>00:00:00.00</entry>
	<entry>23:59:59.99</entry>
	<entry>1 microsec</entry>
       </row>
       <row>
	<entry>timespan</entry>
	<entry>-178000000 years</entry>
	<entry>178000000 years</entry>
	<entry>1 microsec (14 digits)</entry>
       </row>
       <row>
	<entry>timestamp</entry>
	<entry>1901-12-14</entry>
	<entry>2038-01-19</entry>
	<entry>1 sec</entry>
       </row>
      </tbody>
     </tgroup>
    </table>
   </para>

   <sect2>
    <title>SQL92 Conventions</title>

    <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.
       </para>
      </listitem>

      <listitem>
       <para>
	The default time zone is specified as a constant integer offset 
	from GMT/UTC.
       </para>
      </listitem>

     </itemizedlist>

     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>

    <para>
     To address these difficulties, <productname>Postgres</productname> 
     associates time zones only with date and time
     types which contain both date and time,
     and 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>
     In future releases, the number of date/time types will decrease, 
     with the current implementation of 
     <type>datetime</type> becoming <type>timestamp</type>, 
     <type>timespan</type> becoming <type>interval</type>,
     and (possibly) <type>abstime</type> and <type>reltime</type> 
     being deprecated in favor of <type>timestamp</type> and <type>interval</type>.
     The more arcane features of the date/time definitions from 
     the <acronym>SQL92</acronym> standard are not likely to be pursued.
    </para>
   </sect2>

<sect2>
<title>Date/Time Styles</title>

<para>
Output formats can be set to one of four styles: 
ISO-8601, <acronym>SQL</acronym> (Ingres), traditional
Postgres, and German.

<table tocentry="1">
<title><productname>Postgres</productname> Date Styles</title>
<titleabbrev>Styles</titleabbrev>
<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><acronym>SQL</acronym></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 <acronym>SQL</acronym> style has European and non-European (US) variants, 
which determines whether month follows day or vica versa.

<table tocentry="1">
<title><productname>Postgres</productname> Date Order Conventions</title>
<titleabbrev>Order</titleabbrev>
<tgroup cols="3">
<thead>
  <row>
    <entry>Style Specification</entry>
    <entry>Description</entry>
    <entry>Example</entry>
  </row>
</thead>
<tbody>
  <row>
    <entry>European</entry>
    <entry>Regional convention</entry>
    <entry>17/12/1997 15:37:16.00 MET</entry>
  </row>
  <row>
    <entry>NonEuropean</entry>
    <entry>Regional convention</entry>
    <entry>12/17/1997 07:37:16.00 PST</entry>
  </row>
  <row>
    <entry>US</entry>
    <entry>Regional convention</entry>
    <entry>12/17/1997 07:37:16.00 PST</entry>
  </row>
</tbody>
</tgroup>
</table>
</para>

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

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

<para>
For <productname>Postgres</productname> v6.4 (and earlier) 
the default date/time style is
"non-European traditional Postgres". 
In future releases, the default may become "ISO" (compatible with ISO-8601),
which alleviates date specification ambiguities and Y2K collation problems.
</para>

</sect2>

   <sect2>
    <title>Calendar</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 something like 4013BC
     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 consistant enough to warrant coding into a date/time handler.
    </para>
   </sect2>

   <sect2>
    <title>Time Zones</title>

    <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 Universal UTC,
     alternately 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 TZ environment variable used by the backend directly
	on postmaster startup as the default time zone.
       </para>
      </listitem>
      <listitem>
       <para>
	The PGTZ environment variable set at the client used by libpq 
	to send time zone information 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>
     </itemizedlist></para>

    <para>
     If an invalid time zone is specified,
     the time zone becomes GMT (on most systems anyway).
    </para>
   </sect2>

   <sect2>
    <title>Date/Time Input</title>

    <para>
     General-use date and time is input using a wide range of
     styles, including ISO-compatible, <acronym>SQL</acronym>-compatible, 
     traditional <productname>Postgres</productname> 
     and other permutations of date and time. In cases where interpretation
     can be ambiguous (quite possible with many traditional styles of date 
     specification) <productname>Postgres</productname> uses a style setting 
     to resolve the ambiguity.
    </para>

    <para>
     Most date and time types share code for data input. For those types
     the input can have any of a wide variety of styles. For numeric date 
     representations,
     European and US conventions can differ, and the proper interpretation 
     is obtained by using the <command>SET DATESTYLE</command>
     command before entering data.
     Note that the style setting does not preclude use of various styles for input;
     it is used primarily to determine the output style and to resolve ambiguities.
    </para>

    <para>
     The special values <literal>current</literal>,
     <literal>infinity</literal> and <literal>-infinity</literal> are provided.
     <literal>infinity</literal> specifies a time later than any other valid time, and
     <literal>-infinity</literal> specifies a time earlier than any other valid time.
     <literal>current</literal> indicates that the current time should be
     substituted whenever this value appears in a computation.
    </para>

    <para>
     The strings
     <literal>now</literal>,
     <literal>today</literal>,
     <literal>yesterday</literal>,
     <literal>tomorrow</literal>,
     and <literal>epoch</literal>
     can be used to specify time values.
     <literal>now</literal>
     means the current transaction time, and differs from
     <literal>current</literal>
     in that the current time is immediately substituted for it.
     <literal>epoch</literal> means <literal>Jan 1 00:00:00 1970 GMT</literal>.
    </para>

    <para>
     <table tocentry="1">
      <title><productname>Postgres</productname> Date/Time Special Constants</title>
      <titleabbrev>Constants</titleabbrev>
      <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>
    </para>

    <para>
     <table tocentry="1">
      <title><productname>Postgres</productname> Date Input</title>
      <titleabbrev>Date Inputs</titleabbrev>
      <tgroup cols="2">
       <thead>
	<row>
	 <entry>Example</entry>
	 <entry>Description</entry>
	</row>
       </thead>
       <tbody>
	<row>
	 <entry>January 8, 1999</entry>
	 <entry>Unambiguous text month</entry>
	</row>
	<row>
	 <entry>1999-01-08</entry>
	 <entry>ISO-8601</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>1999.008</entry>
	 <entry>Year and day of year</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 Christian Era</entry>
	</row>
       </tbody>
      </tgroup>
     </table>
    </para>

    <para>
     <table tocentry="1">
      <title><productname>Postgres</productname> Month Abbreviations</title>
      <titleabbrev>Month Abbreviations</titleabbrev>
      <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><productname>Postgres</productname> Day of Week Abbreviations</title>
      <titleabbrev>Day of Week Abbreviations</titleabbrev>
      <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>

    <para>
     <table tocentry="1">
      <title><productname>Postgres</productname> Time Input</title>
      <titleabbrev>Time Inputs</titleabbrev>
      <tgroup cols="2">
       <thead>
	<row>
	 <entry>Example</entry>
	 <entry>Description</entry>
	</row>
       </thead>
       <tbody>
	<row>
	 <entry>04:05:06.789</entry>
	 <entry>ISO-8601, with all time fields</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>

    <para>
     <table tocentry="1">
      <title><productname>Postgres</productname> Time Zone Input</title>
      <titleabbrev>Time Zone Inputs</titleabbrev>
      <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>

    <para>
     <table tocentry="1">
      <title><productname>Postgres</productname> Recognized Time Zones</title>
      <titleabbrev>Time Zones</titleabbrev>
      <tgroup cols="3">
       <thead>
	<row>
	 <entry>Time Zone</entry>
	 <entry>Offset from UTC</entry>
	 <entry>Description</entry>
	</row>
       </thead>
       <tbody>
	<row>
	 <entry>NZDT</entry>
	 <entry>+13:00</entry>
	 <entry>New Zealand Daylight Time</entry>
	</row>
	<row>
	 <entry>IDLE</entry>
	 <entry>+12:00</entry>
	 <entry>International Date Line, East</entry>
	</row>
	<row>
	 <entry>NZST</entry>
	 <entry>+12:00</entry>
	 <entry>New Zealand Std Time</entry>
	</row>
	<row>
	 <entry>NZT</entry>
	 <entry>+12:00</entry>
	 <entry>New Zealand Time</entry>
	</row>
	<row>
	 <entry>AESST</entry>
	 <entry>+11:00 </entry>
	 <entry>Australia Eastern Summer Std Time</entry>
	</row>
	<row>
	 <entry>ACSST</entry>
	 <entry>+10:30 </entry>
	 <entry>Central Australia Summer Std Time</entry>
	</row>
	<row>
	 <entry>CADT</entry>
	 <entry>+10:30 </entry>
	 <entry>Central Australia Daylight Savings Time</entry>
	</row>
	<row>
	 <entry>SADT</entry>
	 <entry>+10:30</entry>
	 <entry>South Australian Daylight Time</entry>
	</row>
	<row>
	 <entry>AEST</entry>
	 <entry>+10:00 </entry>
	 <entry>Australia Eastern Std Time</entry>
	</row>
	<row>
	 <entry>EAST</entry>
	 <entry>+10:00 </entry>
	 <entry>East Australian Std Time</entry>
	</row>
	<row>
	 <entry>GST</entry>
	 <entry>+10:00</entry>
	 <entry>Guam Std Time, USSR Zone 9</entry>
	</row>
	<row>
	 <entry>LIGT</entry>
	 <entry>+10:00</entry>
	 <entry>Melbourne, Australia</entry>
	</row>
	<row>
	 <entry>ACST</entry>
	 <entry>+09:30 </entry>
	 <entry>Central Australia Std Time</entry>
	</row>
	<row>
	 <entry>CAST</entry>
	 <entry>+09:30 </entry>
	 <entry>Central Australia Std Time</entry>
	</row>
	<row>
	 <entry>SAT</entry>
	 <entry>+9:30</entry>
	 <entry>South Australian Std Time</entry>
	</row>
	<row>
	 <entry>AWSST</entry>
	 <entry>+9:00 </entry>
	 <entry>Australia Western Summer Std Time</entry>
	</row>
	<row>
	 <entry>JST</entry>
	 <entry>+9:00</entry>
	 <entry>Japan Std Time,USSR Zone 8</entry>
	</row>
	<row>
	 <entry>KST</entry>
	 <entry>+9:00</entry>
	 <entry>Korea Standard Time</entry>
	</row>
	<row>
	 <entry>WDT</entry>
	 <entry>+9:00</entry>
	 <entry>West Australian Daylight Time</entry>
	</row>
	<row>
	 <entry>MT</entry>
	 <entry>+8:30</entry>
	 <entry>Moluccas Time</entry>
	</row>
	<row>
	 <entry>AWST</entry>
	 <entry>+8:00 </entry>
	 <entry>Australia Western Std Time</entry>
	</row>
	<row>
	 <entry>CCT</entry>
	 <entry>+8:00 </entry>
	 <entry>China Coastal Time</entry>
	</row>
	<row>
	 <entry>WADT</entry>
	 <entry>+8:00</entry>
	 <entry>West Australian Daylight Time</entry>
	</row>
	<row>
	 <entry>WST</entry>
	 <entry>+8:00</entry>
	 <entry>West Australian Std Time</entry>
	</row>
	<row>
	 <entry>JT</entry>
	 <entry>+7:30</entry>
	 <entry>Java Time</entry>
	</row>
	<row>
	 <entry>WAST</entry>
	 <entry>+7:00</entry>
	 <entry>West Australian Std Time</entry>
	</row>
	<row>
	 <entry>IT</entry>
	 <entry>+3:30</entry>
	 <entry>Iran Time</entry>
	</row>
	<row>
	 <entry>BT</entry>
	 <entry>+3:00 </entry>
	 <entry>Baghdad Time</entry>
	</row>
	<row>
	 <entry>EETDST</entry>
	 <entry>+3:00 </entry>
	 <entry>Eastern Europe Daylight Savings Time</entry>
	</row>
	<row>
	 <entry>CETDST</entry>
	 <entry>+2:00 </entry>
	 <entry>Central European Daylight Savings Time</entry>
	</row>
	<row>
	 <entry>EET</entry>
	 <entry>+2:00 </entry>
	 <entry>Eastern Europe, USSR Zone 1</entry>
	</row>
	<row>
	 <entry>FWT</entry>
	 <entry>+2:00</entry>
	 <entry>French Winter Time</entry>
	</row>
	<row>
	 <entry>IST</entry>
	 <entry>+2:00</entry>
	 <entry>Israel Std Time</entry>
	</row>
	<row>
	 <entry>MEST</entry>
	 <entry>+2:00</entry>
	 <entry>Middle Europe Summer Time</entry>
	</row>
	<row>
	 <entry>METDST</entry>
	 <entry>+2:00</entry>
	 <entry>Middle Europe Daylight Time</entry>
	</row>
	<row>
	 <entry>SST</entry>
	 <entry>+2:00</entry>
	 <entry>Swedish Summer Time</entry>
	</row>
	<row>
	 <entry>BST</entry>
	 <entry>+1:00 </entry>
	 <entry>British Summer Time</entry>
	</row>
	<row>
	 <entry>CET</entry>
	 <entry>+1:00 </entry>
	 <entry>Central European Time</entry>
	</row>
	<row>
	 <entry>DNT</entry>
	 <entry>+1:00 </entry>
	 <entry>Dansk Normal Tid</entry>
	</row>
	<row>
	 <entry>DST</entry>
	 <entry>+1:00 </entry>
	 <entry>Dansk Standard Time (?)</entry>
	</row>
	<row>
	 <entry>FST</entry>
	 <entry>+1:00 </entry>
	 <entry>French Summer Time</entry>
	</row>
	<row>
	 <entry>MET</entry>
	 <entry>+1:00</entry>
	 <entry>Middle Europe Time</entry>
	</row>
	<row>
	 <entry>MEWT</entry>
	 <entry>+1:00</entry>
	 <entry>Middle Europe Winter Time</entry>
	</row>
	<row>
	 <entry>MEZ</entry>
	 <entry>+1:00</entry>
	 <entry>Middle Europe Zone</entry>
	</row>
	<row>
	 <entry>NOR</entry>
	 <entry>+1:00</entry>
	 <entry>Norway Standard Time</entry>
	</row>
	<row>
	 <entry>SET</entry>
	 <entry>+1:00</entry>
	 <entry>Seychelles Time</entry>
	</row>
	<row>
	 <entry>SWT</entry>
	 <entry>+1:00</entry>
	 <entry>Swedish Winter Time</entry>
	</row>
	<row>
	 <entry>WETDST</entry>
	 <entry>+1:00</entry>
	 <entry>Western Europe Daylight Savings Time</entry>
	</row>
	<row>
	 <entry>GMT</entry>
	 <entry>0:00</entry>
	 <entry>Greenwish Mean Time</entry>
	</row>
	<row>
	 <entry>WET</entry>
	 <entry>0:00</entry>
	 <entry>Western Europe</entry>
	</row>
	<row>
	 <entry>WAT</entry>
	 <entry>-1:00</entry>
	 <entry>West Africa Time</entry>
	</row>
	<row>
	 <entry>NDT</entry>
	 <entry>-2:30</entry>
	 <entry>Newfoundland Daylight Time</entry>
	</row>
	<row>
	 <entry>ADT</entry>
	 <entry>-03:00 </entry>
	 <entry>Atlantic Daylight Time</entry>
	</row>
	<row>
	 <entry>NFT</entry>
	 <entry>-3:30</entry>
	 <entry>Newfoundland Standard Time</entry>
	</row>
	<row>
	 <entry>NST</entry>
	 <entry>-3:30</entry>
	 <entry>Newfoundland Standard Time</entry>
	</row>
	<row>
	 <entry>AST</entry>
	 <entry>-4:00 </entry>
	 <entry>Atlantic Std Time (Canada)</entry>
	</row>
	<row>
	 <entry>EDT</entry>
	 <entry>-4:00 </entry>
	 <entry>Eastern Daylight Time</entry>
	</row>
	<row>
	 <entry>ZP4</entry>
	 <entry>-4:00</entry>
	 <entry>GMT +4 hours</entry>
	</row>
	<row>
	 <entry>CDT</entry>
	 <entry>-5:00 </entry>
	 <entry>Central Daylight Time</entry>
	</row>
	<row>
	 <entry>EST</entry>
	 <entry>-5:00 </entry>
	 <entry>Eastern Standard Time</entry>
	</row>
	<row>
	 <entry>ZP5</entry>
	 <entry>-5:00</entry>
	 <entry>GMT +5  hours</entry>
	</row>
	<row>
	 <entry>CST</entry>
	 <entry>-6:00 </entry>
	 <entry>Central Std Time</entry>
	</row>
	<row>
	 <entry>MDT</entry>
	 <entry>-6:00</entry>
	 <entry>Mountain Daylight Time</entry>
	</row>
	<row>
	 <entry>ZP6</entry>
	 <entry>-6:00</entry>
	 <entry>GMT +6  hours</entry>
	</row>
	<row>
	 <entry>MST</entry>
	 <entry>-7:00</entry>
	 <entry>Mountain Standard Time</entry>
	</row>
	<row>
	 <entry>PDT</entry>
	 <entry>-7:00</entry>
	 <entry>Pacific Daylight Time</entry>
	</row>
	<row>
	 <entry>PST</entry>
	 <entry>-8:00</entry>
	 <entry>Pacific Std Time</entry>
	</row>
	<row>
	 <entry>YDT</entry>
	 <entry>-8:00</entry>
	 <entry>Yukon Daylight Time</entry>
	</row>
	<row>
	 <entry>HDT</entry>
	 <entry>-9:00</entry>
	 <entry>Hawaii/Alaska Daylight Time</entry>
	</row>
	<row>
	 <entry>YST</entry>
	 <entry>-9:00</entry>
	 <entry>Yukon Standard Time</entry>
	</row>
	<row>
	 <entry>AHST</entry>
	 <entry>-10:00 </entry>
	 <entry>Alaska-Hawaii Std Time</entry>
	</row>
	<row>
	 <entry>CAT</entry>
	 <entry>-10:00 </entry>
	 <entry>Central Alaska Time</entry>
	</row>
	<row>
	 <entry>NT</entry>
	 <entry>-11:00</entry>
	 <entry>Nome Time</entry>
	</row>
	<row>
	 <entry>IDLW</entry>
	 <entry>-12:00</entry>
	 <entry>International Date Line, West</entry>
	</row>
       </tbody>
      </tgroup>
     </table>
     <note>
      <para>
       If the compiler option USE_AUSTRALIAN_RULES is set 
       then <literal>EST</literal> refers to Australia Eastern Std Time,
       which has an offset of +10:00 hours from UTC.
      </para>
     </note>
</para>

    <para>
     Australian time zones and their naming variants
     account for fully one quarter of all time zones in the 
     <productname>Postgres</productname> time zone lookup table.
    </para>

    <procedure>
     <title>Date/Time Input Interpretation</title>

     <para>
      The date/time types are all decoded using a common set of routines.
     </para>

     <step>
      <para>
       Break the input string into tokens and categorize each token as
       a string, time, time zone, or number.
      </para>

      <substeps>
       <step>
	<para>
	 If the token contains a colon (":"), this is a time string.
	</para>
       </step>

       <step>
	<para>
	 If the token contains a dash ("-"), slash ("/"), or dot ("."),
	 this is a date string which may have a text month.
	</para>
       </step>

       <step>
	<para>
	 If the token is numeric only, then it is either a single field
	 or an ISO-8601 concatenated date (e.g. "19990113" for January 13, 1999)
	 or time (e.g. 141516 for 14:15:16).
	</para>
       </step>
       <step>
	<para>
	 If the token starts with a plus ("+") or minus ("-"),
	 then it is either a time zone or a special field.
	</para>
       </step>
      </substeps>
     </step>

     <step>
      <para>
       If the token is a text string, match up with possible strings.
      </para>

      <substeps>
       <step>
	<para>
	 Do a binary-search table lookup for the token
	 as either a special string (e.g. <literal>today</literal>),
	 day (e.g. <literal>Thursday</literal>),
	 month (e.g. <literal>January</literal>), or noise word (e.g. <literal>on</literal>).
	</para>
	<para>
	 Set field values and bit mask for fields.
	 For example, set year, month, day for <literal>today</literal>, and additionally
	 hour, minute, second for <literal>now</literal>.
	</para>
       </step>

       <step>
	<para>
	 If not found, do a similar binary-search table lookup to match
	 the token with a time zone.
	</para>
       </step>

       <step>
	<para>
	 If not found, throw an error.
	</para>
       </step>
      </substeps>
     </step>

     <step>
      <para>
       The token is a number or number field.
       If there are more than 4 digits, 
       and if no other date fields have been previously read, then interpret 
       as a "concatenated date" (e.g. <literal>19990118</literal>).
      </para>

      <substeps>
       <step>
	<para>
	 If there are more than 4 digits, 
	 and if no other date fields have been previously read, then interpret 
	 as a "concatenated date" (e.g. <literal>19990118</literal>).
	</para>
       </step>

       <step>
	<para>
	 If three digits and a year has already been decoded, then interpret as day of year.
	</para>
       </step>

       <step>
	<para>
	 If longer than two digits, then interpret as a year.
	</para>
       </step>

       <step>
	<para>
	 If in European date mode, and if the day field has not yet been read,
	 and if the value is less than or equal to 31, then interpret as a day.
	</para>
       </step>

       <step>
	<para>
	 If in non-European (US) date mode, and if the month field has not yet been read,
	 and if the value is less than or equal to 12, then interpret as a month.
	</para>
       </step>

       <step>
	<para>
	 If the day field has not yet been read,
	 and if the value is less than or equal to 31, then interpret as a month.
	</para>
       </step>

       <step>
	<para>
	 If the month field has not yet been read,
	 and if the value is less than or equal to 12, then interpret as a month.
	</para>
       </step>

       <step>
	<para>
	 Otherwise, interpret as a year.
	</para>
       </step>
      </substeps>
     </step>

     <step>
      <para>
       If BC has been specified, negate the year and offset by one 
       (there is no year zero in the Gregorian calendar).
      </para>
     </step>

     <step>
      <para>
       If BC was not specified, and if the year field was two digits in length, then
       adjust the year to 4 digits. If the field was less than 70, then add 2000;
       otherwise, add 1900.
      </para>
     </step>

    </procedure>
   </sect2>

   <sect2>
    <title>datetime</title>

<para>
General-use date and time is input using a wide range of
styles, including ISO-compatible, <acronym>SQL</acronym>-compatible, traditional
<productname>Postgres</productname> (see section on "absolute time")
and other permutations of date and time. Output styles can be ISO-compatible,
<acronym>SQL</acronym>-compatible, or traditional 
<productname>Postgres</productname>, with the default set to be compatible
with <productname>Postgres</productname> v6.0.
</para>

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

<programlisting>
Year-Month-Day [ Hour : Minute : Second ]      [AD,BC] [ Timezone ]
  YearMonthDay [ Hour : Minute : Second ]      [AD,BC] [ Timezone ]
     Month Day [ Hour : Minute : Second ] Year [AD,BC] [ Timezone ]
where	
    Year is 4013 BC, ..., very large
    Month is Jan, Feb, ..., Dec or 1, 2, ..., 12
    Day is 1, 2, ..., 31
    Hour is 00, 02, ..., 23
    Minute is 00, 01, ..., 59
    Second is 00, 01, ..., 59 (60 for leap second)
    Timezone is 3 characters or ISO offset to GMT
</programlisting>
    </para>

<para>
Valid dates are from Nov 13 00:00:00 4013 BC GMT to far into the future.
Timezones are either three characters (e.g. "GMT" or "PST") or ISO-compatible
offsets to GMT (e.g. "-08" or "-08:00" when in Pacific Standard Time).
Dates are stored internally in Greenwich Mean Time. Input and output routines 
translate time to the local time zone of the server.
</para></sect2>

<sect2>
<title><type>timespan</type></title>

<para>
General-use time span is input using a wide range of
syntaxes, including ISO-compatible, <acronym>SQL</acronym>-compatible, 
traditional
<productname>Postgres</productname> (see section on "relative time")
 and other permutations of time span. Output formats can be ISO-compatible,
<acronym>SQL</acronym>-compatible, or traditional 
<productname>Postgres</productname>, 
with the default set to be <productname>Postgres</productname>-compatible.
Months and years are a "qualitative" time interval, and are stored separately
from the other "quantitative" time intervals such as day or hour. 
For date arithmetic,
the qualitative time units are instantiated in the context of the 
relevant date or time.
    </para>

<para>
Time span is specified with the following syntax:

<programlisting>
  Quantity Unit [Quantity Unit...] [Direction]
@ Quantity Unit [Direction]
where 	
    Quantity is ..., <literal>-1</literal>, <literal>0</literal>, <literal>1</literal>, <literal>2</literal>, ...
    Unit is <literal>second</literal>, <literal>minute</literal>, <literal>hour</literal>, <literal>day</literal>, <literal>week</literal>, <literal>month</literal>, <literal>year</literal>,
      <literal>decade</literal>, <literal>century</literal>, <literal>millenium</literal>, or abbreviations or plurals of these units.
    Direction is <literal>ago</literal>.
</programlisting>
</para>
</sect2>

<sect2>
<title>abstime</title>

    <para>
     Absolute time (<type>abstime</type>) is a limited-range (+/- 68 years) and 
     limited-precision (1 sec)
     date data type. <type>datetime</type> may be preferred, since it
     covers a larger range with greater precision.
    </para>

<para>
Absolute time is specified using the following syntax:

<programlisting>
Month  Day [ Hour : Minute : Second ]  Year [ Timezone ]
where	
    Month is Jan, Feb, ..., Dec
    Day is 1, 2, ..., 31
    Hour is 01, 02, ..., 24
    Minute is 00, 01, ..., 59
    Second is 00, 01, ..., 59
    Year is 1901, 1902, ..., 2038
</programlisting>
</para>

<para>
     Valid dates are from <literal>Dec 13 20:45:53 1901 GMT</literal>
     to <literal>Jan 19 03:14:04 2038 GMT</literal>. 

<note>
<title>Historical Note</title>
<para>
As of Version 3.0, times are no longer read and written
using Greenwich Mean Time; the input and output routines default to
the local time zone.</para>
</note>

All special values allowed for <type>datetime</type> are also 
allowed for "absolute time".
</para>

</sect2>

<sect2>
<title>reltime</title>

<para>
Relative time <type>reltime</type> is a limited-range (+/- 68 years)
 and limited-precision (1 sec) time span data type.
<type>timespan</type> should be preferred, since it
covers a larger range with greater precision and, more importantly, 
can distinguish between
relative units (months and years) and quantitative units (days, hours, etc). 
Instead, reltime
must force months to be exactly 30 days, so time arithmetic does not 
always work as expected.
     For example, adding one reltime <literal>year</literal> to abstime <literal>today</literal> does not 
produce today's date one year from
now, but rather a date 360 days from today.
</para>

<para>
<type>reltime</type> shares input and output routines with the other 
time span types.
The section on <type>timespan</type> covers this in more detail.
</para>

</sect2>

<sect2>
<title><type>timestamp</type></title>

<para>
This is currently a limited-range absolute time which closely resembles the
abstime
data type. It shares the general input parser with the other date/time types.
In future releases this type will absorb the capabilities of the 
<type>datetime</type> type
and will move toward <acronym>SQL92</acronym> compliance.
</para>

<para>
<type>timestamp</type> is specified using the same syntax as for 
<type>datetime</type>.
</para>
</sect2>

<sect2>
<title><type>interval</type></title>

<para>
<type>interval</type> is an <acronym>SQL92</acronym> data type which is
currently mapped to the <type>timespan</type> 
<productname>Postgres</productname> data type.
</para>
</sect2>

<sect2>
<title>tinterval</title>

<para>
Time ranges are specified as:

<programlisting>
[ 'abstime' 'abstime']
where
    abstime is a time in the absolute time format.
</programlisting>

Special abstime values such as 
<literal>current', <literal>infinity' and <literal>-infinity' can be used.</literal></literal></literal>
</para></sect2>

</sect1>

<sect1>
<title>Boolean Type</title>

<para>
<productname>Postgres</productname> supports <type>bool</type> as
the <acronym>SQL3</acronym> boolean type.
<type>bool</type> can have one of only two states: 'true' or 'false'. 
A third state, 'unknown', is not
implemented and is not suggested in <acronym>SQL3</acronym>; 
<acronym>NULL</acronym> is an
effective substitute. <type>bool</type> can be used in any boolean expression, 
and boolean expressions
always evaluate to a result compatible with this type.</para>

<para>
<type>bool</type> uses 1 byte of storage.
</para>

<para>
<table tocentry="1">
<title><productname>Postgres</productname> Boolean Type</title>
<titleabbrev>Booleans</titleabbrev>
<tgroup cols="3">
<thead>
  <row>
    <entry>State</entry>
    <entry>Output</entry>
    <entry>Input</entry>
  </row>
</thead>
<tbody>
  <row>
    <entry>True</entry>
    <entry>'t'</entry>
    <entry>TRUE, 't', 'true', 'y', 'yes', '1'</entry>
  </row>
  <row>
    <entry>False</entry>
    <entry>'f'</entry>
    <entry>FALSE, 'f', 'false', 'n', 'no', '0'</entry>
  </row>
</tbody>
</tgroup>
</table>
</para>
</sect1>

<sect1>
<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><productname>Postgres</productname> Geometric Types</title>
<titleabbrev>Geometrics</titleabbrev>
<tgroup cols="4">
<thead>
  <row>
    <entry>Geometric Type</entry>
    <entry>Storage</entry>
    <entry>Representation</entry>
    <entry>Description</entry>
  </row>
</thead>
<tbody>
  <row>
    <entry>point</entry>
    <entry>16 bytes</entry>
    <entry>(x,y)</entry>
    <entry>Point in space</entry>
  </row>
  <row>
    <entry>line</entry>
    <entry>32 bytes</entry>
    <entry>((x1,y1),(x2,y2))</entry>
    <entry>Infinite line</entry>
  </row>
  <row>
    <entry>lseg</entry>
    <entry>32 bytes</entry>
    <entry>((x1,y1),(x2,y2))</entry>
    <entry>Finite line segment</entry>
  </row>
  <row>
    <entry>box</entry>
    <entry>32 bytes</entry>
    <entry>((x1,y1),(x2,y2))</entry>
    <entry>Rectangular box</entry>
  </row>
  <row>
    <entry>path</entry>
    <entry>4+32n bytes</entry>
    <entry>((x1,y1),...)</entry>
    <entry>Closed path (similar to polygon)</entry>
  </row>
  <row>
    <entry>path</entry>
    <entry>4+32n bytes</entry>
    <entry>[(x1,y1),...]</entry>
    <entry>Open path</entry>
  </row>
  <row>
    <entry>polygon</entry>
    <entry>4+32n bytes</entry>
    <entry>((x1,y1),...)</entry>
    <entry>Polygon (similar to closed path)</entry>
  </row>
  <row>
    <entry>circle</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>

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

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

<programlisting>
( x , y )
  x , y
where
    x is the x-axis coordinate as a floating point number
    y is the y-axis coordinate as a floating point number
</programlisting>
</para>
</sect2>

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

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

<para>
<type>lseg</type> is specified using the following syntax:
<programlisting>
( ( x1 , y1 ) , ( x2 , y2 ) )
  ( x1 , y1 ) , ( x2 , y2 )  
    x1 , y1   ,   x2 , y2    
where
    (x1,y1) and (x2,y2) are the endpoints of the segment
</programlisting>
</para>
</sect2>

<sect2>
<title>Box</title>

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

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

<programlisting>
( ( x1 , y1 ) , ( x2 , y2 ) )
  ( x1 , y1 ) , ( x2 , y2 )  
    x1 , y1   ,   x2 , y2    
where
    (x1,y1) and (x2,y2) are opposite corners
</programlisting>

Boxes are output using the first syntax.
The corners are reordered on input to store
the lower left corner first and the upper right corner last.
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>

<para>
Paths are represented by connected sets of points. Paths can be "open", where
the first and last points in the set are not connected, and "closed",
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 select either type in a query.
</para>

<para>
path is specified using the following syntax:

<programlisting>
( ( x1 , y1 ) , ... , ( xn , yn ) )
[ ( x1 , y1 ) , ... , ( xn , yn ) ]
  ( x1 , y1 ) , ... , ( xn , yn )  
  ( x1 , y1   , ... ,   xn , yn )  
    x1 , y1   , ... ,   xn , yn    
where
    (x1,y1),...,(xn,yn) are points 1 through n
    a leading "[" indicates an open path
    a leading "(" indicates a closed path
</programlisting>
Paths are output using the first syntax.
Note that <productname>Postgres</productname> versions prior to
v6.1 used a format for paths which had a single leading parenthesis, 
a "closed" flag,
an integer count of the number of points, then the list of points followed by a
closing parenthesis. 
The built-in function <function>upgradepath</function> is supplied to convert
paths dumped and reloaded from pre-v6.1 databases.
</para>
</sect2>

<sect2>
<title>Polygon</title>

<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:

<programlisting>
( ( x1 , y1 ) , ... , ( xn , yn ) )
  ( x1 , y1 ) , ... , ( xn , yn )  
  ( x1 , y1   , ... ,   xn , yn )  
    x1 , y1   , ... ,   xn , yn    
where
    (x1,y1),...,(xn,yn) are points 1 through n
</programlisting>

Polygons are output using the first syntax.
Note that <productname>Postgres</productname> versions prior to
v6.1 used a format for polygons which had a single leading parenthesis, the list
of x-axis coordinates, the list of y-axis coordinates, 
followed by a closing parenthesis.
The built-in function <function>upgradepoly</function> is supplied to convert
polygons dumped and reloaded from pre-v6.1 databases.
</para>
</sect2>

<sect2>
<title>Circle</title>

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

<para>
circle is specified using the following syntax:

<programlisting>
< ( x , y ) , r >
( ( x , y ) , r )
  ( x , y ) , r  
    x , y   , r  
where
    (x,y) is the center of the circle
    r is the radius of the circle
</programlisting>

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

</sect1>

<sect1>
<title>IP Version 4 Networks and Host Addresses</title>

<para>
The <type>cidr</type> type stores networks specified 
in <acronym>CIDR</acronym> (Classless Inter-Domain Routing) notation.
The <type>inet</type> type stores hosts and networks in CIDR notation using a simple
variation in representation to represent simple host TCP/IP addresses.
</para>

<para>
<table tocentry="1">
<title><productname>Postgres</productname>IP Version 4 Types</title>
<titleabbrev>IPV4</titleabbrev>
<tgroup cols="4">
<thead>
  <row>
    <entry>IPV4 Type</entry>
    <entry>Storage</entry>
    <entry>Description</entry>
    <entry>Range</entry>
  </row>
</thead>
<tbody>
  <row>
    <entry>cidr</entry>
    <entry>variable</entry>
    <entry>CIDR networks</entry>
    <entry>Valid IPV4 CIDR blocks</entry>
  </row>
  <row>
    <entry>inet</entry>
    <entry>variable</entry>
    <entry>nets and hosts</entry>
    <entry>Valid IPV4 CIDR blocks</entry>
  </row>
</tbody>
</tgroup>
</table>
</para>

<sect2>
<title>CIDR</title>

<para>
The <type>cidr</type> type holds a CIDR network.
The format for specifying classless networks is <replaceable class="parameter">x.x.x.x/y</replaceable>
 where <replaceable class="parameter">x.x.x.x</replaceable> is the
network and <replaceable class="parameter">/y</replaceable> is the number of bits in the netmask.
If <replaceable class="parameter">/y</replaceable> omitted, it is calculated using assumptions from
the older classfull naming system except that it is extended to include at least
all of the octets in the input.
</para>

<para> 
Here are some examples:

     <table tocentry="1">
      <title><productname>Postgres</productname>IP Types Examples</title>
      <tgroup cols="2">
       <thead> 
	<row> 
	 <entry>CIDR Input</entry>
	 <entry>CIDR Displayed</entry>
	</row>
       </thead>
       <tbody>
	<row>
	 <entry>192.168.1</entry>
	 <entry>192.168.1/24</entry>
	</row>
	<row>
	 <entry>192.168</entry>
	 <entry>192.168.0/24</entry>
	</row>
	<row>
	 <entry>128.1</entry>
	 <entry>128.1/16</entry>
	</row>
	<row>
	 <entry>128</entry>
	 <entry>128.0/16</entry>
	</row>
	<row>
	 <entry>128.1.2</entry>
	 <entry>128.1.2/24</entry>
	</row>
	<row>
	 <entry>10.1.2</entry>
	 <entry>10.1.2/24</entry>
	</row>
	<row>
	 <entry>10.1</entry>
	 <entry>10.1/16</entry>
	</row>
	<row>
	 <entry>10</entry>
	 <entry>10/8</entry>
	</row>
       </tbody>
      </tgroup>
     </table>
    </para>
   </sect2>

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

<para>
The <type>inet</type> type is designed to hold, in one field, all of the information
about a host including the CIDR-style subnet that it is in.
Note that if you want to store proper CIDR networks,
you should use the <type>cidr</type> type.
The <type>inet</type> type is similar to the <type>cidr</type> type except that the bits in the
host part can be non-zero.
Functions exist to extract the various elements of the field.
</para>

<para>  
     The input format for this function is
 <replaceable class="parameter">x.x.x.x/y</replaceable> 
where <replaceable class="parameter">x.x.x.x</replaceable> is
an internet host 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, 
it is treated as <literal>/32</literal>.
On output, the <replaceable class="parameter">/y</replaceable> part is not printed 
if it is <literal>/32</literal>.
This allows the type to be used as a straight host type by just leaving off
the bits part.
</para></sect2>
</sect1>

</chapter>

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