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<chapter id="advanced">
</sect2>
</sect1>
-<!--
-
-We haven't had Time Travel for two or three years, so let's stop
-mentioning it. - thomas 2000-04-02
-
- <sect1>
- <title>Time Travel</title>
-
- <para>
- As of <productname>Postgres</productname> v6.2, <emphasis>time
- travel is no longer supported</emphasis>. There are
- several reasons for this: performance impact, storage size, and a
- pg_time file which grows
- toward infinite size in a short period of time.
- </para>
-
- <para>
- New features such as triggers allow one to mimic the behavior of
- time travel when desired, without
- incurring the overhead when it is not needed (for most users, this
- is most of the time).
- See examples in the <filename>contrib</filename> directory for
- more information.
- </para>
-
- <note>
- <title>Time travel is deprecated</title>
- <para>
- The remaining text in this section is retained only until it can
- be rewritten in the context
- of new techniques to accomplish the same purpose.
- Volunteers? - thomas 1998-01-12
- </para>
- </note>
-
- <para>
- <productname>Postgres</productname> supports the notion of time
- travel. This feature
- allows a user to run historical queries. For
- example, to find the current population of Mariposa
- city, one would query:
-
- <programlisting>
-SELECT * FROM cities WHERE name = 'Mariposa';
-
-+---------+------------+----------+
-|name | population | altitude |
-+---------+------------+----------+
-|Mariposa | 1320 | 1953 |
-+---------+------------+----------+
- </programlisting>
-
- <productname>Postgres</productname> will automatically find the
- version of Mariposa's
- record valid at the current time.
- One can also give a time range. For example to see the
- past and present populations of Mariposa, one would
- query:
-
- <programlisting>
-SELECT name, population
- FROM cities['epoch', 'now']
- WHERE name = 'Mariposa';
-</programlisting>
-
- where "epoch" indicates the beginning of the system
- clock.
-
- <note>
- <para>
- On Unix systems, this is always midnight, January 1, 1970 GMT.
- </para>
- </note>
- </para>
-
- <para>
- If you have executed all of the examples so
- far, then the above query returns:
-
- <programlisting>
-+---------+------------+
-|name | population |
-+---------+------------+
-|Mariposa | 1200 |
-+---------+------------+
-|Mariposa | 1320 |
-+---------+------------+
- </programlisting>
- </para>
-
- <para>
- The default beginning of a time range is the earliest
- time representable by the system and the default end is
- the current time; thus, the above time range can be
- abbreviated as ``[,].''
- </para>
- </sect1>
--->
-
<sect1>
<title>More Advanced Features</title>
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<chapter id="query">
flavor of <acronym>SQL</acronym> and is in no way a complete tutorial on
<acronym>SQL</acronym>. Numerous books have been written on
<acronym>SQL92</acronym>, including
- <xref linkend="MELT93" endterm="MELT93-title"> and
- <xref linkend="DATE97" endterm="DATE97-title">.
+ <xref linkend="MELT93" endterm="MELT93"> and
+ <xref linkend="DATE97" endterm="DATE97">.
You should be aware that some language features
are extensions to the <acronym>ANSI</acronym> standard.
</para>
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