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The original titles only had the module name, which is not very useful when scanning the list. By adding a very brief description to each title, the table of contents becomes friendlier. Also amend the introduction in the "additional modules" appendix, using the word "Extension" more extensively. Nowadays, almost all contrib modules are extensions, so this is also helpful. Author: Karl O. Pinc <kop@karlpinc.com> Reviewed-by: Brar Piening <brar@gmx.de> Discussion: https://postgr.es/m/20230102180015.372995a9@slate.karlpinc.com
73 lines
2.3 KiB
Plaintext
73 lines
2.3 KiB
Plaintext
<!-- doc/src/sgml/tsm-system-time.sgml -->
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<sect1 id="tsm-system-time" xreflabel="tsm_system_time">
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<title>tsm_system_time —
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the <literal>SYSTEM_TIME</literal> sampling method for <literal>TABLESAMPLE</literal></title>
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<indexterm zone="tsm-system-time">
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<primary>tsm_system_time</primary>
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</indexterm>
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<para>
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The <filename>tsm_system_time</filename> module provides the table sampling method
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<literal>SYSTEM_TIME</literal>, which can be used in
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the <literal>TABLESAMPLE</literal> clause of a <link linkend="sql-select"><command>SELECT</command></link>
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command.
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</para>
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<para>
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This table sampling method accepts a single floating-point argument that
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is the maximum number of milliseconds to spend reading the table. This
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gives you direct control over how long the query takes, at the price that
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the size of the sample becomes hard to predict. The resulting sample will
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contain as many rows as could be read in the specified time, unless the
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whole table has been read first.
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</para>
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<para>
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Like the built-in <literal>SYSTEM</literal> sampling
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method, <literal>SYSTEM_TIME</literal> performs block-level sampling, so
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that the sample is not completely random but may be subject to clustering
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effects, especially if only a small number of rows are selected.
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</para>
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<para>
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<literal>SYSTEM_TIME</literal> does not support
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the <literal>REPEATABLE</literal> clause.
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</para>
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<para>
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This module is considered <quote>trusted</quote>, that is, it can be
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installed by non-superusers who have <literal>CREATE</literal> privilege
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on the current database.
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</para>
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<sect2 id="tsm-system-time-examples">
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<title>Examples</title>
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<para>
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Here is an example of selecting a sample of a table with
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<literal>SYSTEM_TIME</literal>. First install the extension:
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</para>
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<programlisting>
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CREATE EXTENSION tsm_system_time;
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</programlisting>
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<para>
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Then you can use it in a <command>SELECT</command> command, for instance:
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<programlisting>
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SELECT * FROM my_table TABLESAMPLE SYSTEM_TIME(1000);
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</programlisting>
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</para>
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<para>
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This command will return as large a sample of <structname>my_table</structname> as
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it can read in 1 second (1000 milliseconds). Of course, if the whole
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table can be read in under 1 second, all its rows will be returned.
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</para>
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</sect2>
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</sect1>
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