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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
71 lines
2.1 KiB
Plaintext
71 lines
2.1 KiB
Plaintext
<!-- doc/src/sgml/tsm-system-rows.sgml -->
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<sect1 id="tsm-system-rows" xreflabel="tsm_system_rows">
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<title>tsm_system_rows —
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the <literal>SYSTEM_ROWS</literal> sampling method for <literal>TABLESAMPLE</literal></title>
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<indexterm zone="tsm-system-rows">
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<primary>tsm_system_rows</primary>
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</indexterm>
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<para>
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The <filename>tsm_system_rows</filename> module provides the table sampling method
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<literal>SYSTEM_ROWS</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 integer argument that is the
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maximum number of rows to read. The resulting sample will always contain
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exactly that many rows, unless the table does not contain enough rows, in
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which case the whole table is selected.
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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_ROWS</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 requested.
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</para>
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<para>
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<literal>SYSTEM_ROWS</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-rows-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_ROWS</literal>. First install the extension:
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</para>
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<programlisting>
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CREATE EXTENSION tsm_system_rows;
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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_ROWS(100);
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</programlisting>
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</para>
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<para>
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This command will return a sample of 100 rows from the
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table <structname>my_table</structname> (unless the table does not have 100
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visible rows, in which case all its rows are returned).
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</para>
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</sect2>
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</sect1>
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