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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