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Add docs for tablesample system_rows()
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doc/src/sgml/tsm-system-rows.sgml
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doc/src/sgml/tsm-system-rows.sgml
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<!-- 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</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</> module provides the tablesample method
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<literal>SYSTEM_ROWS</literal>, which can be used inside the
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<command>TABLESAMPLE</command> clause of a <command>SELECT</command>.
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</para>
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<para>
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This tablesample method uses a linear probing algorithm to read sample
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of a table and uses actual number of rows as limit (unlike the
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<literal>SYSTEM</literal> tablesample method which limits by percentage
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of a table).
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</para>
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<sect2>
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<title>Examples</title>
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<para>
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Here is an example of selecting sample of a table with
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<literal>SYSTEM_ROWS</>. 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 <command>SELECT</command> command same way as other
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tablesample methods:
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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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The above command will return a sample of 100 rows from the table my_table
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(less if the table does not have 100 visible rows).
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</para>
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</sect2>
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</sect1>
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