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Add docs for tablesample system_rows()

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Simon Riggs 2015-05-15 21:44:53 -04:00
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<!-- doc/src/sgml/tsm-system-rows.sgml -->
<sect1 id="tsm-system-rows" xreflabel="tsm_system_rows">
<title>tsm_system_rows</title>
<indexterm zone="tsm-system-rows">
<primary>tsm_system_rows</primary>
</indexterm>
<para>
The <filename>tsm_system_rows</> module provides the tablesample method
<literal>SYSTEM_ROWS</literal>, which can be used inside the
<command>TABLESAMPLE</command> clause of a <command>SELECT</command>.
</para>
<para>
This tablesample method uses a linear probing algorithm to read sample
of a table and uses actual number of rows as limit (unlike the
<literal>SYSTEM</literal> tablesample method which limits by percentage
of a table).
</para>
<sect2>
<title>Examples</title>
<para>
Here is an example of selecting sample of a table with
<literal>SYSTEM_ROWS</>. First install the extension:
</para>
<programlisting>
CREATE EXTENSION tsm_system_rows;
</programlisting>
<para>
Then you can use it in <command>SELECT</command> command same way as other
tablesample methods:
<programlisting>
SELECT * FROM my_table TABLESAMPLE SYSTEM_ROWS(100);
</programlisting>
</para>
<para>
The above command will return a sample of 100 rows from the table my_table
(less if the table does not have 100 visible rows).
</para>
</sect2>
</sect1>