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3c49c6facb
Since some preparation work had already been done, the only source changes left were changing empty-element tags like <xref linkend="foo"> to <xref linkend="foo"/>, and changing the DOCTYPE. The source files are still named *.sgml, but they are actually XML files now. Renaming could be considered later. In the build system, the intermediate step to convert from SGML to XML is removed. Everything is build straight from the source files again. The OpenSP (or the old SP) package is no longer needed. The documentation toolchain instructions are updated and are much simpler now. Peter Eisentraut, Alexander Lakhin, Jürgen Purtz
64 lines
1.8 KiB
Plaintext
64 lines
1.8 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</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 <xref linkend="sql-select"/>
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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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<sect2>
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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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