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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
66 lines
2.0 KiB
Plaintext
66 lines
2.0 KiB
Plaintext
<!-- doc/src/sgml/tsm-system-time.sgml -->
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<sect1 id="tsm-system-time" xreflabel="tsm_system_time">
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<title>tsm_system_time</title>
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<indexterm zone="tsm-system-time">
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<primary>tsm_system_time</primary>
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</indexterm>
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<para>
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The <filename>tsm_system_time</filename> module provides the table sampling method
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<literal>SYSTEM_TIME</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 floating-point argument that
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is the maximum number of milliseconds to spend reading the table. This
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gives you direct control over how long the query takes, at the price that
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the size of the sample becomes hard to predict. The resulting sample will
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contain as many rows as could be read in the specified time, unless the
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whole table has been read first.
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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_TIME</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 selected.
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</para>
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<para>
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<literal>SYSTEM_TIME</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_TIME</literal>. First install the extension:
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</para>
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<programlisting>
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CREATE EXTENSION tsm_system_time;
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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_TIME(1000);
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</programlisting>
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</para>
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<para>
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This command will return as large a sample of <structname>my_table</structname> as
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it can read in 1 second (1000 milliseconds). Of course, if the whole
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table can be read in under 1 second, all its rows will be returned.
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</para>
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</sect2>
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</sect1>
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