66 lines
1.9 KiB
Plaintext
66 lines
1.9 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</> 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</> 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</>. 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</> 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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