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52 lines
1.4 KiB
Plaintext
52 lines
1.4 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 tablesample method
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<literal>SYSTEM_TIME</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 time in milliseconds as limit (unlike the
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<literal>SYSTEM</literal> tablesample method which limits by percentage
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of a table). This gives you some control over the length of execution
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of your query.
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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_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 the same way as
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other tablesample methods:
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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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The above command will return as large a sample of my_table as it can read in
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1 second (or less if it reads whole table faster).
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</para>
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</sect2>
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</sect1>
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