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85 lines
2.5 KiB
85 lines
2.5 KiB
<sect1 id="functions-statistics">
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<title>Statistics Information Functions</title>
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<indexterm zone="functions-statistics">
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<primary>function</primary>
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<secondary>statistics</secondary>
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</indexterm>
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<para>
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<productname>PostgreSQL</productname> provides a function to inspect complex
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statistics defined using the <command>CREATE STATISTICS</command> command.
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</para>
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<sect2 id="functions-statistics-mcv">
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<title>Inspecting MCV Lists</title>
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<indexterm>
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<primary>pg_mcv_list_items</primary>
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</indexterm>
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<synopsis>
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<function>pg_mcv_list_items</function> ( <type>pg_mcv_list</type> ) <returnvalue>setof record</returnvalue>
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</synopsis>
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<para>
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<function>pg_mcv_list_items</function> returns a set of records describing
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all items stored in a multi-column <acronym>MCV</acronym> list. It
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returns the following columns:
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<informaltable>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Name</entry>
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<entry>Type</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><literal>index</literal></entry>
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<entry><type>integer</type></entry>
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<entry>index of the item in the <acronym>MCV</acronym> list</entry>
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</row>
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<row>
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<entry><literal>values</literal></entry>
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<entry><type>text[]</type></entry>
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<entry>values stored in the MCV item</entry>
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</row>
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<row>
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<entry><literal>nulls</literal></entry>
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<entry><type>boolean[]</type></entry>
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<entry>flags identifying <literal>NULL</literal> values</entry>
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</row>
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<row>
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<entry><literal>frequency</literal></entry>
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<entry><type>double precision</type></entry>
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<entry>frequency of this <acronym>MCV</acronym> item</entry>
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</row>
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<row>
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<entry><literal>base_frequency</literal></entry>
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<entry><type>double precision</type></entry>
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<entry>base frequency of this <acronym>MCV</acronym> item</entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable>
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</para>
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<para>
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The <function>pg_mcv_list_items</function> function can be used like this:
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<programlisting>
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SELECT m.* FROM pg_statistic_ext join pg_statistic_ext_data on (oid = stxoid),
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pg_mcv_list_items(stxdmcv) m WHERE stxname = 'stts';
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</programlisting>
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Values of the <type>pg_mcv_list</type> type can be obtained only from the
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<structname>pg_statistic_ext_data</structname>.<structfield>stxdmcv</structfield>
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column.
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</para>
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</sect2>
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</sect1>
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