doc/src/sgml/func/func-statistics.sgml - master - githubmirror/postgres

doc/src/sgml/func/func-statistics.sgml
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Andrew Dunstan
Split func.sgml into more manageable pieces
04 авг 2025, 16:04
04 авг 2025, 16:044e23c9e
  <sect1 id="functions-statistics">
   <title>Statistics Information Functions</title>

   <indexterm zone="functions-statistics">
    <primary>function</primary>
    <secondary>statistics</secondary>
   </indexterm>

   <para>
    <productname>PostgreSQL</productname> provides a function to inspect complex
    statistics defined using the <command>CREATE STATISTICS</command> command.
   </para>

  <sect2 id="functions-statistics-mcv">
   <title>Inspecting MCV Lists</title>

   <indexterm>
    <primary>pg_mcv_list_items</primary>
   </indexterm>

<synopsis>
<function>pg_mcv_list_items</function> ( <type>pg_mcv_list</type> ) <returnvalue>setof record</returnvalue>
</synopsis>

   <para>
    <function>pg_mcv_list_items</function> returns a set of records describing
    all items stored in a multi-column <acronym>MCV</acronym> list.  It
    returns the following columns:

    <informaltable>
     <tgroup cols="3">
      <thead>
       <row>
        <entry>Name</entry>
        <entry>Type</entry>
        <entry>Description</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><literal>index</literal></entry>
        <entry><type>integer</type></entry>
        <entry>index of the item in the <acronym>MCV</acronym> list</entry>
       </row>
       <row>
        <entry><literal>values</literal></entry>
        <entry><type>text[]</type></entry>
        <entry>values stored in the MCV item</entry>
       </row>
       <row>
        <entry><literal>nulls</literal></entry>
        <entry><type>boolean[]</type></entry>
        <entry>flags identifying <literal>NULL</literal> values</entry>
       </row>
       <row>
        <entry><literal>frequency</literal></entry>
        <entry><type>double precision</type></entry>
        <entry>frequency of this <acronym>MCV</acronym> item</entry>
       </row>
       <row>
        <entry><literal>base_frequency</literal></entry>
        <entry><type>double precision</type></entry>
        <entry>base frequency of this <acronym>MCV</acronym> item</entry>
       </row>
      </tbody>
     </tgroup>
    </informaltable>
   </para>

   <para>
    The <function>pg_mcv_list_items</function> function can be used like this:

<programlisting>
SELECT m.* FROM pg_statistic_ext join pg_statistic_ext_data on (oid = stxoid),
                pg_mcv_list_items(stxdmcv) m WHERE stxname = 'stts';
</programlisting>

    Values of the <type>pg_mcv_list</type> type can be obtained only from the
    <structname>pg_statistic_ext_data</structname>.<structfield>stxdmcv</structfield>
    column.
   </para>
  </sect2>

  </sect1>