<!--
doc/src/sgml/ref/create_statistics.sgml
PostgreSQL documentation
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<refentry id="sql-createstatistics">
 <indexterm zone="sql-createstatistics">
  <primary>CREATE STATISTICS</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>CREATE STATISTICS</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CREATE STATISTICS</refname>
  <refpurpose>define extended statistics</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
CREATE STATISTICS [ IF NOT EXISTS ] <replaceable class="parameter">statistics_name</replaceable>
    [ ( <replaceable class="parameter">statistics_kind</replaceable> [, ... ] ) ]
    ON <replaceable class="parameter">column_name</replaceable>, <replaceable class="parameter">column_name</replaceable> [, ...]
    FROM <replaceable class="parameter">table_name</replaceable>
</synopsis>

 </refsynopsisdiv>

 <refsect1 id="sql-createstatistics-description">
  <title>Description</title>

  <para>
   <command>CREATE STATISTICS</command> will create a new extended statistics
   object tracking data about the specified table, foreign table or
   materialized view.  The statistics object will be created in the current
   database and will be owned by the user issuing the command.
  </para>

  <para>
   If a schema name is given (for example, <literal>CREATE STATISTICS
   myschema.mystat ...</literal>) then the statistics object is created in the
   specified schema.  Otherwise it is created in the current schema.
   The name of the statistics object must be distinct from the name of any
   other statistics object in the same schema.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>

   <varlistentry>
    <term><literal>IF NOT EXISTS</literal></term>
    <listitem>
     <para>
      Do not throw an error if a statistics object with the same name already
      exists.  A notice is issued in this case.  Note that only the name of
      the statistics object is considered here, not the details of its
      definition.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">statistics_name</replaceable></term>
    <listitem>
     <para>
      The name (optionally schema-qualified) of the statistics object to be
      created.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">statistics_kind</replaceable></term>
    <listitem>
     <para>
      A statistics kind to be computed in this statistics object.
      Currently supported kinds are
      <literal>ndistinct</literal>, which enables n-distinct statistics,
      <literal>dependencies</literal>, which enables functional
      dependency statistics, and <literal>mcv</literal> which enables
      most-common values lists.
      If this clause is omitted, all supported statistics kinds are
      included in the statistics object.
      For more information, see <xref linkend="planner-stats-extended"/>
      and <xref linkend="multivariate-statistics-examples"/>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">column_name</replaceable></term>
    <listitem>
     <para>
      The name of a table column to be covered by the computed statistics.
      At least two column names must be given;  the order of the column names
      is insignificant.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">table_name</replaceable></term>
    <listitem>
     <para>
      The name (optionally schema-qualified) of the table containing the
      column(s) the statistics are computed on;  see <xref
      linkend="sql-analyze"/> for an explanation of the handling of
      inheritance and partitions.
     </para>
    </listitem>
   </varlistentry>

  </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   You must be the owner of a table to create a statistics object
   reading it.  Once created, however, the ownership of the statistics
   object is independent of the underlying table(s).
  </para>

  <para>
   Extended statistics are not currently used by the planner for selectivity
   estimations made for table joins.  This limitation will likely be removed
   in a future version of <productname>PostgreSQL</productname>.
  </para>
 </refsect1>

 <refsect1 id="sql-createstatistics-examples">
  <title>Examples</title>

  <para>
   Create table <structname>t1</structname> with two functionally dependent columns, i.e.,
   knowledge of a value in the first column is sufficient for determining the
   value in the other column. Then functional dependency statistics are built
   on those columns:

<programlisting>
CREATE TABLE t1 (
    a   int,
    b   int
);

INSERT INTO t1 SELECT i/100, i/500
                 FROM generate_series(1,1000000) s(i);

ANALYZE t1;

-- the number of matching rows will be drastically underestimated:
EXPLAIN ANALYZE SELECT * FROM t1 WHERE (a = 1) AND (b = 0);

CREATE STATISTICS s1 (dependencies) ON a, b FROM t1;

ANALYZE t1;

-- now the row count estimate is more accurate:
EXPLAIN ANALYZE SELECT * FROM t1 WHERE (a = 1) AND (b = 0);
</programlisting>

   Without functional-dependency statistics, the planner would assume
   that the two <literal>WHERE</literal> conditions are independent, and would
   multiply their selectivities together to arrive at a much-too-small
   row count estimate.
   With such statistics, the planner recognizes that the <literal>WHERE</literal>
   conditions are redundant and does not underestimate the row count.
  </para>

  <para>
   Create table <structname>t2</structname> with two perfectly correlated columns
   (containing identical data), and a MCV list on those columns:

<programlisting>
CREATE TABLE t2 (
    a   int,
    b   int
);

INSERT INTO t2 SELECT mod(i,100), mod(i,100)
                 FROM generate_series(1,1000000) s(i);

CREATE STATISTICS s2 (mcv) ON a, b FROM t2;

ANALYZE t2;

-- valid combination (found in MCV)
EXPLAIN ANALYZE SELECT * FROM t2 WHERE (a = 1) AND (b = 1);

-- invalid combination (not found in MCV)
EXPLAIN ANALYZE SELECT * FROM t2 WHERE (a = 1) AND (b = 2);
</programlisting>

   The MCV list gives the planner more detailed information about the
   specific values that commonly appear in the table, as well as an upper
   bound on the selectivities of combinations of values that do not appear
   in the table, allowing it to generate better estimates in both cases.
  </para>

 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   There is no <command>CREATE STATISTICS</command> command in the SQL standard.
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-alterstatistics"/></member>
   <member><xref linkend="sql-dropstatistics"/></member>
  </simplelist>
 </refsect1>
</refentry>
