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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>CREATE STATISTICS</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="sql-createserver.html" title="CREATE SERVER" /><link rel="next" href="sql-createsubscription.html" title="CREATE SUBSCRIPTION" /></head><body id="docContent" class="container-fluid col-10"><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">CREATE STATISTICS</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-createserver.html" title="CREATE SERVER">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><th width="60%" align="center">SQL Commands</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-createsubscription.html" title="CREATE SUBSCRIPTION">Next</a></td></tr></table><hr></hr></div><div class="refentry" id="SQL-CREATESTATISTICS"><div class="titlepage"></div><a id="id-1.9.3.83.1" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">CREATE STATISTICS</span></h2><p>CREATE STATISTICS — define extended statistics</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis">
CREATE STATISTICS [ IF NOT EXISTS ] <em class="replaceable"><code>statistics_name</code></em>
[ ( <em class="replaceable"><code>statistics_kind</code></em> [, ... ] ) ]
ON <em class="replaceable"><code>column_name</code></em>, <em class="replaceable"><code>column_name</code></em> [, ...]
FROM <em class="replaceable"><code>table_name</code></em>
</pre></div><div class="refsect1" id="SQL-CREATESTATISTICS-DESCRIPTION"><h2>Description</h2><p>
<code class="command">CREATE STATISTICS</code> 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.
</p><p>
If a schema name is given (for example, <code class="literal">CREATE STATISTICS
myschema.mystat ...</code>) 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.
</p></div><div class="refsect1" id="id-1.9.3.83.6"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">IF NOT EXISTS</code></span></dt><dd><p>
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.
</p></dd><dt><span class="term"><em class="replaceable"><code>statistics_name</code></em></span></dt><dd><p>
The name (optionally schema-qualified) of the statistics object to be
created.
</p></dd><dt><span class="term"><em class="replaceable"><code>statistics_kind</code></em></span></dt><dd><p>
A statistics kind to be computed in this statistics object.
Currently supported kinds are
<code class="literal">ndistinct</code>, which enables n-distinct statistics,
<code class="literal">dependencies</code>, which enables functional
dependency statistics, and <code class="literal">mcv</code> 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 <a class="xref" href="planner-stats.html#PLANNER-STATS-EXTENDED" title="14.2.2. Extended Statistics">Section 14.2.2</a>
and <a class="xref" href="multivariate-statistics-examples.html" title="71.2. Multivariate Statistics Examples">Section 71.2</a>.
</p></dd><dt><span class="term"><em class="replaceable"><code>column_name</code></em></span></dt><dd><p>
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.
</p></dd><dt><span class="term"><em class="replaceable"><code>table_name</code></em></span></dt><dd><p>
The name (optionally schema-qualified) of the table containing the
column(s) the statistics are computed on.
</p></dd></dl></div></div><div class="refsect1" id="id-1.9.3.83.7"><h2>Notes</h2><p>
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).
</p></div><div class="refsect1" id="SQL-CREATESTATISTICS-EXAMPLES"><h2>Examples</h2><p>
Create table <code class="structname">t1</code> 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:
</p><pre class="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);
</pre><p>
Without functional-dependency statistics, the planner would assume
that the two <code class="literal">WHERE</code> 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 <code class="literal">WHERE</code>
conditions are redundant and does not underestimate the row count.
</p><p>
Create table <code class="structname">t2</code> with two perfectly correlated columns
(containing identical data), and a MCV list on those columns:
</p><pre class="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);
</pre><p>
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.
</p></div><div class="refsect1" id="id-1.9.3.83.9"><h2>Compatibility</h2><p>
There is no <code class="command">CREATE STATISTICS</code> command in the SQL standard.
</p></div><div class="refsect1" id="id-1.9.3.83.10"><h2>See Also</h2><span class="simplelist"><a class="xref" href="sql-alterstatistics.html" title="ALTER STATISTICS"><span class="refentrytitle">ALTER STATISTICS</span></a>, <a class="xref" href="sql-dropstatistics.html" title="DROP STATISTICS"><span class="refentrytitle">DROP STATISTICS</span></a></span></div></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-createserver.html" title="CREATE SERVER">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-createsubscription.html" title="CREATE SUBSCRIPTION">Next</a></td></tr><tr><td width="40%" align="left" valign="top">CREATE SERVER </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> CREATE SUBSCRIPTION</td></tr></table></div></body></html>
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