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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!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 Vsnapshot" /><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 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 15.7 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 /></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>
ON ( <em class="replaceable"><code>expression</code></em> )
FROM <em class="replaceable"><code>table_name</code></em>
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>expression</code></em> ) }, { <em class="replaceable"><code>column_name</code></em> | ( <em class="replaceable"><code>expression</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>
The <code class="command">CREATE STATISTICS</code> command has two basic forms. The
first form allows univariate statistics for a single expression to be
collected, providing benefits similar to an expression index without the
overhead of index maintenance. This form does not allow the statistics
kind to be specified, since the various statistics kinds refer only to
multivariate statistics. The second form of the command allows
multivariate statistics on multiple columns and/or expressions to be
collected, optionally specifying which statistics kinds to include. This
form will also automatically cause univariate statistics to be collected on
any expressions included in the list.
</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 multivariate 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. Univariate expression statistics are
built automatically if the statistics definition includes any complex
expressions rather than just simple column references.
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="75.2. Multivariate Statistics Examples">Section 75.2</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.
This is only allowed when building multivariate statistics. At least
two column names or expressions must be specified, and their order is
not significant.
</p></dd><dt><span class="term"><em class="replaceable"><code>expression</code></em></span></dt><dd><p>
An expression to be covered by the computed statistics. This may be
used to build univariate statistics on a single expression, or as part
of a list of multiple column names and/or expressions to build
multivariate statistics. In the latter case, separate univariate
statistics are built automatically for each expression in the list.
</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; see <a class="xref" href="sql-analyze.html" title="ANALYZE"><span class="refentrytitle">ANALYZE</span></a> for an explanation of the handling of
inheritance and partitions.
</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><p>
Expression statistics are per-expression and are similar to creating an
index on the expression, except that they avoid the overhead of index
maintenance. Expression statistics are built automatically for each
expression in the statistics object definition.
</p><p>
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 <span class="productname">PostgreSQL</span>.
</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 an 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><p>
Create table <code class="structname">t3</code> with a single timestamp column,
and run queries using expressions on that column. Without extended
statistics, the planner has no information about the data distribution for
the expressions, and uses default estimates. The planner also does not
realize that the value of the date truncated to the month is fully
determined by the value of the date truncated to the day. Then expression
and ndistinct statistics are built on those two expressions:
</p><pre class="programlisting">
CREATE TABLE t3 (
a timestamp
);
INSERT INTO t3 SELECT i FROM generate_series('2020-01-01'::timestamp,
'2020-12-31'::timestamp,
'1 minute'::interval) s(i);
ANALYZE t3;
-- the number of matching rows will be drastically underestimated:
EXPLAIN ANALYZE SELECT * FROM t3
WHERE date_trunc('month', a) = '2020-01-01'::timestamp;
EXPLAIN ANALYZE SELECT * FROM t3
WHERE date_trunc('day', a) BETWEEN '2020-01-01'::timestamp
AND '2020-06-30'::timestamp;
EXPLAIN ANALYZE SELECT date_trunc('month', a), date_trunc('day', a)
FROM t3 GROUP BY 1, 2;
-- build ndistinct statistics on the pair of expressions (per-expression
-- statistics are built automatically)
CREATE STATISTICS s3 (ndistinct) ON date_trunc('month', a), date_trunc('day', a) FROM t3;
ANALYZE t3;
-- now the row count estimates are more accurate:
EXPLAIN ANALYZE SELECT * FROM t3
WHERE date_trunc('month', a) = '2020-01-01'::timestamp;
EXPLAIN ANALYZE SELECT * FROM t3
WHERE date_trunc('day', a) BETWEEN '2020-01-01'::timestamp
AND '2020-06-30'::timestamp;
EXPLAIN ANALYZE SELECT date_trunc('month', a), date_trunc('day', a)
FROM t3 GROUP BY 1, 2;
</pre><p>
Without expression and ndistinct statistics, the planner has no information
about the number of distinct values for the expressions, and has to rely
on default estimates. The equality and range conditions are assumed to have
0.5% selectivity, and the number of distinct values in the expression is
assumed to be the same as for the column (i.e. unique). This results in a
significant underestimate of the row count in the first two queries. Moreover,
the planner has no information about the relationship between the expressions,
so it assumes the two <code class="literal">WHERE</code> and <code class="literal">GROUP BY</code>
conditions are independent, and multiplies their selectivities together to
arrive at a severe overestimate of the group count in the aggregate query.
This is further exacerbated by the lack of accurate statistics for the
expressions, forcing the planner to use a default ndistinct estimate for the
expression derived from ndistinct for the column. With such statistics, the
planner recognizes that the conditions are correlated, and arrives at much
more accurate estimates.
</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 class="navfooter"><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 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> CREATE SUBSCRIPTION</td></tr></table></div></body></html>
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