CREATE STATISTICS
CREATE STATISTICS
CREATE STATISTICS  define extended statistics
Synopsis
CREATE STATISTICS [ IF NOT EXISTS ]statistics_name
ON (expression
) FROMtable_name
CREATE STATISTICS [ IF NOT EXISTS ]statistics_name
[ (statistics_kind
[, ... ] ) ] ON {column_name
 (expression
) }, {column_name
 (expression
) } [, ...] FROMtable_name
Description
CREATE STATISTICS
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.
The
CREATE STATISTICS
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.
If a schema name is given (for example,
CREATE STATISTICS
myschema.mystat ...
) 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.
Parameters

IF NOT EXISTS

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.

statistics_name

The name (optionally schemaqualified) of the statistics object to be created.

statistics_kind

A multivariate statistics kind to be computed in this statistics object. Currently supported kinds are
ndistinct
, which enables ndistinct statistics,dependencies
, which enables functional dependency statistics, andmcv
which enables mostcommon 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 Section 14.2.2 and Section 72.2 . 
column_name

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.

expression

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.

table_name

The name (optionally schemaqualified) of the table containing the column(s) the statistics are computed on; see ANALYZE for an explanation of the handling of inheritance and partitions.
Notes
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).
Expression statistics are perexpression 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.
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 PostgreSQL .
Examples
Create table
t1
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:
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);
Without functionaldependency statistics, the planner would assume
that the two
WHERE
conditions are independent, and would
multiply their selectivities together to arrive at a muchtoosmall
row count estimate.
With such statistics, the planner recognizes that the
WHERE
conditions are redundant and does not underestimate the row count.
Create table
t2
with two perfectly correlated columns
(containing identical data), and an MCV list on those columns:
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);
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.
Create table
t3
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:
CREATE TABLE t3 ( a timestamp ); INSERT INTO t3 SELECT i FROM generate_series('20200101'::timestamp, '20201231'::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) = '20200101'::timestamp; EXPLAIN ANALYZE SELECT * FROM t3 WHERE date_trunc('day', a) BETWEEN '20200101'::timestamp AND '20200630'::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 (perexpression  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) = '20200101'::timestamp; EXPLAIN ANALYZE SELECT * FROM t3 WHERE date_trunc('day', a) BETWEEN '20200101'::timestamp AND '20200630'::timestamp; EXPLAIN ANALYZE SELECT date_trunc('month', a), date_trunc('day', a) FROM t3 GROUP BY 1, 2;
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
WHERE
and
GROUP BY
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.
Compatibility
There is no
CREATE STATISTICS
command in the SQL standard.