CREATE STATISTICS(7)              PostgreSQL 12.3 Documentation              CREATE STATISTICS(7)



NAME
       CREATE_STATISTICS - define extended statistics

SYNOPSIS
       CREATE STATISTICS [ IF NOT EXISTS ] statistics_name
           [ ( statistics_kind [, ... ] ) ]
           ON column_name, column_name [, ...]
           FROM table_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.

       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 schema-qualified) of the statistics object to be created.

       statistics_kind
           A statistics kind to be computed in this statistics object. Currently supported kinds
           are ndistinct, which enables n-distinct statistics, dependencies, which enables
           functional dependency statistics, and mcv 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 Section 14.2.2 and Section 70.2.

       column_name
           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.

       table_name
           The name (optionally schema-qualified) of the table containing the column(s) the
           statistics are computed on.

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).

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 functional-dependency statistics, the planner would assume that the two WHERE
       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
       WHERE conditions are redundant and does not underestimate the row count.

       Create table t2 with two perfectly correlated columns (containing identical data), and a
       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.

COMPATIBILITY
       There is no CREATE STATISTICS command in the SQL standard.

SEE ALSO
       ALTER STATISTICS (ALTER_STATISTICS(7)), DROP STATISTICS (DROP_STATISTICS(7))



PostgreSQL 12.3                                2020                          CREATE STATISTICS(7)