NAME

       CREATE_STATISTICS - define extended statistics

SYNOPSIS

       CREATE STATISTICS [ [ IF NOT EXISTS ] statistics_name ]
           ON ( expression )
           FROM table_name

       CREATE STATISTICS [ [ IF NOT EXISTS ] statistics_name ]
           [ ( statistics_kind [, ... ] ) ]
           ON { column_name | ( expression ) }, { column_name | ( expression ) } [, ...]
           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.

       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. If given, 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 is required when IF NOT EXISTS is specified.

       statistics_name
           The name (optionally schema-qualified) of the statistics object to be created. If the name is
           omitted, PostgreSQL chooses a suitable name based on the parent table's name and the defined column
           name(s) and/or expression(s).

       statistics_kind
           A multivariate 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. 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 76.2.

       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 schema-qualified) of the table containing the column(s) the statistics are
           computed on; see ANALYZE(7) 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 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.

       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 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 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('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;

       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.

SEE ALSO

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