bionic (7) CREATE_STATISTICS.7.gz

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, and dependencies, which enables functional dependency
           statistics. 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 69.2.

       column_name
           The name of a table column to be covered by the computed statistics. At least two column names must
           be given.

       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).

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 rowcount.

COMPATIBILITY

       There is no CREATE STATISTICS command in the SQL standard.

SEE ALSO

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