PostgreSQL STATISTICS

CREATE STATISTICS

CREATE STATISTICS — define extended statistics
CREATE STATISTICS [ IF NOT EXISTS ] statistics_name
[ statistics_kind [, … ] ) ]
ON column_name, column_name [, …]
FROM table_name

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.

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.

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.

ALTER STATISTICS

ALTER STATISTICS — change the definition of an extended statistics object

ALTER STATISTICS name OWNER TO { new_owner | CURRENT_USER | SESSION_USER }
ALTER STATISTICS name RENAME TO new_name
ALTER STATISTICS name SET SCHEMA new_schema
ALTER STATISTICS name SET STATISTICS new_target

ALTER STATISTICS changes the parameters of an existing extended statistics object. Any parameters not specifically set in the ALTER STATISTICS command retain their prior settings.

You must own the statistics object to use ALTER STATISTICS. To change a statistics object’s schema, you must also have CREATE privilege on the new schema. To alter the owner, you must also be a direct or indirect member of the new owning role, and that role must have CREATE privilege on the statistics object’s schema. (These restrictions enforce that altering the owner doesn’t do anything you couldn’t do by dropping and recreating the statistics object. However, a superuser can alter ownership of any statistics object anyway.)

Parameters

name

The name (optionally schema-qualified) of the statistics object to be altered.

new_owner

The user name of the new owner of the statistics object.

new_name

The new name for the statistics object.

new_schema

The new schema for the statistics object.

new_target

The statistic-gathering target for this statistics object for subsequent ANALYZE operations. The target can be set in the range 0 to 10000; alternatively, set it to -1 to revert to using the maximum of the statistics target of the referenced columns, if set, or the system default statistics target (default_statistics_target).

DROP STATISTICS

DROP STATISTICS — remove extended statistics

DROP STATISTICS [ IF EXISTS ] name [, …]

DROP STATISTICS removes statistics object(s) from the database. Only the statistics object’s owner, the schema owner, or a superuser can drop a statistics object.

Parameters

IF EXISTS

Do not throw an error if the statistics object does not exist. A notice is issued in this case.

name

The name (optionally schema-qualified) of the statistics object to drop.

Examples

To destroy two statistics objects in different schemas, without failing if they don’t exist:

DROP STATISTICS IF EXISTS
    accounting.users_uid_creation,
    public.grants_user_role;

Also See:

PostgreSQL SCHEMA