CREATE STATISTICS(7) PostgreSQL 10.1 Documentation CREATE STATISTICS(7)NAMECREATE_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 68.2.
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.
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))
PostgreSQL 10.1 2017 CREATE STATISTICS(7)
