CREATE STATISTICS - define extended statistics
CREATE STATISTICS [ IF NOT EXISTS ]
statistics_kind[, ... ] ) ] ON
column_name[, ...] FROM
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,
) 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.
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.
The name (optionally schema-qualified) of the statistics object to be created.
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 71.2 .
The name of a table column to be covered by the computed statistics. At least two column names must be given.
The name (optionally schema-qualified) of the table containing the column(s) the statistics are computed on; see ANALYZE for an explanation of the handling of inheritance and partitions.
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).
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 .
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
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
conditions are redundant and does not underestimate the row count.
There is no
command in the SQL standard.