CREATE STATISTICS — define extended statistics
CREATE STATISTICS [ IF NOT EXISTS ]
statistics_kind[, ... ] ) ] ON
column_name[, ...] FROM
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.
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,
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
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.
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).
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
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
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.
There is no
command in the SQL standard.
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