CREATE STATISTICS — define extended statistics
CREATE STATISTICS [ IF NOT EXISTS ]statistics_name
[ (statistic_type
[, ... ] ) ] ONcolumn_name
,column_name
[, ...] FROMtable_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.
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.
statistic_type
A statistic type to be computed in this statistics
object. Currently supported types are ndistinct
, which enables n-distinct
statistics, and dependencies
, which enables functional
dependency statistics. If this clause is omitted, all
supported statistic types 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.
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).
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.
There is no CREATE STATISTICS
command in the SQL standard.
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