Grzegorz Jaśkiewicz wrote:
> Can correlation be negative ?
Yes, if the data in the column are in descending order. For example:
postgres=# CREATE TABLE foo(id int4);
CREATE TABLE
postgres=# INSERT INTO foo SELECT 1000 - generate_series(1, 1000);
INSERT 0 1000
postgres=# ANALYZE foo;
ANALYZE
postgres=# SELECT attname, correlation FROM pg_stats WHERE tablename='foo';
attname | correlation
---------+-------------
id | -1
(1 row)
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com