Re: stats are way off on 8.4 b1 - Mailing list pgsql-performance

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)

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   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com