2009/4/18 Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>:
> 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)
aye, thanks.
--
GJ