Re: Bad n_distinct estimation; hacks suggested?

> > Solaris is unknown to me. Maybe the used random number generator there
> > isn't good enough?
>
> Hmmm. Good point. Will have to test on Linux.

Nope:

Linux 2.4.20:

test=# select tablename, attname, n_distinct from pg_stats where tablename =
'web_site_activity_fa';
tablename | attname | n_distinct
----------------------+---------------------+------------
web_site_activity_fa | session_id | 626127

test=# select count(distinct session_id) from web_site_activity_fa;
count
---------
3174813
(1 row)

... I think the problem is in our heuristic sampling code. I'm not the first
person to have this kind of a problem. Will be following up with tests ...

--
--Josh

Josh Berkus
Aglio Database Solutions
San Francisco