Performance Anomalies in 7.4.5

I'm seeing some weird behavior on a repurposed server that was wiped
clean and set up to run as a database and application server with
postgres and Apache, as well as some command-line PHP scripts.

The box itself is a quad processor (2.4 GHz Intel Xeons) Debian woody
GNU/Linux (2.6.2) system.

postgres is crawling on some fairly routine queries. I'm wondering if
this could somehow be related to the fact that this isn't a
database-only server, but Apache is not really using any resources when
postgres slows to a crawl.

Here's an example of analysis of a recent query:

EXPLAIN ANALYZE SELECT COUNT(DISTINCT u.id)
FROM userdata as u, userdata_history as h
WHERE h.id = '18181'
AND h.id = u.id;

QUERY PLAN
------------------------------------------------------------------------
------------------------------------------------------------------------
Aggregate (cost=0.02..0.02 rows=1 width=8) (actual
time=298321.421..298321.422 rows=1 loops=1)
-> Nested Loop (cost=0.00..0.01 rows=1 width=8) (actual
time=1.771..298305.531 rows=2452 loops=1)
Join Filter: ("inner".id = "outer".id)
-> Seq Scan on userdata u (cost=0.00..0.00 rows=1 width=8)
(actual time=0.026..11.869 rows=2452 loops=1)
-> Seq Scan on userdata_history h (cost=0.00..0.00 rows=1
width=8) (actual time=0.005..70.519 rows=41631 loops=2452)
Filter: (id = 18181::bigint)
Total runtime: 298321.926 ms
(7 rows)

userdata has a primary/foreign key on id, which references
userdata_history.id, which is a primary key.

At the time of analysis, the userdata table had < 2,500 rows.
userdata_history had < 50,000 rows. I can't imagine how even a seq scan
could result in a runtime of nearly 5 minutes in these circumstances.

Also, doing a count( * ) from each table individually returns nearly
instantly.

I can provide details of postgresql.conf and kernel settings if
necessary, but I'm using some pretty well tested settings that I use
any time I admin a postgres installation these days based on box
resources and database size. I'm more interested in knowing if there
are any bird's eye details I should be checking immediately.

Thanks.

-tfo

--
Thomas F. O'Connell
Co-Founder, Information Architect
Sitening, LLC
http://www.sitening.com/
110 30th Avenue North, Suite 6
Nashville, TN 37203-6320
615-260-0005