Re: Performance Anomalies in 7.4.5

Thomas F.O'Connell wrote:
> 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

Is your enable_seqscan set to true?
Try it after issuing set enable_seqscan to off;