On Sun, Mar 12, 2006 at 11:36:23PM -0800, Casey Duncan wrote:
> SELECT count(*) FROM webhits
> WHERE path LIKE '/radio/tuner_%.swf' AND status = 200
> AND date_recorded >= '3/10/2006'::TIMESTAMP
> AND date_recorded < '3/11/2006'::TIMESTAMP;
[...]
> Aggregate (cost=794775.08..794775.09 rows=1 width=0)
[...]
> According to the planner it should take <15 minutes which is typical in
> practice.
The planner's cost estimate is in units of disk page fetches, not
time. The above estimate isn't 794775.09 ms (~13.25 min) but rather
794775.09 times the cost of a single page fetch, however much that
is. See "Using EXPLAIN" in the "Performance Tips" chapter of the
documentation.
http://www.postgresql.org/docs/8.1/interactive/performance-tips.html#USING-EXPLAIN
> About half the times it runs, however, it never terminates
> (even after days) and just spins consuming 99+% of CPU with no disk
> activity. This query was never a problem in postgres versions < 8.1.2,
> however the data has grown substantially since that time. I notice it
> uses the recent in-memory bitmap feature, so I wondered if it was
> exposing a bug.
If the problem happens half the time then you have a somewhat
repeatable test case. Do you get more consistent performance if
you set enable_bitmapscan to off? What's the query plan if you do
that?
If you narrow the search criteria so the query returns fewer rows,
do you still see the problem? Can you identify a "sour spot" where
the problem starts to happen?
> If I restart the postmaster, the query will complete in the expected
> time.
Does the problem eventually start happening again? If so, after
how long? How did you determine that the restart is relevant? Do
you consistently see different (presumably better) performance after
a restart than if you don't restart?
--
Michael Fuhr
