Re: slow query

Thanks everybody for your help.
VACUUM FULL did the job, and now the query performance is the same in
both databases. I am surprised that FULL option makes such a dramatic
change to the query performance: from 4min. to 5sec.!!! It also changed
planner stats from ~9 sec to ~8sec.
I haven't tried to REINDEX yet, though.
Regarding IN vs. EXISTS. The sub-query in the IN clause will always
return fewer records than 12. I tried using EXISTS instead of IN with
Postgres7.2.1 and it slowed down query performance. With postgres 7.3,
when I use EXISTS instead of IN the planner returns the same stats and
query performance does not improve. However, if I use
m.mediatype=(SELECT objectid FROM mediatype WHERE medianame='Audio')
the planner returns ~7 sec., which is the same as if I the query is
changed like this:
SELECT * FROM media m, speccharacter c, mediatype mt
WHERE mt.objectid=m.mediatype and mt.medianame='Audio'
...
So, using JOIN and =(SELECT ...) is better than using IN and EXISTS in
this case.
Thanks.

Oleg


-----Original Message-----
From: Josh Berkus [mailto:josh@agliodbs.com]
Sent: Sunday, February 23, 2003 1:53 PM
To: Oleg Lebedev; pgsql-performance@postgresql.org
Subject: Re: [PERFORM] slow query
Importance: Low


Oleg,

> I VACUUM ANALYZED both databases and made sure they have same indexes
> on the tables.

Have you VACUUM FULL the main database?  And how about REINDEX?

> Here is the query:
> SELECT * FROM media m, speccharacter c
> WHERE m.mediatype IN (SELECT objectid FROM mediatype WHERE
> medianame='Audio')

The above should use an EXISTS clause, not IN, unless you are absolutely
sure
that the subquery will never return more than 12 rows.

--
Josh Berkus
Aglio Database Solutions
San Francisco

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