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Re: slow query

From: Oleg Lebedev <oleg(dot)lebedev(at)waterford(dot)org>
To: Josh Berkus <josh(at)agliodbs(dot)com>,"pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: slow query
Date: 2003-02-24 15:59:11
Message-ID: 993DBE5B4D02194382EC8DF8554A5273033587@postoffice.waterford.org (view raw or flat)
Thread:
Lists: pgsql-performance
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(at)agliodbs(dot)com] 
Sent: Sunday, February 23, 2003 1:53 PM
To: Oleg Lebedev; pgsql-performance(at)postgresql(dot)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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