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Re: Performance problems with 8.1.1 compared to 7.4.7

From: Albert Cervera Areny <albert(at)sedifa(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Performance problems with 8.1.1 compared to 7.4.7
Date: 2005-12-27 18:02:17
Message-ID: 200512271902.18023.albert@sedifa.com (view raw or flat)
Thread:
Lists: pgsql-performance

A Dimarts 27 Desembre 2005 18:13, Michael Fuhr va escriure:
> On Tue, Dec 27, 2005 at 05:09:28PM +0100, Albert Cervera Areny wrote:
> > 	However, now we have a query that is much slower with 8.1 compared to
> > 7.4. The query lasts 7minutes (all the times we try) with 8.1, keeping
> > CPU usage at 93~97% while it lasts 25 seconds in 7.4 the first time going
> > down to 4 seconds the following tries.
> > 	We're not experts at all but we can't see anything strange with the
> > differences of EXPLAIN in the queries. Below I paste the query and the
> > EXPLAIN output.
>
> Could you post the EXPLAIN ANALYZE output of the query on both
> systems?  That'll show how accurate the planner's estimates are.
>
> Have you run ANALYZE (or VACUUM ANALYZE) on the tables in both
> versions?  The row count estimates in the 8.1.1 query differ from
> those in the 7.4.7 query.  Are the two versions using the same data
> set?
>
> Are your configuration settings the same in both versions?  You
> mentioned increasing work_mem, but what about others like
> effective_cache_size, random_page_cost, and shared_buffers?

Hey, thank you for your fast response, I found what the problem was.

I thought the settings were the same but work_mem was still higher in 7.4, 
30Mb, so I increased 8.1 to 30Mb and it worked faster, down to 17 seconds the 
first time, 2.5 seconds for the others. 

Are there any "rules of thumb" to let a begginer give reasonable values to 
these parameters? Not only work_mem, but also random_page_cost, and so on. 
Are there any tests one can run to determine "good" values?

Thanks a lot!

-- 
Albert Cervera Areny
Dept. Informàtica Sedifa, S.L.

Av. Can Bordoll, 149
08202 - Sabadell (Barcelona)
Tel. 93 715 51 11
Fax. 93 715 51 12

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