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Re: count(*) performance

From: "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>
To: G?briel ?kos <akos(dot)gabriel(at)i-logic(dot)hu>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: count(*) performance
Date: 2006-03-27 13:41:04
Message-ID: 20060327134103.GI80726@pervasive.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Mon, Mar 27, 2006 at 03:34:32PM +0200, G?briel ?kos wrote:
> Hi,
> 
> I guess this is an age-old 100times answered question, but I didn't find 
> the answer to it yet (neither in the FAQ nor in the mailing list archives).
> 
> Question: I have a table with 2.5M rows. count(*) on this table is 
> running 4 minutes long. (dual opteron, 4gig ram, db on 4 disk raid10 
> array (sata, not scsi)) Is this normal? How could I make it run faster?
> Maybe make it run faster for the 2nd time? Which parameters should I 
> change in postgresql.conf and how?

First, count(*) on PostgreSQL tends to be slow because you can't do
index covering[1].

But in this case, I'd bet money that if it's taking 4 minutes something
else is wrong. Have you been vacuuming that table frequently enough?
What's SELECT relpages FROM pg_class WHERE relname='tablename' show?

[1] http://www.pervasive-postgres.com/lp/newsletters/2006/Insights_postgres_Feb.asp#5
-- 
Jim C. Nasby, Sr. Engineering Consultant      jnasby(at)pervasive(dot)com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461

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