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Re: Performance of count(*)

From: Andreas Tille <tillea(at)rki(dot)de>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Performance of count(*)
Date: 2007-03-22 11:48:08
Message-ID: Pine.LNX.4.62.0703221239570.13747@wr-linux02 (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
On Thu, 22 Mar 2007, Andreas Kostyrka wrote:

> Which version of PG?

Ahh, sorry, forgot that.  The issue occurs in Debian (Etch) packaged
version 7.4.16.  I plan to switch soon to 8.1.8.

> That's the reason why PG (check the newest releases, I seem to
> remember that there has been some aggregate optimizations there),

I'll verify this once I moved to the new version.

> does
> a SeqScan for select count(*) from table. btw, depending upon your
> data, doing a select count(*) from table where user=X will use an
> Index, but will still need to fetch the rows proper to validate them.

I have an index on three (out of 7 columns) of this table.  Is there
any chance to optimize indexing regarding this.

Well, to be honest I'm not really interested in the performance of
count(*).  I was just discussing general performance issues on the
phone line and when my colleague asked me about the size of the
database he just wonderd why this takes so long for a job his
MS-SQL server is much faster.  So in principle I was just asking
a first question that is easy to ask.  Perhaps I come up with
more difficult optimisation questions.

Kind regards



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Subject: Potential memory usage issue
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