To: "Jim C. Nasby" <jnasby@pervasive.com>
cc: Greg Stark <gsstark@mit.edu>, Cristian Prieto <cristian@clickdiario.com>,
	pgsql-performance@postgresql.org
Subject: Re: Any way to optimize GROUP BY queries? 
In-reply-to: <20051220194710.GJ28771@pervasive.com> 
References: <00d701c604c1$e6945d80$6500a8c0@gt.ClickDiario.local>
	<87wti0ak60.fsf@stark.xeocode.com>
	<20051220194710.GJ28771@pervasive.com>
Comments: In-reply-to "Jim C. Nasby" <jnasby@pervasive.com>
	message dated "Tue, 20 Dec 2005 13:47:10 -0600"
Date: Tue, 20 Dec 2005 16:44:47 -0500
Message-ID: <6542.1135115087@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

"Jim C. Nasby" <jnasby@pervasive.com> writes:
> On Mon, Dec 19, 2005 at 03:47:35PM -0500, Greg Stark wrote:
>> Increase your work_mem (or sort_mem in older postgres versions), you can do
>> this for the server as a whole or just for this one session and set it back
>> after this one query. You can increase it up until it starts causing swapping
>> at which point it would be counter productive.

> Just remember that work_memory is per-operation, so it's easy to push
> the box into swapping if the workload increases. You didn't say how much
> memory you have, but I'd be careful if work_memory * max_connections
> gets very much larger than your total memory.

It's considered good practice to have a relatively small default
work_mem setting (in postgresql.conf), and then let individual sessions
push up the value locally with "SET work_mem" if they are going to
execute queries that need it.  This works well as long as you only have
one or a few such "heavy" sessions at a time.

			regards, tom lane