To: Jeff Janes <jeff.janes@gmail.com>
cc: lars <lhofhansl@yahoo.com>, Kevin Grittner <Kevin.Grittner@wicourts.gov>,
	Ivan Voras <ivoras@freebsd.org>, pgsql-performance@postgresql.org
Subject: Re: UPDATEDs slowing SELECTs in a fully cached database 
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Comments: In-reply-to Jeff Janes <jeff.janes@gmail.com>
	message dated "Tue, 12 Jul 2011 20:35:54 -0400"
Date: Tue, 12 Jul 2011 21:16:37 -0400
Message-ID: <21639.1310519797@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Jeff Janes <jeff.janes@gmail.com> writes:
> On 7/12/11, lars <lhofhansl@yahoo.com> wrote:
>> The fact that a select (maybe a big analytical query we'll run) touching
>> many rows will update the WAL and wait
>> (apparently) for that IO to complete is making a fully cached database
>> far less useful.
>> I just artificially created this scenario.

> I can't think of any reason that that WAL would have to be flushed
> synchronously.

Maybe he's running low on shared_buffers?  We would have to flush WAL
before writing a dirty buffer out, so maybe excessive pressure on
available buffers is part of the issue here.

			regards, tom lane