From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
cc: Jeff Janes <jeff.janes@gmail.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>,
	Daniel Farina <daniel@heroku.com>, Craig Ringer <ringerc@ringerc.id.au>,
	=?ISO-8859-1?Q?Harold_A=2E_Gim=E9nez?= <harold.gimenez@gmail.com>
Subject: Re: [PERFORM] DELETE vs TRUNCATE explanation
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Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Mon, 16 Jul 2012 14:39:58 -0400"
Date: Mon, 16 Jul 2012 15:03:03 -0400
Message-ID: <15645.1342465383@sss.pgh.pa.us>

Robert Haas <robertmhaas@gmail.com> writes:
> Unfortunately, there are lots of important operations (like bulk
> loading, SELECT * FROM bigtable, and VACUUM notverybigtable) that
> inevitably end up writing out their own dirty buffers.  And even when
> the background writer does write something, it's not always clear that
> this is a positive thing.  Here's Greg Smith commenting on the
> more-is-worse phenonmenon:

> http://archives.postgresql.org/pgsql-hackers/2012-02/msg00564.php

> Jeff Janes and I came up with what I believe to be a plausible
> explanation for the problem:

> http://archives.postgresql.org/pgsql-hackers/2012-03/msg00356.php

> I kinda think we ought to be looking at fixing that for 9.2, and
> perhaps even back-patching further, but nobody else seemed terribly
> excited about it.

I'd be fine with back-patching something like that into 9.2 if we had
(a) a patch and (b) experimental evidence that it made things better.
Unless I missed something, we have neither.  Also, I read the above
two messages to say that you, Greg, and Jeff have three different ideas
about exactly what should be done, which is less than comforting for
a last-minute patch...

			regards, tom lane