To: Alvaro Herrera <alvherre@commandprompt.com>
cc: Heikki Linnakangas <heikki@enterprisedb.com>,
	Greg Smith <gsmith@gregsmith.com>,
	Pavan Deolasee <pavan.deolasee@gmail.com>, pgsql-performance@postgresql.org
Subject: Re: I/O on select count(*) 
In-reply-to: <20080515154932.GB8221@alvh.no-ip.org> 
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Comments: In-reply-to Alvaro Herrera <alvherre@commandprompt.com>
	message dated "Thu, 15 May 2008 11:49:32 -0400"
Date: Thu, 15 May 2008 12:19:14 -0400
Message-ID: <6227.1210868354@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Alvaro Herrera <alvherre@commandprompt.com> writes:
> Heikki Linnakangas escribió:
>> We know what kind of a relation we're dealing with in ReadBuffer, so we  
>> could add a flag to BufferDesc to mark heap pages.

> Hmm, I was thinking that it would need access to the catalogs to know
> where the tuples are, but that's certainly not true, so perhaps it could
> be made to work.

The issue in my mind is not so much could bgwriter physically do it
as that it's a violation of module layering.  That has real
consequences, like potential for deadlocks.  It'll become particularly
pressing if we go forward with the plans to get rid of the separate
dedicated buffers for pg_clog etc and have them work in the main
shared-buffer pool.

			regards, tom lane