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Re: Dead Space Map

From: "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>,pgsql-hackers(at)postgresql(dot)org
Subject: Re: Dead Space Map
Date: 2006-02-28 16:14:01
Message-ID: 20060228161401.GP82012@pervasive.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tue, Feb 28, 2006 at 01:18:14AM -0500, Greg Stark wrote:
> But I think the thought process went the other direction. If you have the bit
> intended for index scans indicating that the tuple is not "in doubt" ie, it's
> visible to every transaction, then that also implies the tuple doesn't need to
> be visited by vacuum.
> 
> Skipping pages that don't contain any in doubt tuples would be a huge win.
> Even if there might be some additional pages that vacuum could in theory be
> skipping too.

Agreed. IMO, *anything* that improves the efficiency of vacuum would be
of huge benefit, and keeping a bitmap of pages that are known to be 100%
visible would be a big start in that direction. For many large tables,
this case would cover a large percentage of pages, speeding up not only
vacuum but also index scans.

ISTM that the continuing debate about how to improve vacuum is due
largely to the fact that there are a very large number of possibilities.
I would very much like to see a decision on one to impliment as a
starting point. Ideas about some kind of dead-space-map, or a
known-clean-map have been floating around for at least 2 versions now.
-- 
Jim C. Nasby, Sr. Engineering Consultant      jnasby(at)pervasive(dot)com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461

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