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Re: Dead Space Map

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>
Cc: Greg Stark <gsstark(at)mit(dot)edu>, 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 16:58:44
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Lists: pgsql-hackers
Jim C. Nasby wrote:
> 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.

Right, we should discuss all the possibilities and do something.  I
think we just don't know what yet.

One idea Simon and I had was to reuse heap rows where all indexed values
in the old row and the new row were the same, meaning the heap value
could be replaced without changing the indexes at all.  We thought this
would be very useful for frequently-updated rows.  It could also be used
if no index are on the table.

  Bruce Momjian
  SRA OSS, Inc.

  + If your life is a hard drive, Christ can be your backup. +

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