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Re: Dead Space Map

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Dead Space Map
Date: 2006-02-28 18:12:41
Message-ID: 200602281812.k1SICfc10337@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Greg Stark wrote:
> 
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> 
> > I should be clearer.  Suppose you have a table with a single index on
> > the primary key.  You are updating the row over and over again (a
> > typical case).  You create the first row, commit, then it is updated
> > (two copies), commit, then you update it again.  That first created row
> > might not be visible to anyone, but has the same index value as the new
> > row you are about to add.  Why not reused that heap tuple?
> 
> If you commit each update then your tuple might well be visible to other
> transactions, how would you check that?

You check the xmin/xmax using standard visibility rules.

> I originally thought you meant if you are repeatedly updating the same record
> within the same transaction. In that case sure you could reuse the space but
> a) only if it's big enough for the new record and b) how often do you really
> do that?

Right, that is a rare case.

-- 
  Bruce Momjian   http://candle.pha.pa.us
  SRA OSS, Inc.   http://www.sraoss.com

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

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