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Re: Dead Space Map for vacuum

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Simon Riggs" <simon(at)2ndquadrant(dot)com>
Cc: "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com>, "Gavin Sherry" <swm(at)linuxworld(dot)com(dot)au>, "ITAGAKI Takahiro" <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Dead Space Map for vacuum
Date: 2006-12-29 21:41:11
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Lists: pgsql-hackers
"Simon Riggs" <simon(at)2ndquadrant(dot)com> writes:
> On Fri, 2006-12-29 at 10:49 -0500, Tom Lane wrote:
>> Counterexample: table in which all tuples exceed half a page.

> Current FSM code will ignore those too, if they are less than the
> average size of the tuple so far requested. Thats a pretty wierd
> counterexample, even if it is a case that needs handling.

Better read it again.  The number that's passed to the FSM is the
free space *after* vacuuming, which in this scenario will be
BLCKSZ-less-page-header.  This case is not broken now, but it will
be if we adopt your proposal.

			regards, tom lane

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