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Re: Resurrecting per-page cleaner for btree

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)postgresql(dot)org, ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>
Cc: pgsql-patches(at)postgresql(dot)org, teramoto(dot)junji(at)lab(dot)ntt(dot)co(dot)jp
Subject: Re: Resurrecting per-page cleaner for btree
Date: 2006-07-24 16:46:52
Message-ID: 11860.1153759612@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp> writes:
> This is a revised patch originated by Junji TERAMOTO for HEAD.
>   [BTree vacuum before page splitting]
>   http://archives.postgresql.org/pgsql-patches/2006-01/msg00301.php
> I think we can resurrect his idea because we will scan btree pages
> at-atime now; the missing-restarting-point problem went away.
> Have I missed something? Comments welcome.

I think the only serious objection to this would be that it'd mean that
tuples that should have an index entry might not have one.  The current
form of VACUUM does not care, but people keep raising the idea of doing
"retail" vacuuming that operates by looking up index entries explicitly.
You could certainly make a retail vacuumer do nothing if it fails to
find the expected index entry, but ISTM that'd be a rather serious loss
of consistency checking --- you could not tell the someone-already-
deleted-it case apart from a bug in the vacuumer's index value
computation or lookup.

Personally I don't think retail vacuuming in that form will ever fly
anyway, so I have no problem with installing the proposed patch,
but I thought I'd better throw this comment out to see if anyone
thinks it's a big deal.

			regards, tom lane

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