Re: Resurrecting per-page cleaner for btree

Tom Lane wrote:
> ITAGAKI Takahiro <itagaki.takahiro@oss.ntt.co.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.

Agreed.  Reverse lookup of index entries will always be too slow.

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  Bruce Momjian   bruce@momjian.us
  EnterpriseDB    http://www.enterprisedb.com

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