Re: PATCH to allow concurrent VACUUMs to not lock each

This has been saved for the 8.2 release:

http://momjian.postgresql.org/cgi-bin/pgpatches_hold

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Hannu Krosing wrote:
> On E, 2005-07-04 at 10:24 +0300, Hannu Krosing wrote:
> > On P, 2005-07-03 at 12:19 -0400, Tom Lane wrote:
> > > Hannu Krosing <hannu(at)skype(dot)net> writes:
> > > > Ok, this is a new version of the vacuum patch with the following changes
> > > > following some suggestions in this thread.
> > >
> > > The more I look at this, the uglier it looks ... and I still haven't
> > > seen any convincing demonstration that it *works*, ie doesn't have
> > > bad side-effects on the transaction-is-in-progress logic.
>
> Ok, I changed GetOldestXmin() to use proc->inVacuum only when
> determining the oldest visible xid for vacuum and index (i.e. which
> tuples are safe to delete and which tuples there is no need to index).
>
> The third use on GetOldestXmin() in xlog.c is changed to use old
> GetOldestXmin() logic.
>
>
> My reasoning for why the patch should work is as follows:
>
> 1) the only transaction during which inVacuum is set is the 2nd
> transaction (of originally 3, now 4) of lazy VACUUM, which does simple
> heap scanning and old tuple removal (lazy_vacuum_rel()), and does no
> externally visible changes to the data. It only removes tuples which are
> already invisible to all running transactions.
>
> 2) That transaction never deletes, updates or inserts any tuples on it
> own.
>
> 3) As it can't add any tuples or change any existing tuples to have its
> xid as either xmin or xmax, it already does run logically "outside of
> transactions".
>
> 4) The only use made of of proc->inVacuum is when determining which
> tuples are safe to remove (in vacuum.c) or not worth indexing (in
> index.c) and thus can't affect anything else.
>
>
>
> I can easily demonstrate that it "works" in the sense that it allows
> several concurrent vacuums to clean out old tuples, and I have thus far
> been unable to construct the counterexample where it does anything bad.
>
> Could you tell me which part of my reasoning is wrong or what else do I
> overlook.
>
> --
> Hannu Krosing <hannu(at)tm(dot)ee>

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