Re: BUG #17257: (auto)vacuum hangs within lazy_scan_prune()

From: Peter Geoghegan <pg(at)bowt(dot)ie>
To: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Cc: Andres Freund <andres(at)anarazel(dot)de>, Noah Misch <noah(at)leadboat(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>, Alexander Lakhin <exclusion(at)gmail(dot)com>, PostgreSQL mailing lists <pgsql-bugs(at)lists(dot)postgresql(dot)org>
Subject: Re: BUG #17257: (auto)vacuum hangs within lazy_scan_prune()
Date: 2024-04-26 21:28:06
Message-ID: CAH2-Wz=_FWEs2YSa=ggKuXhZtdCPiORKhk2JWu1bcVNb+hJdxw@mail.gmail.com
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On Fri, Apr 26, 2024 at 4:46 PM Melanie Plageman
<melanieplageman(at)gmail(dot)com> wrote:
> I have a more basic question. How could GlobalVisState->maybe_needed
> going backwards cause a problem with relfrozenxid? Yes, if
> maybe_needed goes backwards, we may not remove a tuple whose xmin/xmax
> are older than VacuumCutoffs->OldestXmin. But, if that tuple's
> xmin/xmax are older than OldestXmin, then wouldn't we freeze it?

You can't freeze every XID older than OldestXmin.
heap_prepare_freeze_tuple() isn't prepared for HEAPTUPLE_DEAD tuples,
and expects that those will be taken care of by the time it is called.

--
Peter Geoghegan

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