From: | Peter Geoghegan <pg(at)bowt(dot)ie> |
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To: | Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com> |
Cc: | Andres Freund <andres(at)anarazel(dot)de>, Michael Paquier <michael(at)paquier(dot)xyz>, Justin Pryzby <pryzby(at)telsasoft(dot)com>, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Subject: | Re: pg14b1 stuck in lazy_scan_prune/heap_page_prune of pg_statistic |
Date: | 2021-06-16 16:46:07 |
Message-ID: | CAH2-Wz=C6X52MuLHN3-5EonETSrP30mcct9nUTqc9LTpOM_e3A@mail.gmail.com |
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On Wed, Jun 16, 2021 at 9:03 AM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> On Wed, Jun 16, 2021 at 3:59 AM Matthias van de Meent
> > So the implicit assumption in heap_page_prune that
> > HeapTupleSatisfiesVacuum(OldestXmin) is always consistent with
> > heap_prune_satisfies_vacuum(vacrel) has never been true. In that case,
> > we'll need to redo the condition in heap_page_prune as well.
>
> I don't think that this shows that the assumption within
> lazy_scan_prune() (the assumption that both "satisfies vacuum"
> functions agree) is wrong, with the obvious exception of cases
> involving the bug that Justin reported. GlobalVis*.maybe_needed is
> supposed to be conservative.
I suppose it's true that they can disagree because we call
vacuum_set_xid_limits() to get an OldestXmin inside vacuumlazy.c
before calling GlobalVisTestFor() inside vacuumlazy.c to get a
vistest. But that only implies that a tuple that would have been
considered RECENTLY_DEAD inside lazy_scan_prune() (it just missed
being considered DEAD according to OldestXmin) is seen as an LP_DEAD
stub line pointer. Which really means it's DEAD to lazy_scan_prune()
anyway. These days the only way that lazy_scan_prune() can consider a
tuple fully DEAD is if it's no longer a tuple -- it has to actually be
an LP_DEAD stub line pointer.
It's really no different to an opportunistic prune that concurrently
prunes tuples that VACUUM would have seen as RECENTLY_DEAD if it was
going solely on the OldestXmin cutoff. There are certain kinds of
tables where non-HOT updates and opportunistic pruning constantly
leave behind loads of LP_DEAD items. Pruning inside VACUUM won't do
much of the total required pruning at all. That'll mean that some
DEAD/LP_DEAD items will become dead long after a VACUUM starts, while
nevertheless being removed by the same VACUUM. Of course there is no
way for lazy_scan_prune() to distinguish one LP_DEAD item from another
-- they're all stubs without tuple storage, and without a tuple header
with XIDs.
--
Peter Geoghegan
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