From: Tom Lane <tgl@sss.pgh.pa.us>
To: Peter Geoghegan <pg@bowt.ie>
cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
        Andres Freund <andres@anarazel.de>
Subject: Re: Intermittent buildfarm failures on wrasse
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Comments: In-reply-to Peter Geoghegan <pg@bowt.ie>
	message dated "Wed, 13 Apr 2022 15:20:30 -0700"
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Date: Wed, 13 Apr 2022 18:54:06 -0400
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Peter Geoghegan <pg@bowt.ie> writes:
> On Wed, Apr 13, 2022 at 3:08 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I'm tempted to add something like
>> SELECT relallvisible = relpages FROM pg_class WHERE relname = 'tenk1';
>> so that we can confirm or refute the theory that relallvisible is
>> the driving factor.

> It would be fairly straightforward to commit a temporary debugging
> patch that has the autovacuum logging stuff report directly on how
> VACUUM set new_rel_allvisible in pg_class. We should probably be doing
> that already, just because it's useful information that is already
> close at hand.

Doesn't look like wrasse has autovacuum logging enabled, though.

After a bit more navel-contemplation I see a way that the pgstats
work could have changed timing in this area.  We used to have a
rate limit on how often stats reports would be sent to the
collector, which'd ensure half a second or so delay before a
transaction's change counts became visible to the autovac daemon.
I've not looked at the new code, but I'm betting that that's gone
and the autovac launcher might start a worker nearly immediately
after some foreground process finishes inserting some rows.
So that could result in autovac activity occurring concurrently
with test_setup where it didn't before.

As to what to do about it ... maybe apply the FREEZE and
DISABLE_PAGE_SKIPPING options in test_setup's vacuums?
It seems like DISABLE_PAGE_SKIPPING is necessary but perhaps
not sufficient.

			regards, tom lane