From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: Amit Kapila <amit.kapila16@gmail.com>,
        Tomas Vondra <tomas.vondra@enterprisedb.com>,
        pgsql-hackers@lists.postgresql.org
Subject: Re: Stabilizing the test_decoding checks, take N
In-reply-to: <20220417143236.lp3hksv2h2z7qn6p@alap3.anarazel.de>
References: <1958043.1650129119@sss.pgh.pa.us>
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Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Sun, 17 Apr 2022 07:32:36 -0700"
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Date: Sun, 17 Apr 2022 12:01:53 -0400
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Andres Freund <andres@anarazel.de> writes:
>> On 2022-04-16 13:11:59 -0400, Tom Lane wrote:
>>> We've seen this sort of problem before [2], and attempted to fix it [3]
>>> by making these tests ignore empty transactions.  But of course
>>> auto-analyze's transaction wasn't empty, so that didn't help.

>> I don't quite understand this bit - the logic test_decoding uses to
>> decide if a transaction is "empty" is just whether a tuple was
>> output. And there shouldn't be any as part of auto-analyze, because we
>> don't decode catalog changes.  I suspect there's something broken in the
>> streaming logic (potentially just in test_decoding) around
>> skip_empty_xacts.

Hmm, I'll defer to somebody who knows that code better about whether
there's an actual bug.  However ...

>>> I think the most expedient way to prevent this type of failure is to run
>>> the test_decoding tests with autovacuum_naptime cranked up so far as to
>>> make it a non-issue, like maybe a day.

>> I'm a bit worried about this approach - we've IIRC had past bugs that
>> came only to light because of autovacuum starting. I wonder if we rather
>> should do the opposite and reduce naptime so it'll be seen on fast
>> machines, rather than very slow ones.

It seems likely to me that trying to make a test like this one blind to
autovacuum/autoanalyze activity will make it less useful, not more so.
Why is such blindness desirable?

			regards, tom lane