Re: Possible fails in pg_stat_statements test

From: "Anton A(dot) Melnikov" <aamelnikov(at)inbox(dot)ru>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Andres Freund <andres(at)anarazel(dot)de>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Possible fails in pg_stat_statements test
Date: 2022-03-31 15:08:01
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On 30.03.2022 22:36, Robert Haas wrote:
> I don't think that the idea of "extra" WAL records is very principled.
> It's pretty vague what "extra" means, and your definition seems to be
> basically "whatever would be needed to make this test case pass." I
> think the problem is basically with the test cases's idea that # of
> WAL records and # of table rows ought to be equal. I think that's just
> false. In general, we'd also have to worry about index insertions,
> which would provoke variable numbers of WAL records depending on
> whether they cause a page split. And we'd have to worry about TOAST
> table insertions, which could produce different numbers of records
> depending on the size of the data, the configured block size and TOAST
> threshold, and whether the TOAST table index incurs a page split.

Thank you very much for this information. I really didn't take it into

> If it's true that this test case sometimes randomly fails, then we
> ought to fix that somehow, maybe by just removing this particular
> check from the test case, or changing it to >=, or something like
> that. But I don't think adding a new counter is the right idea.

Indeed. Then there is a very simple solution for this particular case as
wal_records counter may only sometime becomes greater but never less.
The corresponding patch is attached.

With best regards,
Anton A. Melnikov
Postgres Professional:
The Russian Postgres Company

Attachment Content-Type Size
v4-0001-Fix-possible-fails-in-pg_stat_statements-test.patch text/x-patch 3.4 KB

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