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Subject: Re: Fix bug in multixact Oldest*MXactId initialization and access
From: Chao Li <li.evan.chao@gmail.com>
In-Reply-To: <c3eaaac1-d244-4e4d-b5d7-06c284108eac@postgrespro.ru>
Date: Thu, 26 Feb 2026 10:21:10 +0800
Cc: "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>,
 Heikki Linnakangas <hlinnaka@iki.fi>,
 Andres Freund <andres@anarazel.de>,
 Sami Imseih <samimseih@gmail.com>
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To: Yura Sokolov <y.sokolov@postgrespro.ru>
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> On Feb 25, 2026, at 23:16, Yura Sokolov <y.sokolov@postgrespro.ru> =
wrote:
>=20
> Good day.
>=20
> Chao Li and Sami Imseih, thank you for looking at.
>=20
> After thinking a bit, I've decided to make sizes of arrays precise:
> - OldestMemberMXactId's size remains MaxBackends + max_prepared_xacts.
>  Instead of changing its size, procno is now adjusted to not include
> auxiliary procs.
> - OldestVisibleMXactId contains only MaxBackends elemenents now.
>  It is used only for real backends and not prepared transactions.
>=20
> All accesses are validated with asserts certainly.
>=20
> I believe, index transformation in access of OldestMemberMXactId will =
not
> cost much since all this operations are quite rare.
> In the loops arrays are accessed directly since limiting loop index is =
enough.
>=20
> --=20
> regards
> Yura Sokolov aka =
funny-falcon<v3-0001-Fix-multixacts-OldestMemberMXactId-and-OldestVisi.pat=
ch>

Actually, while I reviewing v1, I was thinking over the same way as v3, =
as the NUM_AUXILIARY_PROCS portions in OldestMemberMXactId and =
OldestVisibleMXactId arrays are not used at all. I didn=E2=80=99t raise =
the idea because that given these code are in hot paths, I wasn't sure =
if the complexity was worth the shared memory optimization. It=E2=80=99s =
a classic trade-off, and I=E2=80=99m curious to see which direction the =
senior committers prefer.

However, I agree that the new helper function names in v3 are a =
significant improvement over v1.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/