Horton
Menu

Re: Fix bug in multixact Oldest*MXactId initialization and access

Chao Li <li.evan.chao@gmail.com>

View thread
From: Chao Li <li.evan.chao@gmail.com>
To: Yura Sokolov <y.sokolov@postgrespro.ru>
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>
Date: 2026-02-26T02:21:10Z
Lists: pgsql-hackers 

> On Feb 25, 2026, at 23:16, Yura Sokolov <y.sokolov@postgrespro.ru> wrote:
> 
> Good day.
> 
> Chao Li and Sami Imseih, thank you for looking at.
> 
> 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.
> 
> All accesses are validated with asserts certainly.
> 
> 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.
> 
> -- 
> regards
> Yura Sokolov aka funny-falcon<v3-0001-Fix-multixacts-OldestMemberMXactId-and-OldestVisi.patch>

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’t 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’s a classic trade-off, and I’m 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/

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Add test for row-locking and multixids with prepared transactions

  2. Skip prepared_xacts test if max_prepared_transactions < 2

  3. Fix OldestMemberMXactId and OldestVisibleMXactId array usage

  4. Redefine backend ID to be an index into the proc array

  5. Fix an old bug in multixact and two-phase commit. Prepared transactions can