Horton
Menu

Re: IPC/MultixactCreation on the Standby server

Álvaro Herrera <alvherre@kurilemu.de>

View thread
From: Álvaro Herrera <alvherre@kurilemu.de>
To: Andrey Borodin <x4mmm@yandex-team.ru>
Cc: Dmitry <dsy.075@yandex.ru>, pgsql-hackers@lists.postgresql.org
Date: 2025-07-18T11:53:45Z
Lists: pgsql-hackers 
Hello,

Andrey and I discussed this on IM, and after some back and forth, he
came up with a brilliant idea: modify the WAL record for multixact
creation, so that the offset of the next multixact is transmitted and
can be replayed.  (We know it when we create each multixact, because the
number of members is known).  So the replica can store the offset of the
next multixact right away, even though it doesn't know the members for
that multixact.  On replay of the next multixact we can cross-check that
the offset matches what we had written previously.  This allows reading
the first multixact, without having to wait for the replay of creation
of the second multixact.

One concern is: if we write the offset for the second mxact, but haven't
written its members, what happens if another process looks up the
members for that multixact?  We'll have to make it wait (retry) somehow.
Given what was described upthread, it's possible for the multixact
beyond that one to be written already, so we won't have the zero offset
that would make us wait.

Anyway, he's going to try and implement this.

Andrey, please let me know if I misunderstood the idea.

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.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. Fix test to work with non-8kB block sizes

  2. Fix setting next multixid's offset at offset wraparound

  3. Add test for multixid wraparound

  4. Set next multixid's offset when creating a new multixid

  5. psql: Improve tab completion for large objects.

  6. Refactor some repetitive SLRU code

  7. Transaction IDs wrap around, per my proposal of 13-Aug-01. More