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Re: IPC/MultixactCreation on the Standby server

Andrey Borodin <x4mmm@yandex-team.ru>

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From: Andrey Borodin <x4mmm@yandex-team.ru>
To: Álvaro Herrera <alvherre@kurilemu.de>
Cc: Dmitry <dsy.075@yandex.ru>, pgsql-hackers@lists.postgresql.org
Date: 2025-07-25T18:33:39Z
Lists: pgsql-hackers 

> On 21 Jul 2025, at 19:58, Andrey Borodin <x4mmm@yandex-team.ru> wrote:
> 
> I'm planning to prepare tests and fixes for all supported branches

This is a status update message. I've reproduced problem on REL_13_STABLE and verified that proposed fix works there.

Also I've discovered one more serious problem.
If a backend crashes just before WAL-logging multi, any heap tuple that uses this multi will become unreadable. Any attempt to read it will hang forever.

I've reproduced the problem and now I'm working on scripting this scenario. Basically, I modify code to hang forever after assigning multi number 2. Then execute in first psql:

create table x as select i,0 v from generate_series(1,10) i;
create unique index on x(i);

\set id 1
begin;
select * from x where i = :id for no key update;
savepoint s1;
update x set v = v+1 where i = :id; -- multi 1
commit;

\set id 2
begin;
select * from x where i = :id for no key update;
savepoint s1;
update x set v = v+1 where i = :id; -- multi 2 -- will hang
commit;

Then in second psql:

create table y as select i,0 v from generate_series(1,10) i;
create unique index on y(i);

\set id 1
begin;
select * from y where i = :id for no key update;
savepoint s1;
update y set v = v+1 where i = :id;
commit;

After this I pkill -9 postgres. Recovered installation cannot execute select * from x; because multi 1 cannot be read without recovery of multi 2 which was never logged.


Luckily fix is the same: just restore offset of multi 2 when multi 1 is recovered.


Best regards, Andrey Borodin.

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