Re: POC: make mxidoff 64 bits

Hi, Heikki!

On Tue, Nov 25, 2025 at 12:07 PM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
> Looking at the upgrade code, in light of the "IPC/MultixactCreation on
> the Standby server" thread [1], I think we need to make it more
> tolerant. It's possible that there are 0 offsets in
> pg_multixact/offsets. That might or might not be a problem: it's OK as
> long as those multixids don't appear in any heap table, or you might
> actually have lost those multixids, which is bad but the damage has
> already been done and upgrade should not get stuck on it.
> GetOldMultiXactIdSingleMember() currently asserts that the offset is
> never zero, but it should try to do something sensible in that case
> instead of just failing.

Thank you for your work on this subject. It's very much appreciated.

I'd like to raise the question about compression again. You have
fairly criticized non-deterministic compression, but what do you think
about deterministic one that I've proposed [1]. I understand that
multixact offsets are subject of growth and their limit is not
removed. However, it's still several extra gigabytes for multixact
offsets, which we could save.

Links.
1. https://www.postgresql.org/message-id/CAPpHfduDFLXATvBkUiOjyvZUBZXhK_pj5zjVpxvrJzkRVq%2B8Lw%40mail.gmail.com

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Regards,
Alexander Korotkov
Supabase