Horton

Re: POC: make mxidoff 64 bits

Alexander Korotkov <aekorotkov@gmail.com>

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From: Alexander Korotkov <aekorotkov@gmail.com>
To: Maxim Orlov <orlovmg@gmail.com>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, wenhui qiu <qiuwenhuifx@gmail.com>, Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, Postgres hackers <pgsql-hackers@lists.postgresql.org>
Date: 2025-11-04T15:10:31Z
Lists: pgsql-hackers

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 partial read handling in pg_upgrade's multixact conversion

  2. Increase timeout in multixid_conversion upgrade test

  3. Improve sanity checks on multixid members length

  4. Clarify comment on multixid offset wraparound check

  5. Never store 0 as the nextMXact

  6. Add runtime checks for bogus multixact offsets

  7. Widen MultiXactOffset to 64 bits

  8. Move pg_multixact SLRU page format definitions to a separate header

  9. Convert confusing macros in multixact.c to static inline functions

  10. Index SLRUs by 64-bit integers rather than by 32-bit integers

  11. Cope with possible failure of the oldest MultiXact to exist.

Attachments

On Thu, Oct 30, 2025 at 6:17 PM Maxim Orlov <orlovmg@gmail.com> wrote:
> On Thu, 30 Oct 2025 at 12:10, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>>
>>
>> Oh I see, the 'base' is not necessarily the base offset of the first
>> multixact on the page, it's the base offset of the first multixid that
>> is written to the page. And the (short) offsets can be negative. That's
>> a frighteningly clever encoding scheme. One upshot of that is that WAL
>> redo might get construct the page with a different 'base'. I guess that
>> works, but it scares me. Could we come up with a more deterministic scheme?
>>
> Definitely! The most stable approach is the one we had before, which
> used actual 64-bit offsets in the SLRU. To be honest, I'm completely
> happy with it. After all, what's most important for me is to have 64-bit
> xids in Postgres, and this patch is a step towards that goal.

Yes, but why can't we have an encoding scheme which would both be
deterministic and provide compression?  The attached is what I meant
in [1].  It's based on v19 and provide deterministic conversion of
each 8 of 64-bit offsets into a chunks containing 64-bit base and 7 of
24-bit increments.  I didn't touch pg_upgrade code though.

Links.
1. https://www.postgresql.org/message-id/CAPpHfdtPybyMYBj-x3-Z5%3D4bj_vhYk2R0nezfy%3DVjcz4QBMDgw%40mail.gmail.com

------
Regards,
Alexander Korotkov
Supabase