Re: XID formatting and SLRU refactorings (was: Add 64-bit XIDs into PostgreSQL 15)

On 07/03/2023 13:38, Maxim Orlov wrote:
> As for making pg_multixact 64 bit, I spend the last couple of days to 
> make proper pg_upgrade for pg_multixact's and for pg_xact's
> with wraparound and I've understood, that it is not a simple task 
> compare to pg_xact's. The problem is, we do not have epoch for
> multixacts, so we do not have ability to "overcome" wraparound. The 
> solution may be adding some kind of epoch for multixacts or
> make them 64 bit in "main" 64-xid patch, but in perspective of this 
> thread, in my view, this should be last in line here.

That is true for pg_multixact/offsets. We will indeed need to add an 
epoch and introduce the concept of FullMultiXactIds for that. However, 
we can change pg_multixact/members independently of that. We can extend 
MultiXactOffset from 32 to 64 bits, and eliminate pg_multixact/members 
wraparound, while keeping multi-xids 32 bits wide.

- Heikki