Re: Add 64-bit XIDs into PostgreSQL 15

On 2021/12/30 21:15, Maxim Orlov wrote:
> Hi, hackers!
>
> Long time wraparound was a really big pain for highly loaded systems. One source of performance degradation is the need to vacuum before every wraparound.
> And there were several proposals to make XIDs 64-bit like [1], [2], [3] and [4] to name a few.
>
> The approach [2] seems to have stalled on CF since 2018. But meanwhile it was successfully being used in our Postgres Pro fork all time since then. We have hundreds of customers using 64-bit XIDs. Dozens of instances are under load that require wraparound each 1-5 days with 32-bit XIDs.
> It really helps the customers with a huge transaction load that in the case of 32-bit XIDs could experience wraparounds every day. So I'd like to propose this approach modification to CF.
>
> PFA updated working patch v6 for PG15 development cycle.
> It is based on a patch by Alexander Korotkov version 5 [5] with a few fixes, refactoring and was rebased to PG15.

Thanks a lot! I'm really happy to see this proposal again!!

Is there any documentation or README explaining this whole 64-bit XID mechanism?

Could you tell me what happens if new tuple with XID larger than xid_base + 0xFFFFFFFF is inserted into the page? Such new tuple is not allowed to be inserted into that page? Or xid_base and xids of all existing tuples in the page are increased? Also what happens if one of those xids (of existing tuples) cannot be changed because the tuple still can be seen by very-long-running transaction?

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION