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Re: lastOverflowedXid does not handle transaction ID wraparound - Mailing list pgsql-hackers

From Stan Hu
Subject Re: lastOverflowedXid does not handle transaction ID wraparound
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Msg-id CAMBWrQn8VSPCW+_AtLMNanLRwUp-fixEJEJXT+OmDgzeaupdkA@mail.gmail.com
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In response to Re: lastOverflowedXid does not handle transaction ID wraparound  (Andrey Borodin <x4mmm@yandex-team.ru>)
Responses Re: lastOverflowedXid does not handle transaction ID wraparound
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On Wed, Oct 20, 2021 at 4:00 AM Andrey Borodin <x4mmm@yandex-team.ru> wrote:


> 17 окт. 2021 г., в 21:55, Dmitry Dolgov <9erthalion6@gmail.com> написал(а):
> I wonder what would be side
> effects of clearing it when the snapshot is not suboverfloved anymore?

I think we should just invalidate lastOverflowedXid on every XLOG_RUNNING_XACTS if subxid_overflow == false. I can't find a reason not to do so.


On a replica, I think it's possible for lastOverflowedXid to be set even if subxid_overflow is false on the primary and secondary (https://github.com/postgres/postgres/blob/dc899146dbf0e1d23fb24155a5155826ddce34c9/src/backend/storage/ipc/procarray.c#L1327). I thought subxid_overflow only gets set if there are more than PGPROC_MAX_CACHED_SUBXIDS (64) used in a given transaction.

Should the replica be invalidating lastOverflowedXid if subxcnt goes to zero in XLOG_RUNNING_XACTS? But if there's an outstanding snapshot with an xmin that precedes lastOverflowedXid we might violate MVCC if we invalidate this, so I wonder if we also need to check the snapshot with the lowest xmin?

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