On Wed, Oct 20, 2021 at 4:00 AM Andrey Borodin <x4...@yandex-team.ru> wrote:
> > > > 17 окт. 2021 г., в 21:55, Dmitry Dolgov <9erthali...@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?
