Re: BUG #17257: (auto)vacuum hangs within lazy_scan_prune()

On Fri, Apr 26, 2024 at 5:28 PM Peter Geoghegan <pg@bowt.ie> wrote:
>
> On Fri, Apr 26, 2024 at 4:46 PM Melanie Plageman
> <melanieplageman@gmail.com> wrote:
> > I have a more basic question. How could GlobalVisState->maybe_needed
> > going backwards cause a problem with relfrozenxid? Yes, if
> > maybe_needed goes backwards, we may not remove a tuple whose xmin/xmax
> > are older than VacuumCutoffs->OldestXmin. But, if that tuple's
> > xmin/xmax are older than OldestXmin, then wouldn't we freeze it?
>
> You can't freeze every XID older than OldestXmin.
> heap_prepare_freeze_tuple() isn't prepared for HEAPTUPLE_DEAD tuples,
> and expects that those will be taken care of by the time it is called.

But, the tuple isn't HEAPTUPLE_DEAD -- it's HEAPTUPLE_RECENTLY_DEAD.
It will always be HEAPTUPLE_RECENTLY_DEAD in 17 and in <= 16, if
HeapTupleSatisfiesVacuum() returns HEAPTUPLE_DEAD, we wouldn't call
heap_prepare_freeze_tuple() because of the retry loop.

- Melanie