Re: Correcting freeze conflict horizon calculation

On Fri, May 30, 2025 at 5:16 PM Melanie Plageman
<melanieplageman@gmail.com> wrote:
> On Thu, May 29, 2025 at 11:37 AM Peter Geoghegan <pg@bowt.ie> wrote:
> > How can taking the "Avoids false conflicts when hot_standby_feedback
> > in use" path more often result in fewer unnecessary conflicts on
> > standbys? Isn't it the other way around?
>
> You're right. I forgot that the visibility_cutoff_xid will be older
> than OldestXmin when all the tuples are going to be frozen.

I added an assertion in a number of places that
"Assert(!TransactionIdIsValid(visibility_cutoff_xid))" when we go to
set a page all-frozen in the VM -- it's irrelvant, and recovery cannot
possibly need a conflict when it replays the record. This seemed
logical, since an all-frozen page must already be visible to every
possible MVCC snapshot (it might also have some small performance
benefit, though that's not really why I did it).

> I associate the visibility_cutoff_xid with being younger/newer than
> OldestXmin because it often will be when there are newer live tuples
> we don't freeze.

I'm not sure what you mean by this. visibility_cutoff_xid can only be
set using tuples that HTSV says are HEAPTUPLE_LIVE according to
VACUUM's OldestXmin cutoff. Personally I find it natural to think of
visibility_cutoff_xid as meaningless unless we're actually able to
apply it in some way.

--
Peter Geoghegan