Re: Combine Prune and Freeze records emitted by vacuum

On Tue, Mar 19, 2024 at 9:36 PM Melanie Plageman
<melanieplageman(at)gmail(dot)com> wrote:
> * "Freeze" NewRelfrozenXid/NewRelminMxid trackers.
> *
> * Trackers used when heap_freeze_execute_prepared freezes, or when there
> * are zero freeze plans for a page. It is always valid for vacuumlazy.c
> * to freeze any page, by definition. This even includes pages that have
> * no tuples with storage to consider in the first place. That way the
> * 'totally_frozen' results from heap_prepare_freeze_tuple can always be
> * used in the same way, even when no freeze plans need to be executed to
> * "freeze the page". Only the "freeze" path needs to consider the need
> * to set pages all-frozen in the visibility map under this scheme.
> *
> * When we freeze a page, we generally freeze all XIDs < OldestXmin, only
> * leaving behind XIDs that are ineligible for freezing, if any. And so
> * you might wonder why these trackers are necessary at all; why should
> * _any_ page that VACUUM freezes _ever_ be left with XIDs/MXIDs that
> * ratchet back the top-level NewRelfrozenXid/NewRelminMxid trackers?
> *
> * It is useful to use a definition of "freeze the page" that does not
> * overspecify how MultiXacts are affected. heap_prepare_freeze_tuple
> * generally prefers to remove Multis eagerly, but lazy processing is used
> * in cases where laziness allows VACUUM to avoid allocating a new Multi.
> * The "freeze the page" trackers enable this flexibility.
> */
>
> So, I don't really know if it is right to just check presult->nfrozen >
> 0 when updating relminmxid. I have changed it to the way you suggested.
> But we can change it back.

I think that this is just about safe. I had to check, though. I see
that the FRM_NOOP case (within
FreezeMultiXactId/heap_prepare_freeze_tuple) will ratchet back both
sets of trackers (both the freeze and no freeze variants). However,
it's rather hard to see that this is true.

The intent here was that cases where "presult->nfrozen == 0" would
always take the "freeze" path. That seems more natural to me, at
least, since I think of the freeze path as the default choice. By
definition, lazy_scan_prune() can always take the freeze path -- even
when the page has no tuples with storage. But it cannot always take
the no-freeze path -- "disobeying" pagefrz.freeze_required creates the
risk that relfrozenxid/relminmxid will be advanced to unsafe values at
the end of the VACUUM. IMV you should stick with that approach now,
even if it is currently safe to do it the other way around.

--
Peter Geoghegan