Re: Correcting freeze conflict horizon calculation

On Thu, May 29, 2025 at 10:57 AM Melanie Plageman
<melanieplageman(at)gmail(dot)com> wrote:
> However, we calculate the snapshot conflict horizon for the
> prune/freeze record in master/17 and the freeze record in 16 before
> updating all-frozen and all-visible. That means the horizon may be too
> aggressive if the page cannot actually be set all-frozen. This isn't a
> correctness issue, but it may lead to transactions on the standby
> being unnecessarily canceled for pages which have some tuples frozen
> but not all due to remaining LP_DEAD items.

I don't follow.

Your concern is that the horizon might be overly aggressive/too
conservative. But your patch (for 16) makes us take the
don't-use-snapshotConflictHorizon-twice block *less* frequently (and
the "use OldestXmin conservatively" block *more* frequently):

- if (prunestate->all_visible && prunestate->all_frozen)
+ if (prunestate->all_visible && prunestate->all_frozen && lpdead_items == 0)
{
/* Using same cutoff when setting VM is now unnecessary */
snapshotConflictHorizon = prunestate->visibility_cutoff_xid;
prunestate->visibility_cutoff_xid = InvalidTransactionId;
}
else
{
/* Avoids false conflicts when hot_standby_feedback in use */
snapshotConflictHorizon = vacrel->cutoffs.OldestXmin;
TransactionIdRetreat(snapshotConflictHorizon);
}

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?

--
Peter Geoghegan