Re: Decoupling antiwraparound autovacuum from special rules around auto cancellation

On Wed, Jan 18, 2023 at 6:10 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > This creates an awkward but logical question, though: what if
> > dead_tuples doesn't go down at all? What if VACUUM actually has to
> > increase it, because VACUUM runs so slowly relative to the workload?
>
> Sure, that can happen - but it's not made better by having wrong stats :)

Maybe it's that simple. It's reasonable to wonder how far we want to
go with letting dead tuples grow and grow, even when VACUUM is running
constantly. It's not a question that has an immediate and obvious
answer IMV.

Maybe the real question is: is this an opportunity to signal to the
user (say via a LOG message) that VACUUM cannot keep up? That might be
very useful, in a general sort of way (not just to avoid new
problems).

> We have reasonably sophisticated accounting in pgstats what newly live/dead
> rows a transaction "creates". So an obvious (and wrong) idea is just decrement
> reltuples by the number of tuples removed by autovacuum.

Did you mean dead_tuples?

> But we can't do that,
> because inserted/deleted tuples reported by backends can be removed by
> on-access pruning and vacuumlazy doesn't know about all changes made by its
> call to heap_page_prune().

I'm not following here. Perhaps this is a good sign that I should stop
working for the day. :-)

> But I think that if we add a
> pgstat_count_heap_prune(nredirected, ndead, nunused)
> around heap_page_prune() and a
> pgstat_count_heap_vacuum(nunused)
> in lazy_vacuum_heap_page(), we'd likely end up with a better approximation
> than what vac_estimate_reltuples() does, in the "partially scanned" case.

What does vac_estimate_reltuples() have to do with dead tuples?

--
Peter Geoghegan