Re: Decoupling antiwraparound autovacuum from special rules around auto cancellation

Hi

On 2023-01-18 19:26:22 -0800, Peter Geoghegan wrote:
> On Wed, Jan 18, 2023 at 7:04 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > > You seem to be saying that it's a problem if we don't update reltuples
> > > -- an estimate -- when less than 2% of the table is scanned by VACUUM.
> > > But why? Why can't we just do nothing sometimes? I mean in general,
> > > leaving aside the heuristics I came up with for a moment?
> >
> > The problem isn't that we might apply the heuristic once, that'd be fine. But
> > that there's nothing preventing it from applying until there basically are no
> > tuples left, as long as the vacuum is frequent enough.
> >
> > As a demo: The attached sql script ends up with a table containing 10k rows,
> > but relpages being set 1 million.
>
> I saw that too. But then I checked again a few seconds later, and
> autoanalyze had run, so reltuples was 10k. Just like it would have if
> there was no VACUUM statements in your script.

There's absolutely no guarantee that autoanalyze is triggered
there. Particularly with repeated vacuums triggered due to an relfrozenxid age
that can't be advanced that very well might not happen within days on a large
relation.

Greetings,

Andres Freund