Re: Decoupling antiwraparound autovacuum from special rules around auto cancellation

On Wed, Jan 18, 2023 at 3:28 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> The problems in this area tend to be that vac_estimate_reltuples()
> behaves as if it sees a random sample, when in fact it's far from
> random -- it's the same scanned_pages as last time, and the ten other
> times before that. That's a feature of your test case, and other
> similar vac_estimate_reltuples test cases that I came up with in the
> past. Both scenarios involved skipping using the visibility map in
> multiple successive VACUUM operations.

FWIW, the problem in your test case just goes away if you just change this line:

DELETE FROM foo WHERE i < (10000000 * 0.1)

To this:

DELETE FROM foo WHERE i < (10000000 * 0.065)

Which is not a huge difference, overall. This effect is a consequence
of the heuristics I added in commit 74388a1a, so it's only present on
Postgres 15+.

Whether or not this is sufficient protection is of course open to
interpretation. One problem with those heuristics (as far as your test
case is concerned) is that they either work, or they don't work. For
example they're conditioned on "old_rel_pages == total_page".

--
Peter Geoghegan