Re: Why is parula failing?

On Thu, 21 Mar 2024 at 14:19, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> David Rowley <dgrowleyml@gmail.com> writes:
> > We could also do something like the attached just in case we're
> > barking up the wrong tree.
>
> Yeah, checking indisvalid isn't a bad idea.  I'd put another
> one further down, just before the DROP of table ab, so we
> can see the state both before and after the unstable tests.

So it's taken quite a while to finally fail again.

Effectively, we're getting:

    relname     | relpages | reltuples | indisvalid | autovacuum_count
| autoanalyze_count
----------------+----------+-----------+------------+------------------+-------------------
- ab_a2_b2       |        0 |        -1 |            |
0 |                 0
+ ab_a2_b2       |        0 |        48 |            |
0 |                 0

I see AddNewRelationTuple() does set reltuples to -1, so I can't quite
figure out why 48 is in there.  Even if auto-analyze had somehow
mistakenly run and the autoanalyze_count stats just were not
up-to-date yet, the table has zero blocks, and I don't see how
acquire_sample_rows() would set *totalrows to anything other than 0.0
in this case.  For the vacuum case, I see that reltuples is set from:

/* now we can compute the new value for pg_class.reltuples */
vacrel->new_live_tuples = vac_estimate_reltuples(vacrel->rel, rel_pages,
vacrel->scanned_pages,
vacrel->live_tuples);

Again, hard to see how that could come to anything other than zero
given that rel_pages and scanned_pages should be 0.

Looking at the binary representation of a float of -1 vs 48, they're
not nearly the same. 0xBF800000 vs 0x42400000, so it's not looking
like a flipped bit.

It would be good to have log_autovacuum_min_duration = 0 on this
machine for a while.

David