Why is parula failing?

For the last few days, buildfarm member parula has been intermittently
failing the partition_prune regression test, due to unexpected plan
changes [1][2][3][4]. The symptoms can be reproduced exactly by
inserting a "vacuum" of one or another of the partitions of table
"ab", so we can presume that the underlying cause is an autovacuum run
against one of those tables. But why is that happening? None of
those tables receive any insertions during the test, so I don't
understand why autovacuum would trigger on them.

I suppose we could attach "autovacuum=off" settings to these tables,
but it doesn't seem to me that that should be necessary. These test
cases are several years old and haven't given trouble before.
Moreover, if that's necessary then there are a lot of other regression
tests that would presumably need the same treatment.

I'm also baffled why this wasn't happening before. I scraped the
buildfarm logs for 3 months back and confirmed my impression that
this is a new failure mode. But one of these four runs is on
REL_14_STABLE, eliminating the theory that the cause is a recent
HEAD-only change.

Any ideas?

regards, tom lane

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=parula&dt=2024-03-19%2016%3A09%3A02
[2] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=parula&dt=2024-03-18%2011%3A13%3A02
[3] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=parula&dt=2024-03-14%2011%3A40%3A02
[4] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=parula&dt=2024-03-14%2019%3A00%3A02