Date: Thu, 8 Apr 2021 14:35:51 -0400
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: pgsql: autovacuum: handle analyze for partitioned tables
Message-ID: <20210408183551.GA16359@alvherre.pgsql>
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On 2021-Apr-08, Tom Lane wrote:

> Yeah.  I hit this on another machine that isn't using EXEC_BACKEND,
> and I concur it looks more like a race condition.  I think the problem
> is that autovacuum is calling find_all_inheritors() on a relation it
> has no lock on, contrary to that function's API spec.  find_all_inheritors
> assumes the OID it's given is valid and locked, and adds it to the
> result list automatically.  Then it looks for children, and won't find
> any in the race case where somebody else just dropped the table.

Hmm.  Autovacuum tries hard to avoid grabbing locks on relations until
really needed (at vacuum/analyze time), which is why all these tests
only use data that can be found in the pg_class rows and pgstat entries.
So I tend to think that my initial instinct was the better direction: we
should not be doing any find_all_inheritors() here at all, but instead
rely on pg_class.reltuples to be set for the partitioned table.

I'll give that another look.  Most places already assume that reltuples
isn't set for a partitioned table, so they shouldn't care.  I wonder,
though, whether we should set relpages to some value other than 0 or -1.
(I'm inclined not to, since autovacuum does not use it.)

-- 
Álvaro Herrera       Valdivia, Chile