Re: Removing more vacuumlazy.c special cases, relfrozenxid optimizations

On Thu, Mar 31, 2022 at 9:37 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> Perhaps we should explicitly mask out parts of relcache entries in the shared
> init file that we know to be unreliable. I.e. set relfrozenxid, relminmxid to
> Invalid* or such.

That has the advantage of being more honest. If you're going to break
the abstraction, then it seems best to break it in an obvious way,
that leaves no doubts about what you're supposed to be relying on.

This bug doesn't seem like the kind of thing that should be left
as-is. If only because it makes it hard to add something like a
WARNING when we make relfrozenxid go backwards (on the basis of the
existing value apparently being in the future), which we really should
have been doing all along.

The whole reason why we overwrite pg_class.relfrozenxid values from
the future is to ameliorate the effects of more serious bugs like the
pg_upgrade/pg_resetwal one fixed in commit 74cf7d46 not so long ago
(mid last year). We had essentially the same pg_upgrade "from the
future" bug twice (once for relminmxid in the MultiXact bug era,
another more recent version affecting relfrozenxid).

> The only place that appears to access rd_rel->relfrozenxid outside of DDL is
> heap_abort_speculative().

I wonder how necessary that really is. Even if the XID is before
relfrozenxid, does that in itself really make it "in the future"?
Obviously it's often necessary to make the assumption that allowing
wraparound amounts to allowing XIDs "from the future" to exist, which
is dangerous. But why here? Won't pruning by VACUUM eventually correct
the issue anyway?

--
Peter Geoghegan