Re: Vacuum ERRORs out considering freezing dead tuples from before OldestXmin

On Mon, Jun 24, 2024 at 9:30 PM Noah Misch <noah@leadboat.com> wrote:
> On Mon, Jun 24, 2024 at 03:23:39PM -0400, Melanie Plageman wrote:
> > Right now, in master, we do use a single horizon when determining what
> > is pruned -- that from GlobalVisState. OldestXmin is only used for
> > freezing and full page visibility determinations. Using a different
> > horizon for pruning by vacuum than freezing is what is causing the
> > error on master.
>
> Agreed, and I think using different sources for pruning and freezing is a
> recipe for future bugs.  Fundamentally, both are about answering "is
> snapshot_considers_xid_in_progress(snapshot, xid) false for every snapshot?"
> That's not to say this thread shall unify the two, but I suspect that's the
> right long-term direction.

What does it really mean to unify the two, though?

If the OldestXmin field was located in struct GlobalVisState (next to
definitely_needed and maybe_needed), but everything worked in
essentially the same way as it will with Melanie's patch in place,
would that count as unifying the two? Why or why not?

--
Peter Geoghegan