Horton

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

Noah Misch <noah@leadboat.com>

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From: Noah Misch <noah@leadboat.com>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Melanie Plageman <melanieplageman@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Pg Hackers <pgsql-hackers@postgresql.org>, Andres Freund <andres@anarazel.de>
Date: 2024-06-25T02:09:52Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Test that vacuum removes tuples older than OldestXmin

  2. Lower minimum maintenance_work_mem to 64kB

  3. Add accidentally omitted test to meson build file

  4. Use DELETE instead of UPDATE to speed up vacuum test

  5. Revert "Test that vacuum removes tuples older than OldestXmin"

  6. Ensure vacuum removes all visibly dead tuples older than OldestXmin

On Mon, Jun 24, 2024 at 09:49:53PM -0400, Peter Geoghegan wrote:
> 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?

To me, no, unification would mean removing the data redundancy.  Relocating
the redundant field and/or code that updates the redundant field certainly
could reduce the risk of bugs, so I'm not opposing everything short of
removing the data redundancy.  I'm just agreeing with the "prefer" from
https://postgr.es/m/CA+TgmoYzS_bkt_MrNxr5QrXDKfedmh4tStn8UBTTBXqv=3JTew@mail.gmail.com