Horton

Re: BUG #17257: (auto)vacuum hangs within lazy_scan_prune()

Peter Geoghegan <pg@bowt.ie>

View thread
From: Peter Geoghegan <pg@bowt.ie>
To: Noah Misch <noah@leadboat.com>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>, robertmhaas@gmail.com, Alexander Lakhin <exclusion@gmail.com>, PostgreSQL mailing lists <pgsql-bugs@lists.postgresql.org>, Andres Freund <andres@anarazel.de>
Date: 2024-01-08T18:36:13Z
Lists: pgsql-bugs

Commits

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

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

  2. Combine freezing and pruning steps in VACUUM

  3. Handle non-chain tuples outside of heap_prune_chain()

  4. Fix false reports in pg_visibility

  5. Remove retry loop in heap_page_prune().

  6. vacuumlazy.c: document vistest and OldestXmin.

  7. Deduplicate choice of horizon for a relation procarray.c.

  8. Remove tupgone special case from vacuumlazy.c.

  9. Simplify state managed by VACUUM.

  10. Recycle nbtree pages deleted during same VACUUM.

  11. snapshot scalability: Don't compute global horizons while building snapshots.

  12. Raise error when affecting tuple moved into different partition.

On Mon, Jan 8, 2024 at 1:21 PM Noah Misch <noah@leadboat.com> wrote:
> > I see no reason why it
> > matters if OldestXmin goes backwards across two VACUUM operations, so
> > I haven't tried to avoid that.
>
> That may be fully okay, or we may want to clamp OldestXmin to be no older than
> relfrozenxid.  I don't feel great about the system moving relfrozenxid
> backward unless it observed an older XID, and observing an older XID would be
> a corruption signal.  I don't have a specific way non-monotonic relfrozenxid
> breaks things, though.

We're already prepared for this -- relfrozenxid simply cannot go
backwards, regardless of what vacuumlazy.c thinks. That is,
vac_update_relstats() won't accept a new relfrozenxid that is < its
existing value (unless it's a value "from the future", which is a way
of recovering after historical pg_upgrade-related corruption bugs).

If memory serves it doesn't take much effort to exercise the relevant
code within vac_update_relstats(). I'm pretty sure that the regression
tests will fail if you run them after removing its defensive
no-older-relfrozenxid test (though I haven't checked recently).

--
Peter Geoghegan