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

Peter Geoghegan <pg@bowt.ie>

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: 2024-01-06T21:41:23Z

Lists: pgsql-bugs

Commits

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Ensure vacuum removes all visibly dead tuples older than OldestXmin
- 06bf404cd07b 16.4 landed
- 45ce054c02b8 14.13 landed
- dc6354c67017 15.8 landed
Combine freezing and pruning steps in VACUUM
- 6dbb490261a6 17.0 cited
Handle non-chain tuples outside of heap_prune_chain()
- 6f47f6883151 17.0 cited
Fix false reports in pg_visibility
- e85662df44ff 17.0 cited
Remove retry loop in heap_page_prune().
- 1ccc1e05ae8f 17.0 cited
vacuumlazy.c: document vistest and OldestXmin.
- 73f6ec3d3c8d 15.0 cited
Deduplicate choice of horizon for a relation procarray.c.
- d9d8aa9bb9aa 15.0 cited
Remove tupgone special case from vacuumlazy.c.
- 8523492d4e34 14.0 cited
Simplify state managed by VACUUM.
- b4af70cb2103 14.0 cited
Recycle nbtree pages deleted during same VACUUM.
- 9dd963ae2534 14.0 cited
snapshot scalability: Don't compute global horizons while building snapshots.
- dc7420c2c927 14.0 cited
Raise error when affecting tuple moved into different partition.
- f16241bef7cc 11.0 cited

On Sat, Jan 6, 2024 at 1:30 PM Peter Geoghegan <pg@bowt.ie> wrote:
> On Sat, Jan 6, 2024 at 12:24 PM Noah Misch <noah@leadboat.com> wrote:
> > Fair enough.  While I agree there's a decent chance back-patching would be
> > okay, I think there's also a decent chance that 1ccc1e05ae creates the problem
> > Matthias theorized.  Something like: we update relfrozenxid based on
> > OldestXmin, even though GlobalVisState caused us to retain a tuple older than
> > OldestXmin.  Then relfrozenxid disagrees with table contents.
>
> Either every relevant code path has the same OldestXmin to work off
> of, or the whole NewRelfrozenXid/relfrozenxid-tracking thing can't be
> expected to work as designed. I find it a bit odd that
> pruneheap.c/GlobalVisState has no direct understanding of this
> dependency (none that I can discern, at least).

What do you think of the idea of adding a defensive "can't happen"
error to lazy_scan_prune() that will catch DEAD or RECENTLY_DEAD
tuples with storage that still contain an xmax < OldestXmin? This
probably won't catch every possible problem, but I suspect it'll work
well enough.

-- 
Peter Geoghegan