Horton

Re: relfrozenxid may disagree with row XIDs after 1ccc1e05ae

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Bowen Shi <zxwsbg12138@gmail.com>, Melanie Plageman <melanieplageman@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, Noah Misch <noah@leadboat.com>, Alexander Lakhin <exclusion@gmail.com>, PostgreSQL mailing lists <pgsql-bugs@lists.postgresql.org>
Date: 2024-05-16T20:38:38Z
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.

Hi,

On 2024-05-16 13:29:49 -0700, Andres Freund wrote:
> On 2024-05-16 16:13:35 -0400, Peter Geoghegan wrote:
> > > Now I wonder if there is some codepath triggering catalog lookups during bulk
> > > delete.
> >
> > I don't think that there's any rule that says that VACUUM cannot do
> > catalog lookups during bulk deletions. B-Tree page deletion needs to
> > generate an insertion scan key, so that it can "refind" a page
> > undergoing deletion. That might require catalog lookups.
>
> I'm not saying there's a hard rule against it. Just that there wasn't an
> immediately apparent, nor immediately observable, path for it. As I didn't see
> the path to the horizon recomputation, I didn't know how a btbulkdelete in the
> middle of the scan would potentially trigger the problem.

Hm. Actually. I think it might not be correct to do catalog lookups at that
point. But it's a bigger issue than just catalog lookups during bulkdelete:
Once we've done
     MyProc->statusFlags |= PROC_IN_VACUUM;

the current backend's snapshots don't prevent rows from being removed
anymore.

I first wrote:
> That's not a huge issue for the pg_class entry itself, as the locks should
> prevent it from being updated. But there are a lot of catalog lookups that
> aren't protected by locks, just normal snapshot semantics.

but as it turns out we haven't even locked the relation at the point we set
PROC_IN_VACUUM.

That seems quite broken.


WRT bulkdelete, there's this comment where we set PROC_IN_VACUUM:

		 * In lazy vacuum, we can set the PROC_IN_VACUUM flag, which lets
		 * other concurrent VACUUMs know that they can ignore this one while
		 * determining their OldestXmin.  (The reason we don't set it during a
		 * full VACUUM is exactly that we may have to run user-defined
		 * functions for functional indexes, and we want to make sure that if
		 * they use the snapshot set above, any tuples it requires can't get
		 * removed from other tables.  An index function that depends on the
		 * contents of other tables is arguably broken, but we won't break it
		 * here by violating transaction semantics.)

the parenthetical explains that/why we can't evaluate user defined
functions. Which seems to be violated by doing key comparisons, no?

Greetings,

Andres Freund