Horton

Re: BUG #17255: Server crashes in index_delete_sort_cmp() due to race condition with vacuum

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Dmitry Dolgov <9erthalion6@gmail.com>, Alexander Lakhin <exclusion@gmail.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, PostgreSQL mailing lists <pgsql-bugs@lists.postgresql.org>
Date: 2021-11-11T03:16:41Z
Lists: pgsql-bugs 
Hi,

On 2021-11-10 18:40:13 -0800, Peter Geoghegan wrote:
> On Wed, Nov 10, 2021 at 6:16 PM Andres Freund <andres@anarazel.de> wrote:
> > Hm. To me all of this is more general than vacuum[lazy].c. Or even than
> > anything heap related.
> 
> Here is a sensible compromise: put most of what you want to say
> wherever (I guess procarray.c), and then move the vacuumlazy.c call to
> GlobalVisTestFor() back, so that it comes immediately after the
> vacuum_set_xid_limits() call. Then place a few breadcrumb comments
> that reference the place in procarray.c that has the real discussion.

I don't really think it's a good idea to move it to earlier. If anything I
would want to move it later (and eventually occasionally re-compute it, that'd
be a *huge* win for longrunning vacuums). We can't really do the same with the
hard horizons. It's currently super likely that the horizon moves forward a
lot between vacuum_set_xid_limits() and GlobalVisTestFor(), but moving it
closer still is going in the wrong direction.

So I'm inclined to add a comment saying that we need to do the
GlobalVisTestFor(), pointing to the comment in ComputeXidHorizons(), and
adding a TODO that we should find a heuristic when to recompute horizons?



> We could probably *also* freeze tuples opportunistically (e.g., freeze
> a few tuples on a page early to be able to mark it all-frozen sooner),

We should definitely do that when a page is already being dirtied for other
reasons.

Greetings,

Andres Freund

Commits

  1. vacuumlazy.c: Standardize rel_pages terminology.

  2. vacuumlazy.c: document vistest and OldestXmin.

  3. heap pruning: Only call BufferGetBlockNumber() once.

  4. Fix possible HOT corruption when RECENTLY_DEAD changes to DEAD while pruning.

  5. Assert redirect pointers are sensible after heap_page_prune().

  6. Remove tupgone special case from vacuumlazy.c.

  7. Recycle nbtree pages deleted during same VACUUM.

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

  9. Refactor heap_page_prune so that instead of changing item states on-the-fly,