On Sat, Feb 19, 2022 at 7:47 PM Andres Freund <andres@anarazel.de> wrote:
> I'm not that sure those are that different... Imagine some corruption leading
> to two hot chains ending in the same tid, which our fancy new secure pruning
> algorithm might detect.
I suppose that's possible, but it doesn't seem all that likely to ever
happen, what with the xmin -> xmax cross-tuple matching stuff.
> Either way, I'm a bit surprised about the logic to not allow killing redirect
> items? What if you have a redirect pointing to an unused item?
Again, I simply think it boils down to having to treat HOT chains as a
whole unit when killing TIDs.
> > Let's assume that we don't want to make VACUUM/pruning just treat
> > orphaned heap-only tuples as DEAD, regardless of their true HTSV-wise
> > status
>
> I don't think that'd ever be a good idea. Those tuples are visible to a
> seqscan after all.
I agree (I don't hate it completely, but it seems mostly bad). This is
what leads me to the conclusion that pg_surgery has to be able to do
this instead. Surely it's not okay to have something that makes VACUUM
always end in error, that cannot even be fixed by pg_surgery.
> > -- let's say that we want to err in the direction of doing
> > nothing at all with the page. Now we have to have a weird error in
> > VACUUM instead (not great, but better than just spinning between
> > lazy_scan_prune and heap_prune_page).
>
> Non DEAD orphaned versions shouldn't cause a problem in lazy_scan_prune(). The
> problem here is a DEAD orphaned HOT tuples, and those we should be able to
> delete with the new page pruning logic, right?
Right. But what good does that really do? The problematic page had a
third tuple (at offnum 3) that was LIVE. If we could have done
something about the problematic tuple at offnum 2 (which is where we
got stuck), then we'd still be left with a very unpleasant choice
about what happens to the third tuple.
> I think it might be worth getting rid of the need for the retry approach by
> reusing the same HTSV status array between heap_prune_page and
> lazy_scan_prune. Then the only legitimate reason for seeing a DEAD item in
> lazy_scan_prune() would be some form of corruption. And it'd be a pretty
> decent performance boost, HTSV ain't cheap.
I guess it doesn't actually matter if we leave an aborted DEAD tuple
behind, that we could have pruned away, but didn't. The important
thing is to be consistent at the level of the page.
--
Peter Geoghegan