Re: Removing more vacuumlazy.c special cases, relfrozenxid optimizations

On Sat, Feb 19, 2022 at 4:22 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> This very much looks like a bug in pg_surgery itself now -- attached
> is a draft fix.

Wait, that's not it either. I jumped the gun -- this isn't sufficient
(though the patch I posted might not be a bad idea anyway).

Looks like pg_surgery isn't processing HOT chains as whole units,
which it really should (at least in the context of killing items via
the heap_force_kill() function). Killing a root item in a HOT chain is
just hazardous -- disconnected/orphaned heap-only tuples are liable to
cause chaos, and should be avoided everywhere (including during
pruning, and within pg_surgery).

It's likely that the hardening I already planned on adding to pruning
[1] (as follow-up work to recent bugfix commit 18b87b201f) will
prevent lazy_scan_prune from getting stuck like this, whatever the
cause happens to be. The actual page image I see lazy_scan_prune choke
on (i.e. exhibit the same infinite loop unpleasantness we've seen
before on) is not in a consistent state at all (its tuples consist of
tuples from a single HOT chain, and the HOT chain is totally
inconsistent on account of having an LP_DEAD line pointer root item).
pg_surgery could in principle do the right thing here by always
treating HOT chains as whole units.

Leaving behind disconnected/orphaned heap-only tuples is pretty much
pointless anyway, since they'll never be accessible by index scans.
Even after a REINDEX, since there is no root item from the heap page
to go in the index. (A dump and restore might work better, though.)

[1] https://postgr.es/m/CAH2-WzmNk6V6tqzuuabxoxM8HJRaWU6h12toaS-bqYcLiht16A@mail.gmail.com
--
Peter Geoghegan