Date: Sat, 19 Feb 2022 17:54:16 -0800
From: Andres Freund <andres@anarazel.de>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Robert Haas <robertmhaas@gmail.com>,
	Masahiko Sawada <sawada.mshk@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Removing more vacuumlazy.c special cases, relfrozenxid
 optimizations
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Hi,

On 2022-02-19 17:22:33 -0800, Peter Geoghegan wrote:
> 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).

How does that cause the endless loop?

It doesn't do so on HEAD + 0001-Add-adversarial-ConditionalLockBuff[...] for
me. So something needs have changed with your patch?


> 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.

Yea, we should pick that up again. Not just for robustness or
performance. Also because it's just a lot easier to understand.


> 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.)

Given that heap_surgery's raison d'etre is correcting corruption etc, I think
it makes sense for it to do as minimal work as possible. Iterating through a
HOT chain would be a problem if you e.g. tried to repair a page with HOT
corruption.

Greetings,

Andres Freund