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From: Peter Geoghegan <pg@bowt.ie>
Date: Sat, 19 Feb 2022 19:31:21 -0800
Message-ID: <CAH2-WznpbJQJ5Hp=Te3YH0hcYeLEZwGe6de6E4L8E1vrNuCLKA@mail.gmail.com>
Subject: Re: Removing more vacuumlazy.c special cases, relfrozenxid optimizations
To: Andres Freund <andres@anarazel.de>
Cc: Robert Haas <robertmhaas@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, 
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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On Sat, Feb 19, 2022 at 6:16 PM Peter Geoghegan <pg@bowt.ie> wrote:
> > 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.
>
> I guess that's also true. There is at least a legitimate argument to
> be made for not leaving behind any orphaned heap-only tuples. The
> interface is a TID, and so the user may already believe that they're
> killing the heap-only, not just the root item (since ctid suggests
> that the TID of a heap-only tuple is the TID of the root item, which
> is kind of misleading).

Actually, I would say that heap_surgery's raison d'etre is making
weird errors related to corruption of this or that TID go away, so
that the user can cut their losses. That's how it's advertised.

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 -- 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). And we've just created natural
demand for heap_surgery to deal with the problem by deleting whole HOT
chains (not just root items).

If we allow VACUUM to treat orphaned heap-only tuples as DEAD right
away, then we might as well do the same thing in heap_surgery, since
there is little chance that the user will get to the heap-only tuples
before VACUUM does (not something to rely on, at any rate).

Either way, I think we probably end up needing to teach heap_surgery
to kill entire HOT chains as a group, given a TID.

-- 
Peter Geoghegan