From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <dgrowleyml@gmail.com>
cc: Sawyer Knoblich <scknoblich@gmail.com>, pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #18399: Query plan optimization results in runtime error when
 hoisting cast from inside subquery
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Comments: In-reply-to David Rowley <dgrowleyml@gmail.com>
	message dated "Wed, 20 Mar 2024 17:08:49 +1300"
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Date: Wed, 20 Mar 2024 00:39:05 -0400
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David Rowley <dgrowleyml@gmail.com> writes:
> There are quite a lot of things which would have to be restricted,
> much more than you might think.  Any conditions which could cause an
> error would have to be evaluated last in a WHERE clause and that might
> result in being unable to use indexes because some other (possibly
> unindexed) expression would need to be evaluated first.

That particular aspect might not be too awful, because there's already
a good deal of pressure for index opclass members to not fail ---
certainly I'd expect that all standard btree comparison functions
could be marked non-failing.  (Let us slide quietly past the question
of exactly how strong the guarantee should be; for example any function
that takes toastable argument types is potentially at risk of OOM,
but do you really want to exclude numeric_eq and texteq from the
set of safe operations?  See also past discussions on how strict
we should be about the related concept of leakproofness.)

In any case I agree with your larger point that ordering things such
that potentially-failing tests are always done last would be a
performance disaster, even if it's possible at all.

			regards, tom lane