From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <dgrowleyml@gmail.com>
cc: Vik Fearing <vik@postgresfriends.org>,
        =?UTF-8?Q?Marek_Malevi=C4=8D?= <marek@malevicovi.net>,
        pgsql-bugs@lists.postgresql.org
Subject: Re: Broken type checking for empty subqueries
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Comments: In-reply-to David Rowley <dgrowleyml@gmail.com>
	message dated "Fri, 29 Sep 2023 01:41:11 +1300"
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Date: Thu, 28 Sep 2023 10:57:39 -0400
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David Rowley <dgrowleyml@gmail.com> writes:
> I've not looked at the code to see if this would be practical or not,
> but I wonder if we could reduce these bug reports by using the new
> soft error reporting that's now done in the input functions to have
> constant folding just silently not do any folding for the expression
> if a cast fails.

Sadly, I doubt that would cover enough of the problem space to make
much difference to people who try to do this sort of thing.

> ... I imagine most people who
> have had them fail during constant folding have just redesigned or
> found some hack to prevent the folding from taking place anyway.

Yeah.  The given query looks like it was already hacked to avoid
constant-folding, though I wonder if whoever wrote it really understood
that.  Otherwise it'd be a lot more natural to use something like CASE.
Anyway, adding OFFSET 0 to suppress sub-select folding is probably
the best answer here.

			regards, tom lane