From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <dgrowleyml@gmail.com>
cc: Chengpeng Yan <chengpeng_yan@outlook.com>,
        PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: [PATCH] Fix hashed ScalarArrayOp semantics for NULL LHS with
 non-strict comparators
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Comments: In-reply-to David Rowley <dgrowleyml@gmail.com>
	message dated "Mon, 20 Apr 2026 15:46:10 +1200"
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Date: Mon, 20 Apr 2026 00:14:03 -0400
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David Rowley <dgrowleyml@gmail.com> writes:
> I've attached a version that "probes" the equality function for its
> NULL =3D NULL behaviour and its NULL =3D non-NULL behaviour and returns
> whatever the result of the probe was at the appropriate time.

> What I came up with does feel quite elaborate, so I'd quite like a 2nd o=
pinion.

> The patch does assume that the non-strict function will return the
> same thing for NULL =3D non-NULL as it will for non-NULL =3D NULL.

Meh.  I think we assume that hashable equality functions satisfy the
symmetric law, ie A =3D B if and only if B =3D A, so that part is fine.
However, I do not care for the assumption that any random non-null
input will produce the same answer.  As a quick counter-example,
consider a text-like datatype that tries to emulate Oracle's semantics
that an empty string is the same as NULL.  Your code would arrive at
different results depending on whether the first non-null input
chanced to be an empty string.

(I've not read this whole thread, so I don't have a global opinion
on what we ought to do here.  I suspect it's a tricky subject.)

			regards, tom lane