From: Tom Lane <tgl@sss.pgh.pa.us>
To: "Plettenbacher, Tobias (LWF)" <Tobias.Plettenbacher@lwf.bayern.de>
cc: "pgsql-bugs@lists.postgresql.org" <pgsql-bugs@lists.postgresql.org>
Subject: Re: Bug or strange result of Max() on arrays containing NULL values
In-reply-to: <325d78c062e945ebb286091f0eaa8b72@lwf.bayern.de>
References: <325d78c062e945ebb286091f0eaa8b72@lwf.bayern.de>
Comments: In-reply-to "Plettenbacher,
 Tobias (LWF)" <Tobias.Plettenbacher@lwf.bayern.de>
	message dated "Sat, 24 Aug 2024 12:54:26 -0000"
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Date: Sat, 24 Aug 2024 10:54:48 -0400
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"Plettenbacher, Tobias (LWF)" <Tobias.Plettenbacher@lwf.bayern.de> writes:
> With Max(ARRAY[]) I get a strange result (in this case {NULL,7}):
> SELECT Max(ARRAY[Val, ID]) FROM (VALUES (1, 5), (2, 6), (NULL, 7)) AS T(=
Val, ID);

This is the expected result, because

=3D# select array[null, 7] > array[2, 6];
 ?column? =

----------
 t
(1 row)

When comparing array elements (or members of any container type),
we treat two nulls as equal and a null as larger than any non-null.
You might think that such a comparison should yield null, but if
we did that then the comparisons would fail to provide a total
order for the container type.  That would, among other things,
break the ability to build b-tree indexes on such types.

			regards, tom lane