From: Tom Lane <tgl@sss.pgh.pa.us>
To: Wilm Hoyer <W.Hoyer@dental-vision.de>
cc: "pgsql-bugs@lists.postgresql.org" <pgsql-bugs@lists.postgresql.org>
Subject: Re: XML column causes record to be both NULL and NOT NULL
In-reply-to: <3a116b87d6eb4a6a8040749c991fe961@dental-vision.de>
References: <3a116b87d6eb4a6a8040749c991fe961@dental-vision.de>
Comments: In-reply-to Wilm Hoyer <W.Hoyer@dental-vision.de>
	message dated "Tue, 04 Jul 2023 18:46:23 -0000"
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Date: Tue, 04 Jul 2023 14:59:24 -0400
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Wilm Hoyer <W.Hoyer@dental-vision.de> writes:
> i found a strange behavior and hope it's not a pilot error.

Well, it is strange, but it's acting per SQL spec.
As stated at [1]:

    If the expression is row-valued, then IS NULL is true when the row
    expression itself is null or when all the row's fields are null, while
    IS NOT NULL is true when the row expression itself is non-null and all
    the row's fields are non-null. Because of this behavior, IS NULL and
    IS NOT NULL do not always return inverse results for row-valued
    expressions; in particular, a row-valued expression that contains both
    null and non-null fields will return false for both tests.

The fact that the added column is of XML type isn't relevant here;
what's relevant is that it has a null value.

			regards, tom lane

[1] https://www.postgresql.org/docs/current/functions-comparison.html