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