Hi,
Currently in Denodo Platform, when there is an obligatory field and the query provides a set of possible values for the field using an IN operator in the WHERE clause, then the Virtual DataPort would generate a union query by providing the obligatory field with each of the possible values. And the behavior is the same if the view parameter does not have a default value. But when there is a default value, the view parameter is not OBL any more, so a union is not generated.
Hope this helps!