Allow unique constraint over keys which include one or more nullable fields. Prior to this change Derby only supported unique constraints on keys that included no nullable columns. The new constraint will allow unlimited inserts of any key with one more null columns, but will limit insert of keys with no null columns to 1 unique value per table.
There is no change to existing or newly created unique indexes on null columns (as opposed to unique constraints on null columns). Also there is no change to existing or newly created constraints on keys with no nullable columns.