code: def is_distinct(self):
      
      return bool(self.__group_by and
                  set(self._to_property_names(self.__group_by)) <=
                  set(self._to_property_names(self.__projection)))
docs: True if results are guaranteed to contain a unique set of property
      values.
  
      This happens when every property in the group_by is also in the projection.