code: def _collection_length(self, final_set):
          
          conn = self.cls.get_connection()
  
          # we have a sorted set without need to sort, use zcard
          if self._has_sortedsets:
              return conn.zcard(final_set)
  
          # we have a stored collection, without other filter, use llen
          elif self.stored_key and not self._lazy_collection['sets']\
                  and len(self._lazy_collection['intersects']) == 1:
  
              return conn.llen(final_set)
  
          # normal call
          return super(ExtendedCollectionManager, self)._collection_length(final_set)
docs: Return the length of the final collection, directly asking redis for the
          count without calling sort