Tempo médio de operações armazenadas no banco de dados

SELECT AVG(TIME_TO_SEC(y.creation_time)-TIME_TO_SEC(x.creation_time)) avg_diff
  FROM my_table x 
  JOIN my_table y 
    ON y.operation_id = x.operation_id 
   AND y.operation_type = 'end' 
 WHERE x.operation_type = 'start';