Mysql Optimize Query: Trying to Get Average of Subquery

I have the following query:

SELECT AVG(time) FROM 
(SELECT UNIX_TIMESTAMP(max(datelast)) - UNIX_TIMESTAMP(min(datestart)) AS time
    FROM table
    WHERE id IN 
        (SELECT DISTINCT id
            FROM table
            WHERE product_id = 12394 AND datelast > '2011-04-13 00:26:59'
        )
GROUP BY id
)
as T

The query gets the greatest datelast value and subtracts it from the greatest datestart value for every ID (which is the length of a user session), and th开发者_开发知识库en averages it.

The outer most query is there only to average the resulting times. Is there any way to optimize this query?

Output from EXPLAIN:

id  select_type         table       type            possible_keys           key     key_len ref     rows    extra
1   PRIMARY             <derived2>  ALL             NULL                    NULL    NULL    NULL    7   
2   DERIVED             table       index           NULL                    id      16      NULL    26      Using where
3   DEPENDENT SUBQUERY  table       index_subquery  id,product_id,datelast  id      12      func    2       Using index; Using where

Is the first SELECT really necessary ?

SELECT
  AVG(time)
FROM 
(
  SELECT
    UNIX_TIMESTAMP(max(datelast)) - UNIX_TIMESTAMP(min(datestart)) AS time
  FROM
    table
  WHERE
    product_id = 12394 AND datelast > '2011-04-13 00:26:59'
  GROUP BY
    id
)

I can't test now and I think it would work too. Otherwise, your query looks good.

You can optimize the query by adding a (datelast, product_id) key (always put the most restrictive field first, to increase selectivity).