如何估計SQL查詢時間？

Question

我試圖讓下面的查詢可能需要多久時間採取粗糙（訂單數量級）估計：

mysql> EXPLAIN SELECT t1.col1, t1_col4 FROM t1 LEFT JOIN t2 ON t1.col1=t2.col1 WHERE col2=0 AND col3 IS NULL; 
+----+-------------+--------------------+------+---------------+------------+---------+-----------------------------+---------+--------------------------+ 
| id | select_type | table    | type | possible_keys | key  | key_len | ref       | rows | Extra     | 
+----+-------------+--------------------+------+---------------+------------+---------+-----------------------------+---------+--------------------------+ 
| 1 | SIMPLE  | t1     | ref | foobar  | foobar  | 4  | const      | 9715129 |       | 
| 1 | SIMPLE  | t2     | ref | col1   | col1  | 4  | db2.t1.col1     | 42318 | Using where; Using index | 
+----+-------------+--------------------+------+---------------+------------+---------+-----------------------------+---------+--------------------------+ 
2 rows in set (0.00 sec) 

mysql>