次のクエリにかかる時間の大まかな(桁違いの)見積もりを取得しようとしています。
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>