gistfile1.txt

mysql> explain select max(length(`key`)), max(length(value)) from notifications;
+----+-------------+---------------+------+---------------+------+---------+------+-----------+-------+
| id | select_type | table         | type | possible_keys | key  | key_len | ref  | rows      | Extra |
+----+-------------+---------------+------+---------------+------+---------+------+-----------+-------+
|  1 | SIMPLE      | notifications | ALL  | NULL          | NULL | NULL    | NULL | 262505557 |       |
+----+-------------+---------------+------+---------------+------+---------+------+-----------+-------+
1 row in set (0.00 sec)

mysql> explain select max(length(value)) from notifications;
+----+-------------+---------------+-------+---------------+---------------------+---------+------+-----------+-------------+
| id | select_type | table         | type  | possible_keys | key                 | key_len | ref  | rows      | Extra       |
+----+-------------+---------------+-------+---------------+---------------------+---------+------+-----------+-------------+
|  1 | SIMPLE      | notifications | index | NULL          | notifications_value | 303     | NULL | 262505557 | Using index |
+----+-------------+---------------+-------+---------------+---------------------+---------+------+-----------+-------------+
1 row in set (0.00 sec)

mysql> explain select max(length(`key`)) from notifications;
+----+-------------+---------------+-------+---------------+-------------------+---------+------+-----------+-------------+
| id | select_type | table         | type  | possible_keys | key               | key_len | ref  | rows      | Extra       |
+----+-------------+---------------+-------+---------------+-------------------+---------+------+-----------+-------------+
|  1 | SIMPLE      | notifications | index | NULL          | notifications_key | 303     | NULL | 262505557 | Using index |
+----+-------------+---------------+-------+---------------+-------------------+---------+------+-----------+-------------+