mysql> EXPLAIN SELECT * FROM `user` WHERE SUBSTRING_INDEX(`name`,' ',1) = '山田';
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | user | ALL | NULL | NULL | NULL | NULL | 2 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> EXPLAIN SELECT * FROM `user` WHERE `name` = '山田';
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
| 1 | SIMPLE | user | ref | name | name | 194 | const | 1 | Using where |
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
1 row in set (0.00 sec)