Mysql索引合并
例如有如下表emps，表结构如下：
mysql> desc emps;
+------------+---------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+---------------+------+-----+---------+----------------+
| id | int | NO | PRI | NULL | auto_increment |
| emp_no | int | NO | MUL | NULL | |
| birth_date | date | NO | | NULL | |
| first_name | varchar(14) | NO | | NULL | |
| last_name | varchar(16) | NO | | NULL | |
| gender | enum('m','f') | NO | MUL | NULL | |
| hire_date | date | NO | | NULL | |
+------------+---------------+------+-----+---------+----------------+
7 rows in set (0.00 sec)
