合理使用MySQL索引建立高质量查询语句(17)

　　这时选择性已经很理想了，而这个索引的长度只有18，比短了接近一半，我们把这个前缀索引<first_name, last_name>建上：

ALTER TABLE employees.employees
ADD INDEX `first_name_last_name4` (first_name, last_name(4));

　　此时再执行一遍按名字查询，比较分析一下与建索引前的结果：

SHOW PROFILES;
+----------+------------+---------------------------------------------------------------------------------+
| Query_ID | Duration   | Query                                                                           |
+----------+------------+---------------------------------------------------------------------------------+
|       87 | 0.11941700 | SELECT * FROM employees.employees WHERE first_name='Eric' AND last_name='Anido' |
|       90 | 0.00092400 | SELECT * FROM employees.employees WHERE first_name='Eric' AND last_name='Anido' |
+----------+------------+---------------------------------------------------------------------------------+