索引|一文带你你搞懂索引如何优化( 五 ) |优化|sql|

这时选择性已经很理想了，而这个索引的长度只有18 ，比&ltfirst_name, last_name&gt短了接近一半，我们把这个前缀索引建上：

ALTER TABLEemployees.employeesADD 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" |+----------+------------+---------------------------------------------------------------------------------+