很多人问为什么使用联合索引，为什么不建两个单独的索引呢？

So why not just create two indexes, one on last_name and one on first_name? You could do that, but MySQL won’t use them both at the same time. In fact, MySQL will only ever use one index per table per query—except for UNIONs.[3] This fact is important enough to say again: MySQL will only ever use one index per table per query.

那是因为，mysql通常一次查询只能使用一个索引，这是由B+树的结构决定的。而不是我们自以为的建两条单列的索引就能起到叠加的效果。MySQL will only ever use one index per table per query！！！！！