https://www.cnblogs.com/dreamworlds/p/5398535.html
http://blog.codinglabs.org/articles/theory-of-mysql-index.html
https://www.cnblogs.com/dreamworlds/p/5398535.html
http://blog.codinglabs.org/articles/theory-of-mysql-index.html
原文：https://www.cnblogs.com/hongdada/p/8280211.html