文章参考：
LINUX上MYSQL优化三板斧
http://blog.sae.sina.com.cn/archives/2482
MySQL 索引原理及慢查询优化
https://blog.csdn.net/zhanghongzheng3213/article/details/51722506
B+树索引和哈希索引的区别
http://imysql.com/2016/01/06/mysql-faq-different-between-btree-and-hash-index.shtml
10分钟让你明白MySQL是如何利用索引的
http://www.fordba.com/spend-10-min-to-understand-how-mysql-use-index.html
MySQL索引背后的数据结构及算法原理
http://blog.jobbole.com/24006/
EXPLAIN执行计划中要重点关注哪些要素
http://imysql.com/2015/11/04/mysql-faq-some-important-in-explain.shtml
解读EXPLAIN执行计划中的key_len
http://imysql.com/2015/10/20/mysql-faq-key-len-in-explain.shtml
EXPLAIN 命令详解
http://www.cnblogs.com/gomysql/p/3720123.html
Performance Tuning MySQL
http://www.cnblogs.com/gomysql/p/3833526.html