参考:
https://www.jianshu.com/p/f635a6a94254
https://zhuanlan.zhihu.com/p/37193700
https://juejin.im/post/5c99f0556fb9a070e82c1fcf
https://juejin.im/post/5bbbf7615188255c59672125
https://juejin.im/post/5daac93051882508652e5076

https://www.kancloud.cn/kancloud/theory-of-mysql-index/41844
https://zhuanlan.zhihu.com/p/24309634
https://blog.csdn.net/yin767833376/article/details/81511377
https://www.cnblogs.com/dongguacai/p/7239599.html