https://blog.csdn.net/a519640026/article/details/106940115?spm=1001.2101.3001.6650.1&utm_medium=distribute.wap_relevant.none-task-blog-2%7Edefault%7ECTRLIST%7Edefault-1-106940115-blog-90668696.wap_blog_relevant_default&depth_1-utm_source=distribute.wap_relevant.none-task-blog-2%7Edefault%7ECTRLIST%7Edefault-1-106940115-blog-90668696.wap_blog_relevant_default http://blog.codinglabs.org/articles/theory-of-mysql-index.html