https://dev.mysql.com/doc/refman/5.6/en/index-btree-hash.html
译文：http://itindex.net/detail/54241-tree-索引-hash
https://stackoverflow.com/questions/1687910/advantage-of-btree
http://blog.sina.com.cn/s/blog_6fd335bb0100v1lm.html
http://www.cnblogs.com/vicenteforever/articles/1789613.html