1.索引，主键，唯一索引，联合索引的区别 http://blog.csdn.net/u011341352/article/details/47731255
2.创建‘联合索引’的意义？https://segmentfault.com/q/1010000000342176
3.聚集索引和非聚集索引http://www.cnblogs.com/julin-peng/p/4807843.html
