一、
1.
1). Indexes reduce the amount of data the server has to examine.
2). Indexes help the server avoid sorting and temporary tables.
3). Indexes turn random I/O into sequential I/O.
Lahdenmaki and Leach’s book also introduces a three-star system for grading how suitable an index is for a query. The index earns one star if it places relevant rows adjacent to each other, a second star if its rows are sorted in the order the query needs,and a final star if it contains all the columns needed for the query.
高性能MySQL笔记-第5章Indexing for High Performance-003索引的作用
原文：http://www.cnblogs.com/shamgod/p/5381882.html