The whole point of having an index is to speed up search queries by essentially cutting down the number of records/rows in a table that need to be examined.
an index is a data structure (most commonly a B- tree) that stores the values for a specific column in a table. An index is created on a column of atable. So, the key points to remember are that an index consists of column values from one table, and that those values are stored in a data structure. The index is a data structure
原文：http://www.cnblogs.com/yingzhongwen/p/3537242.html