103.数据库索引的优缺点以及什么时候数据库索引失效

参考http://www.cnblogs.com/mxmbk/articles/5226344.html

http://www.cnblogs.com/simplefrog/archive/2012/07/15/2592527.html

http://www.open-open.com/lib/view/open1418476492792.html

http://blog.csdn.net/colin_liu2009/article/details/7301089

http://www.cnblogs.com/hongfei/archive/2012/10/20/2732589.html