查看mongo索引命中情况

http://database.51cto.com/art/201107/277116_all.htm
http://blog.csdn.net/yyywyr/article/details/26816361
http://www.cnblogs.com/stephen-liu74/archive/2012/08/01/2561557.html

stage 是COLLSCAN 说明没有走索引，走索引的话会显示 IXSCAN
http://blog.csdn.net/aca_jingru/article/details/45312079

http://www.ttlsa.com/mongodb/mongodb-index-explain-profile-4/