mongodb $maxScan didn't equals limit - mongodb

Question

This is my first question on stack overflow, I am so happy and await your answers. My question is:
When I use MongoDB Query Selectors, I want limit results. But $maxScan is not work as I want.
---------This is What I want result.
db.post.find({query:{status:"publish"},$orderby:{date:-1}},{status:1,name:1,date:1,$slice:2}).limit(3)
{ "_id" : ObjectId("519262580cf21fb1647fb765"), "date" : ISODate("2013-05-14T16:12:08.600Z"), "status" : "publish", "name" : "关于多说" }
{ "_id" : ObjectId("519254ad0cf2f064f6ecef82"), "date" : ISODate("2013-05-14T15:13:49.017Z"), "status" : "publish", "name" : "回顾<蜗居>的100句经典台词" }
{ "_id" : ObjectId("519254690cf2f064f6ecef81"), "date" : ISODate("2013-05-14T15:12:41.462Z"), "status" : "publish", "name" : "女人脱光了是什么" }
-----------This is the results I use $maxScan
db.post.find({query:{status:"publish"},$maxScan:3,$orderby:{date:-1}},{status:1,name:1,date:1})
{ "_id" : ObjectId("518e6c690cf21a363df2956e"), "date" : ISODate("2013-05-11T16:06:01.341Z"), "status" : "publish", "name" : "淘宝新店,充值任务" }
I find may be the $maxScan didn't like limit(). it first limit the collection data and then execute the query! but this is not I want. Is anything i wrong? please help.Thanks
--------------All results
db.post.find({query:{},$orderby:{date:-1}},{status:1,name:1,date:1})
{ "_id" : ObjectId("519262580cf21fb1647fb765"), "date" : ISODate("2013-05-14T16:12:08.600Z"), "status" : "publish", "name" : "关于多说" }
{ "_id" : ObjectId("519254ad0cf2f064f6ecef82"), "date" : ISODate("2013-05-14T15:13:49.017Z"), "status" : "publish", "name" : "回顾<蜗居>的100句经典台词" }
{ "_id" : ObjectId("519254690cf2f064f6ecef81"), "date" : ISODate("2013-05-14T15:12:41.462Z"), "status" : "publish", "name" : "女人脱光了是什么" }
{ "_id" : ObjectId("518ee61a0cf22bd326d60215"), "date" : ISODate("2013-05-12T00:45:14.295Z"), "status" : "publish", "name" : "JSTL日期格式化用法(转载)" }
{ "_id" : ObjectId("518e6c690cf21a363df2956e"), "date" : ISODate("2013-05-11T16:06:01.341Z"), "status" : "publish", "name" : "淘宝新店,充值任务" }
{ "_id" : ObjectId("518e21c90cf21a363df2956d"), "date" : ISODate("2013-05-11T10:47:37.803Z"), "status" : "draft", "name" : "一夜没睡" }
{ "_id" : ObjectId("518df75d0cf21a363df2956c"), "date" : ISODate("2013-05-11T07:46:37.726Z"), "status" : "draft", "name" : "飞娥入侵" }
{ "_id" : ObjectId("518d80630cf21a363df2956b"), "date" : ISODate("2013-05-10T23:18:59.323Z"), "status" : "publish", "name" : "Java的日期格式化常用方法" }

Answer 1

To return only the top results, you should use limit(), which will limit the amount of results returned from the cursor. This is commonly used with skip() to paginate the results.
It's not explained very clearly in the docs, but $maxScan as the name suggests limits the number of documents the query will examine. Presumably your query is examining some documents which don't meet the criteria (with status != publish) and then discarding them.
Do you have an index on status? It's possible that could help the query return the results you want while scanning fewer documents, but I still think limit() is what you want.