limit语句放在group之前，会被当做读取的文档的数量。放在group之后，会被当做返回的结果集的指定数量。

> db.click_20170503.aggregate([{$match:{n_id:25}},{$limit:5},{$sort:{total:-1}},{$group : {_id : {"n_id":"$n_id","s1":"$s1"}, total : {$sum : 1}}}])

{ "_id" : { "n_id" : 25, "s1" : "tOuMezjO2gN4F5mgkJ_50772" }, "total" : 3 }

{ "_id" : { "n_id" : 25, "s1" : "nIMwUjDzw3yYkNmHJg_5605" }, "total" : 2 }

> db.click_20170503.aggregate([{$match:{n_id:25}},{$group : {_id : {"n_id":"$n_id","s1":"$s1"}, total : {$sum : 1}}},{$limit:5},{$sort:{total:-1}}])

{ "_id" : { "n_id" : 25, "s1" : "nIMwUjDzw3yYkNmHJg_5984" }, "total" : 94 }

{ "_id" : { "n_id" : 25, "s1" : "wmNynMMOXO3tdIjtYDj3iu_5844" }, "total" : 27 }

{ "_id" : { "n_id" : 25, "s1" : "12ntjUIduNWJHo5emnn_" }, "total" : 6 }

{ "_id" : { "n_id" : 25, "s1" : "wmNynMMOXO3tdIjtYDj3iu_5847" }, "total" : 4 }

{ "_id" : { "n_id" : 25, "s1" : "12ntjUIduNWJHo5emnn_1668dff40369b608b7f776eb719e230a" }, "total" : 3 }

> db.click_20170503.aggregate([{$match:{n_id:25}},{$group : {_id : {"n_id":"$n_id","s1":"$s1"}, total : {$sum : 1}}},{$limit:10},{$sort:{total:-1}}])

{ "_id" : { "n_id" : 25, "s1" : "tOuMezjO2gN4F5mgkJ_46782" }, "total" : 2981 }

{ "_id" : { "n_id" : 25, "s1" : "2HKynZdvFhMdjMfZW2_5588" }, "total" : 235 }

{ "_id" : { "n_id" : 25, "s1" : "nIMwUjDzw3yYkNmHJg_5984" }, "total" : 94 }

{ "_id" : { "n_id" : 25, "s1" : "wmNynMMOXO3tdIjtYDj3iu_5844" }, "total" : 27 }

{ "_id" : { "n_id" : 25, "s1" : "nIMwUjDzw3yYkNmHJg_3988" }, "total" : 7 }

{ "_id" : { "n_id" : 25, "s1" : "12ntjUIduNWJHo5emnn_" }, "total" : 6 }

{ "_id" : { "n_id" : 25, "s1" : "2HKynZdvFhMdjMfZW2_6140" }, "total" : 6 }

{ "_id" : { "n_id" : 25, "s1" : "wmNynMMOXO3tdIjtYDj3iu_5847" }, "total" : 4 }

{ "_id" : { "n_id" : 25, "s1" : "12ntjUIduNWJHo5emnn_1668dff40369b608b7f776eb719e230a" }, "total" : 3 }

{ "_id" : { "n_id" : 25, "s1" : "2HKynZdvFhMdjMfZW2_7307" }, "total" : 2 }