MongoDB sort winingplan overrides hint - mongodb

Question

I create a collection with three fields as described below. After that, I create an index over second field and executed a search using sort and hint operations.
Why - even using a hint over index created previously - MongoDB set sort as winningPlan?
I believe that if we filter data with some criteria and sort the result could be better, right?
Collection
> db.values.find()
{ "_id" : ObjectId("5763ffebe5a81f569b1005e5"), "field1" : "A", "field2" : "B", "field3" : "C" }
Indexes
> db.values.getIndexes()
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "peftest.values"
},
{
"v" : 1,
"key" : {
"field2" : 1
},
"name" : "field2_1",
"ns" : "peftest.values"
}
]
Query and Explain
> db.values.find({field2:"B"}).sort({field1:1}).hint({field2:1}).explain()
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "peftest.values",
"indexFilterSet" : false,
"parsedQuery" : {
"field2" : {
"$eq" : "B"
}
},
"winningPlan" : {
"stage" : "SORT",
"sortPattern" : {
"field1" : 1
},
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"field2" : 1
},
"indexName" : "field2_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"field2" : [
"[\"B\", \"B\"]"
]
}
}
}
}
},
"rejectedPlans" : [ ]
},
"serverInfo" : {
"host" : "apstrd14501d.intraservice.corp",
"port" : 27017,
"version" : "3.2.4",
"gitVersion" : "e2ee9ffcf9f5a94fad76802e28cc978718bb7a30"
},
"ok" : 1
}

Answer 1

I think the plan is what you expect but you look at it from the wrong perspective :)
The input stage of the sort is an index scan so the query plan uses the index at first and the pass the result data to the sort.