distinct vs find - Is there any performance difference? - mongodb

Question

I have the following format of data in my collection,
{
"dName": "d1",
"city": "c1",
"state": "s1"
}, {
"dName": "d2",
"city": "c1",
"state": "s1"
}, {
"dName": "d2",
"city": "c1",
"state": "s2"
}
I have a compound index on all three fields combined.
dName is unique across documents. I want to get list of dNames, given city and state. I have found the following queries does the same,
db.collection.find({city: 'c1', state: 's1'}, {dName: 1, _id: 0}); -> returns [{dName: 'd1'}, {dName: 'd2'}]
db.collection.distinct('dName', {city: 'c1', state: 's1'}); -> returns ['d1', 'd2']
The first one returns an array of objects and seconds one returns an array of string. Other than that is there any performance improvement using one over another. I think distinct is costlier since it is trying to maintain the uniqueness of the response. Is that true?
Winning plans for both queries,
Of find query (1)
{
...
"winningPlan" : {
"stage" : "PROJECTION",
"transformBy" : {
"dName" : 1.0,
"_id" : 0.0
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"city" : 1,
"state" : 1,
"dName" : 1
},
"indexName" : "city_1_state_1_dName_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"city" : [],
"state" : [],
"dName" : []
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"city" : [
"[\"c1\", \"c1\"]"
],
"state" : [
"[\"s1\", \"s1\"]"
],
"dName" : [
"[MinKey, MaxKey]"
]
}
}
}
...
}
Of distinct query (2)
{
...
"winningPlan": {
"stage": "PROJECTION",
"transformBy": {"_id": 0, "dName": 1},
"inputStage": {
"stage": "DISTINCT_SCAN",
"keyPattern": {
"city": 1,
"state": 1,
"dName": 1
},
"indexName": "city_1_state_1_dName_1",
"isMultiKey": false,
"multiKeyPaths": {
"city": [],
"state": [],
"dName": []
},
"isUnique": false,
"isSparse": false,
"isPartial": false,
"indexVersion": 2,
"direction": "forward",
"indexBounds": {
"city": [
"[\"c1\", \"c1\"]"
],
"state": [
"[\"s1\", \"s1\"]"
],
"dName": [
"[MinKey, MaxKey]"
]
}
}
}
...
}