Why indexOnly attribute is false for this covered query - mongodb

Question

I have a test db with fields _id, name, age, date
Indexes:
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "blogger.users"
},
{
"v" : 1,
"key" : {
"name" : 1,
"age" : 1
},
"name" : "name_1_age_1",
"ns" : "blogger.users"
},
{
"v" : 1,
"key" : {
"age" : 1,
"name" : 1
},
"name" : "age_1_name_1",
"ns" : "blogger.users"
}
]
When running the following query:
> db.users.find({"name":"user10"},{"_id":0,"date":0})
.explain()
I get following:
{
"cursor" : "BtreeCursor name_1_age_1",
"isMultiKey" : false,
"n" : 1,
"nscannedObjects" : 1,
"nscanned" : 1,
"nscannedObjectsAllPlans" : 2,
"nscannedAllPlans" : 2,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"name" : [
[
"user10",
"user10"
]
],
"age" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
]
},
"server" : "Johny-PC:27017",
"filterSet" : false
}
Without explain the result is:
{ "name" : "user10", "age" : 68 }
Even though this is a covered query with proper projections, the indexOnly field is still false. I have also tried explicitly providing index using hint, but no change. In that case values of nscannedObjectsAllPlans and nscannedAllPlans are 1 as the query doesnt try other indexes.

Answer 1

For a query to be "indexOnly" or "covered" the only fields returned must be contained in the index. So even though you have an index for "name_1_age_1", the query engine still expects to be "told" that the only fields you want are those in the index. It does not know this about the document until you inspect it:
db.users.find({"name":"user10"},{"_id":0, "name": 1, "age": 1 }).explain()
That will return "indexOnly" as the query engine knows that the selected index contains all of the fields that are required for output. As such there is no need to go back through the collection in case there are other fields to return.