MongoDB convering indexes

gistfile1.js

$ mongo
MongoDB shell version: 2.0.4
connecting to: test
> doc = {_id: 42, name: "propedeutica", power: 0.92}
{ "_id" : 42, "name" : "propedeutica", "power" : 0.92 }
> db.foobar.insert(doc)
> db.foobar.ensureIndex({name: 1})
> db.foobar.find({name: "propedeutica"}).explain()
{
        "cursor" : "BtreeCursor name_1",
        "nscanned" : 1,
        "nscannedObjects" : 1,
        "n" : 1,
        "millis" : 0,
        "nYields" : 0,
        "nChunkSkips" : 0,
        "isMultiKey" : false,
        "indexOnly" : false,
        "indexBounds" : {
                "name" : [
                        [
                                "propedeutica",
                                "propedeutica"
                        ]
                ]
        }
}
> db.foobar.find({name: "propedeutica"}, {_id: 0}).explain()
{
        "cursor" : "BtreeCursor name_1",
        "nscanned" : 1,
        "nscannedObjects" : 1,
        "n" : 1,
        "millis" : 0,
        "nYields" : 0,
        "nChunkSkips" : 0,
        "isMultiKey" : false,
        "indexOnly" : false,
        "indexBounds" : {
                "name" : [
                        [
                                "propedeutica",
                                "propedeutica"
                        ]
                ]
        }
}
> db.foobar.find({name: "propedeutica"}, {_id: 0})
{ "name" : "propedeutica", "power" : 0.92 }
> db.foobar.find({name: "propedeutica"}, {_id: 0, power: 0})
{ "name" : "propedeutica" }
// This is not indexOnly because MongoDB cannot determine that 'name' is the only field in the results
> db.foobar.find({name: "propedeutica"}, {_id: 0, power: 0}).explain()
{
        "cursor" : "BtreeCursor name_1",
        "nscanned" : 1,
        "nscannedObjects" : 1,
        "n" : 1,
        "millis" : 0,
        "nYields" : 0,
        "nChunkSkips" : 0,
        "isMultiKey" : false,
        "indexOnly" : false,
        "indexBounds" : {
                "name" : [
                        [
                                "propedeutica",
                                "propedeutica"
                        ]
                ]
        }
}
// Now note that this query is *indexOnly*, since you only ask for the 'name' field
> db.foobar.find({name: "propedeutica"}, {_id: 0, name: 1}).explain()
{
        "cursor" : "BtreeCursor name_1",
        "nscanned" : 1,
        "nscannedObjects" : 1,
        "n" : 1,
        "millis" : 0,
        "nYields" : 0,
        "nChunkSkips" : 0,
        "isMultiKey" : false,
        "indexOnly" : true,
        "indexBounds" : {
                "name" : [
                        [
                                "propedeutica",
                                "propedeutica"
                        ]
                ]
        }
}