MongoDB index use on find all without hint

Question

I've got a question on how to write an index properly to avoid resorting to a hint.

Sample "Test" Collection Schema

{
   _id: ObjectId(<whatever>),
   a: <whatever>,
   b: <whatever>,
   c: <whatever>,
   d: <whatever>,
   e: {
         f: <whatever>,
         g: <whatever>
      }
}

Index on "Test"

db.test.ensureIndex( { "a": NumberInt(1), "c": NumberInt(1), "_id": NumberInt(1), "d": NumberInt(1) }, 
                     { name: "a_1_c_1__id_1_d_1", background: true } );

Query without hint and query with hint...

> db.test.find({},{d:1}).explain();
{
    "cursor" : "BasicCursor",
    "isMultiKey" : false,
    "n" : 752,
    "nscannedObjects" : 752,
    "nscanned" : 752,
    "nscannedObjectsAllPlans" : 752,
    "nscannedAllPlans" : 752,
    "scanAndOrder" : false,
    "indexOnly" : false,
    "nYields" : 4,
    "nChunkSkips" : 0,
    "millis" : 5,
    "indexBounds" : {

    },
    "server" : <whatever>
}

> db.test.find({},{d:1}).hint("a_1_c_1__id_1_d_1").explain();
{
    "cursor" : "BtreeCursor a_1_c_1__id_1_d_1",
    "isMultiKey" : false,
    "n" : 752,
    "nscannedObjects" : 752,
    "nscanned" : 752,
    "nscannedObjectsAllPlans" : 752,
    "nscannedAllPlans" : 752,
    "scanAndOrder" : false,
    "indexOnly" : true,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "millis" : 0,
    "indexBounds" : {
        "a" : [
            [
                {
                    "$minElement" : 1
                },
                {
                    "$maxElement" : 1
                }
            ]
        ],
        "c" : [
            [
                {
                    "$minElement" : 1
                },
                {
                    "$maxElement" : 1
                }
            ]
        ],
        "_id" : [
            [
                {
                    "$minElement" : 1
                },
                {
                    "$maxElement" : 1
                }
            ]
        ],
        "d" : [
            [
                {
                    "$minElement" : 1
                },
                {
                    "$maxElement" : 1
                }
            ]
        ]
    },
    "server" : <whatever>
}

I'd (obviously) like the query to use the covered index but I don't know how to get there without using the hint. Is it possible? I'd prefer to manipulate the index vs. changing the query but changing the query is an option, if need be.

Answer 1

Turns out this is a known issue. Apologies for the post.

https://jira.mongodb.org/browse/SERVER-2109