I have two questions, I'll give you the facts, then my questions.

The actual index I'm using for one of my queries has the following characteristics :

• Multikey (contains 4 keys).
• The average key length is 14 characters.
• 2 indexes values contains 2 letters (country code and state code).
• 1 index values contain numerical values (between 0 and 100).
• The index size is : ~395 MB.

Here's an example of what should be indexed (I don't know how mongodb actually is storing it's indexes, so I'm going to represent it like if it was a collection document):

{
    "geolocation.statecode": "AL",
    "personnu_field": 50,
    "geolocation.countrycode": "US"
    "field1.sufield": "Awesome value"
}

Informations about the collection concerned by the index :

• The collection is a 6M documents.
• Fast growing.
• It's actually a collection of Twitter users with some additional business related fields.
• field1 (see the example given above) is an array of subdocuments.
• avgObjSize: 3 KB.
• totalIndexSize: ~2.9 GB.
• storageSize: ~19 GB.

I've done an explain to this long query :

db.crawler_users.find(
    {
        "geolocation.statecode": "AL",
        personnu_field: { "$lte": 65, "$gte": 30 },
        "geolocation.countrycode": "US"
    },
    {
        personnu_field:1, _id:0
    }
).hint(
    {
        "geolocation.countrycode" : 1,
        "personnu_field" : -1,
        "geolocation.statecode" : 1,
        "field1.sufield" : 1
    }
).explain()

And here's the result:

{
    "cursor" : "BtreeCursor geolocation.countrycode_1_personnu_field_-1_geolocation.statecode_1_field1.sufield_1",
    "isMultiKey" : true,
    "n" : 216,
    "nscannedObjects" : 788609,
    "nscanned" : 788609,
    "nscannedObjectsAllPlans" : 788609,
    "nscannedAllPlans" : 788609,
    "scanAndOrder" : false,
    "indexOnly" : false,
    "nYields" : 128,
    "nChunkSkips" : 0,
    "millis" : 127451,
    "indexBounds" : {
        "geolocation.countrycode" : [
            [
                "US",
                "US"
            ]
        ],
        "personnu_field" : [
            [
                65,
                -1.7976931348623157e+308
            ]
        ],
        "geolocation.statecode" : [
            [
                    {
                    "$minElement" : 1
                },
                    {
                    "$maxElement" : 1
                }
            ]
        ],
        "search.keyword" : [
            [
                    {
                    "$minElement" : 1
                },
                    {
                    "$maxElement" : 1
                }
            ]
        ]
    },
}

As you can see the query takes much time to be executed (>2 minutes). And it's hitting the collection. even if the chosen fields already exists on the index.

I have two questions somehow related :

1. Why is indexOnly: false. Isn't it supposed to be a covered index query? (see the explain later)
2. I need to retrieve some additional fields from the collection (the id and the profile_picture url). Should I add them to the index to avoid hitting the collection, even if I'll never have to query them?