Multikey sparse index on empty array in MongoDB

Empty arrays are not treated the same as null in MongoDB. As you can see in the following code from the Mongo shell, a sparse index finds the empty array as an empty array as opposed to as null.

> c = db.docs
test.docs
> c.insert({a : []})
> c.ensureIndex({a : 1}, {sparse: true})
> c.find({a : []}).count()
1
> c.find({a : null}).count()
0

As with most interesting questions about MongoDB, using explain will provide a wealth of info. For example, you can see that the test actually uses the index and that the boundaries of the index are null and [], demonstrating their unique treatment.

> c.find({a : null}).explain()
{
    "cursor" : "BtreeCursor a_1",
    "isMultiKey" : false,
    "n" : 0,
    "nscannedObjects" : 0,
    "nscanned" : 0,
    "nscannedObjectsAllPlans" : 0,
    "nscannedAllPlans" : 0,
    "scanAndOrder" : false,
    "indexOnly" : false,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "millis" : 0,
    "indexBounds" : {
        "a" : [
            [
                null,
                null
            ]
        ]
    },
    "server" : "new-host.home:27017"
}
> c.find({a : []}).explain()
{
    "cursor" : "BtreeCursor a_1 multi",
    "isMultiKey" : false,
    "n" : 1,
    "nscannedObjects" : 1,
    "nscanned" : 1,
    "nscannedObjectsAllPlans" : 1,
    "nscannedAllPlans" : 1,
    "scanAndOrder" : false,
    "indexOnly" : false,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "millis" : 0,
    "indexBounds" : {
        "a" : [
            [
                null,
                null
            ],
            [
                [ ],
                [ ]
            ]
        ]
    },
    "server" : "new-host.home:27017"
}