Question

If I have a collection with thousands of elements, is there a way I can easily find which elements are taking up the most space (in terms of MB)?

Answer 1

There’s no built-in query for this, you have to iterate the collection, gather size for each document, and sort afterwards. Here’s how it’d work:

var cursor = db.coll.find(); 
var doc_size = {}; 
cursor.forEach(function (x) { 
    var size = Object.bsonsize(x); 
    doc_size[x._id] = size;
});

At this point you’ll have a hashmap with document ids as keys and their sizes as values.
Note that with this approach you will be fetching the entire collection over the wire. An alternative is to use MapReduce and do this server-side (inside mongo):

> function mapper() {emit(this._id, Object.bsonsize(this));}
> function reducer(obj, size_in_b) { return { id : obj, size : size_in_b}; }
>
> var results = db.coll.mapReduce(mapper, reducer, {out : {inline : 1 }}).results
> results.sort(function(r1, r2) { return r2.value - r1.value; })

inline:1 tells mongo not to create a temporary collection for results, everything will be kept in RAM.

And a sample output from one of my collections:

[
    {
        "_id" : ObjectId("4ce9339942a812be22560634"),
        "value" : 1156115
    },
    {
        "_id" : ObjectId("4ce9340442a812be24560634"),
        "value" : 913413
    },
    {
        "_id" : ObjectId("4ce9340642a812be26560634"),
        "value" : 866833
    },
    {
        "_id" : ObjectId("4ce9340842a812be28560634"),
        "value" : 483614
    },
       ...
    {
        "_id" : ObjectId("4ce9340742a812be27560634"),
        "value" : 61268
    }
]
>