MongoDB 3.0 explain() result undocumented filter field

Question

I run explain on the following query:

db.explain().find({ site_id:1, dimensions:[], create_date: { $gte: new Date(1452603948196) } )

The result contains a 'filter' object over the dimensions field, while it should have filtered that field using the index, what does it mean ? isn't it a redundant stage?

{ "winningPlan" : {
        "stage" : "FETCH",
        "filter" : {
            "dimensions" : {
                "$eq" : [ ]
            }
        },
        "inputStage" : {
            "stage" : "IXSCAN",
            "keyPattern" : {
                "site_id" : 1,
                "dimensions" : 1,
                "create_date" : 1
            }, }

as far as i understand it means that mongo filter the dimensions field again after it is scanning the index and fetch the documents into memory, is it correct?

thanks,

Answer 1

Your results will be filtered based on the criteria that dimensions are equal to values given in the array.