Question

Saying I have compound _id on collection A:

{
    _id:{
       orgUnit:Int64
       ...
    }
}

Querying such shows that index is used:

db.A.find({_id:{orgUnit:1039}}).explain() 

…

    "indexBounds" : {
            "start" : {
                    "_id" : {
                            "orgUnit" : 1039
                    }
            },
            "end" : {
                    "_id" : {
                            "orgUnit" : 1039
                    }
            }

But when I changing query to “dot notation” shows that query became plain.

db.A.find({"_id.orgUnit":1039}).explain()
...
"indexBounds" : {

},

What is wrong with dot notation? And the main: How to leverage indexing to allow me find by "_id.orgUnit"

Answer 1

You would need to create a separate index on '_id.orgUnit' to efficiently query on that field using dot notation.

db.A.ensureIndex({'_id.orgUnit': 1})