Does default _id_ index use by mongoquery?

Question

Let's assume, we have mongo collection mentioned as below

db.test.findOne() { "_id" : ObjectId("52d0313dc62b629cfabe22ff"), "a" : 1, "b" : 1, "c" : 1 }

and this collection has more than million records.

now, if i want to find out records with condition as field "a" greater then 1000 and "b" field less than 9000 then my mongo query could be like ..

db.test.find({'a':{'$gt':1000}, 'b':{'$lt': 9000}}, {'a':1, 'c':1}) and to make above search faster, i have already applied compound index on "a" & "b" field. and it been used by above query ..

now, here as you must aware, all the collection come with default index on "_id_" field for primary key.

does my above mongo query used default index "_id_" ? if yes then how ? and if not then why ?

thanks you javaamtho

Answer 1

No, mongo does not use the default index on _id with your query, because _id does not occur in the condition nor in the sorting of your query.