Is there any performance difference for nested document in mongodb query

Question

If I create an index for username field, which document is better for query a specific user?

The nested one:

 {
        account:{
        username:\'z         


        

Answer 1

It doesn't make a difference
You're either doing:

db.collection.findOne({"username":"zhengyi"});

or

db.collection.findOne({"account.username":"zhengyi"});

Read up on the dot notation for embedded documents

Similarly, indexes use the dot notation to reach inside a document:

db.collection.ensureIndex({"username": 1});

Or

db.collection.ensureIndex({"account.username": 1});