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:'zhengyi',           passwd:'zhengyi',         friends:[]         }         dev:{             os:'iOS',             ver:'6.0',             hw:'iPhone2,1'         },         app:{             ver:'1.0',             pver:'1.0'         }     } 

The unnested one:

  {             username:'zhengyi',         passwd:'zhengyi',         friends:[],         dev:{             os:'iOS',             ver:'6.0',             hw:'iPhone2,1'         },         app:{             ver:'1.0',             pver:'1.0'         }     } 

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});