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' } } ```

Accepted Answer

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});
```

Is there any performance difference for nested document in mongodb query

Solution

Problem