Hi !
We have a collection which is roughly based on a schema similar to :

{
  _id : GUID
  StartTime: DateTime
  // Other Information
  Counters: [  
    {
         Name : string,
        // Others informations...
    }
   ]
}

And I have that index on the collection :

In our application, we want to use the Counters.Name as a filter (basically display it in a dropdown list). However, using :

db.sessions.explain().distinct('Counters.Name')

The winning plan is a full COLLSCAN.