remove repeat contain index from the relevant candidate indexes, this can avoid some useless calculations.
for example:
   {a:1, b:1} contain {a:1}, so we can remove index {a:1} from the candidates
   {a:"hashed", b:1} contain {a:"hashed"}, so we can remove index {a:"hahsed"} from the candidates

 {a:1, b:1} is better than {a:1},because both cases are satisfied for db.collection.find({a:xx}) 
    and db.collection.find({a:xx,b:xx})