This was generated from the Google Groups discussion, "sorting, does not use defined index."
http://groups.google.com/group/mongodb-user/browse_thread/thread/9bc121708a0a5cde

To reproduce:
> db.example.drop();
> db.example.save({_id:1, "a":"aaa"});
> db.example.save({_id:2, "a":"bbb"});
> db.example.save({_id:3, "a":"ccc"});
> db.example.save({_id:4, "a":"ddd"});
> db.example.save({_id:5, "a":"eee"});
> db.example.save({_id:6, "a":"fff"});
> db.example.save({_id:7, "a":"ggg"});
> db.example.save({_id:8, "a":"hhh"});
> db.example.ensureIndex(

{a:1}

);

> db.example.find({$or:[

{a:"aaa"}

,

{a:"bbb"}

]}).explain(true)

The index is used, and "nscanned" : 2

Now sort the results:

> db.example.find({$or:[

{a:"aaa"}

,

{a:"bbb"}

]}).sort(

{a:1}

).explain(true)
The index is still used, but "nscanned" is now 8, indicating that Mongo is accessing every document in the collection.