> db.foo.insert({a:"hello world",b:42})
Insert WriteResult({ "ok" : 1, "n" : 1 })
> db.foo.ensureIndex({a:"text",b:1})
> db.foo.find({$text:{$search:"hello"},b:42},{_id:0,b:42}).explain().nscannedObjects
1 // should be 0, since this query can be covered