Original title: Aggregation explain returns "cursorType" : "QueryOptimizerCursor" instead of cursor name

If the query in $match involves only fields in the index then it shows the expected "cursorType" : "BtreeCursor action_id_1" but when another field is added to the query then it shows "cursorType" : "QueryOptimizerCursor" even if by performance it seems to be using the index (there are no other indexes in the collection other than one on _id and one on action_id).