If I understand correctly, on a clustered collection the _id field is the same as the record ID (RID). Does this mean that any secondary index also includes the _id value? For example, an index on {a: 1} is really a set of (a, _id) entries, almost like an index on {a: 1, _id: 1}.

Could we use an {a: 1} index to satisfy queries like {$sort: {a: 1, _id: 1}}?