If you were to use MongoDB, wouldn't this just be a case of adding an index to the field of the nested child document (i.e. the note within a notebook) you were

If you were to use MongoDB, wouldn't this just be a case of adding an index to the field of the nested child document (i.e. the note within a notebook) you were interested in and then querying on it? e.g. db.notebooks.ensureIndex({"notes.date": 1});