It's not "underusing" an RDBMs, those are just the lions share of queries in many applications. Also note that a join say from some parent item by ID to childre

It's not "underusing" an RDBMs, those are just the lions share of queries in many applications. Also note that a join say from some parent item by ID to children who have a parent_id makes essentially two lookups by ID in two indices, so the same reasoning applies; this is not just SELECT * FROM bla WHERE id = :x.