[bazoom42]

As soon as you have composite types, you will want to filter and join on the individual components, which would require extending the query language to allow drilling down into nested composite types. Basically you are back to hieracical databases. This is why Codd recommends expressing composite types as relations rather than complex values. You can express the same information, but can use relational algebra rather then special-case operators.

Nulls are a special case though - even if null was not allowed in base tables, you would still need them in outer joins.