A good heuristic to consider when designing composite types is whether any part of the type would lead to redundant storage. For example, a currency composite type might consist of an amount field and a type field.

In most cases, a single invoice, order, deal, etc. is unlikely to be generated using multiple currencies. Therefore, a single currency_type field in the invoice table would be sufficient.

If we used a composite currency type, the currency_type field would be highly redundant.