Being able to model a use case with tuples and relations does not mean the database can meet the performance requirements of that use case. If it can't meet the performance requirements, then the use case is unsupported. It's the same way how no single data structure or combination of data structures can support all regular programming use cases. Sometimes you need a map, sometimes you need a list, sometimes you need a set, sometimes you need a combination, and sometimes you need something completely different.