1. A method of controlling access by each of a plurality of contemporaneously active database transactions to a relation in a database, said method comprising the steps of storing a plurality of versions of said relation representing modifications of said relation resulting from previously executed database transactions and at most one of said plurality of active database transactions, storing in a relation dictionary a plurality of relation blocks each associated with one of said plurality of active database transactions and defining one of said plurality of relation versions and permitting access by each of said plurality of active database transactions only to the one of said plurality of relation versions by the relation block associated with that database transaction.