Hybrid relations for database schema evolution

The author describes hybrid relations in relational databases that allow existing relations to be altered by the addition of new attributes without reorganization of the database scheme. The values of new attributes with respect to an existing relation are stored separately from the relation as a set of triples of tuple identifier, attribute name, and value. At query time, a hybrid relation, which has only the attributes requested in a query, is derived virtually by combining the relation and this set of triples. A relation can be reorganized by upgrading its attribute values from these triples. The hybrid relation is defined as an algebraic expression, and equivalent expressions of a query on the hybrid relations are shown for efficient query processing.