Processing Queries Over Generalization Hierarchies in a Multidatabase System

An important task of multidatabase systems is the integration of existing databases. Database Integration is achieved primarily through the use of generalization. Hence, it is important to develop good tactics for processing queries over generalization hierarchies. This paper defines the class of conjunctive generalization queries, and it describes four tactics for processing those queries that have boon developed for the MDLTIDASE system. Since query processing tactics are best describe algebraically, the paper shows how to model generalization as a sequence of algebraic operations. Three of the tactics described here are adapted from convontional distributed query processing techniqaes. However, it is argued that these tactics are of limited applicability to processing queries over generalization hierarchies. A fourth tactic, semioutorjoin, which is more widely applicable. is introduced. This research was jointly supported by the Defense Advanced Research Projects Agency of the Department of Defense and the Naval Electronic Systems Command and was monitored by the Naval Electronic Systems Command under Contract No. NOOO39-82-C-0226. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, oithor expressed or implied, of the DARPA, NAVPLEX, or the U.S. Government.