Processing Queries Over Generalization Hierarchies in a Multidatabase System

Abstract

An important task of multidatabase systems is the integration of existing databases. Data-base Integration is achieved primarily through the use of generalization. Hence, it is impor-tant to develop good tactics for processing queries over generalization hierarchies. This paper defines the class of conjunctive generali-zation 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 convon-tional 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 De-