Type consistency of queries in an object-oriented database system

Abstract

Queries in object-oriented databases can return non-homogeneous sets of objects when no type restrictions are placed on the inputs to the query. The tradition has been to force homogeneity on the result by restricting the types of the inputs. This restricts the range of permissible, and possibly useful, queries. We propose a type consistency theory for queries in object-oriented databases which supports the existence of multiple types in the query result. The technique is illustrated by developing type inference rules for an object algebra. The main result is that the loss of type information associated with a query operation is reduced in most cases. We also show how type information is increased when queries are qualified by conjunctive predicates.