Value propagation in object-oriented database part hierarchies

Derived schema components are an important aspect of traditional semantic data modeling. In this paper, we address the issue of defining such schema constructs in the context of object-oriented database (OODB) part hierarchies. In particular, we present the concept of derived attribute defined with respect to value propagation across a part relationship between two object classes. Three different types of value propagation, namely, invariant, transformational, and cumulative, allow for a high degree of expressiveness in the definition of such derived attributes. We also present the notion of a generalized derived attribute, which may be defined in terms of simultaneous value propagations across many part relationships. The ambiguity problem of multiple value propagation in part hierarchies is solved by this latter construct, an analogue of which is not applicable in ordinary OODB IS-A hierarchies. It allows for the representation of such common expressions as the weight of the whole is the sum of the weights of the parts. To complement the formal definitions, we present a graphical schema notation for the value propagation mechanisms and the accompanying derived attributes. Such notation provides a convenient means for the specification and communication of OODB part schemata.