Incremental maintenance of materialized object-oriented views in MultiView: strategies and performance evaluation

View materialization is a promising technique for achieving the data sharing and virtual restructuring capabilities needed by advanced applications such as data warehousing and workflow management systems. Much existing work addresses the problem of how to maintain the consistency of materialized relational views under update operations. However, little progress has been made thus far regarding the topic of view materialization in object-oriented databases (OODBs). In this paper, we demonstrate that there are several significant differences between the relational and object-oriented paradigms that can be exploited when addressing the object-oriented view materialization problem. First, we propose techniques that prune update propagation by exploiting knowledge of the subsumption relationships between classes to identify branches of classes to which we do not need to propagate updates and by using derivation ordering to eliminate self-cancelling propagation. Second, we use encapsulated interfaces, combined with the fact that any unique database property is inherited from a single location, to provide a "registration service" by which virtual classes can register their interest in specific properties and be notified upon modification of those properties. Third, we introduce the notion of hierarchical registrations that further optimizes update propagation by organizing the registration structures according to the class generalization hierarchy, thereby pruning the set of classes that are notified of updates. We have successfully implemented all proposed techniques in the MultiView system on top of the GemStone OODBMS. To the best of our knowledge, MultiView is the first OODB view system to provide updatable materialized virtual classes and virtual schemata. In this paper, we also present a cost model for our update algorithms, and we report results from the experimental studies we have run on the MultiView system, measuring the impact of various optimization strategies incorporated into our materialization update algorithms.