The authors describe a time-based approach to distributed concurrency control and recovery that alleviates the high cost of optimistic methods by combining the solutions to concurrency control, recovery management, and localized control into a single flexible yet powerful and efficient mechanism. The approach adapts the object-oriented Timewarp mechanism to handle competing processes rather than the cooperating processes for which it was originally intended. The result is a completely decentralized, nonblocking concurrency and recovery protocol that supports more general features of other desirable aspects of distributed applications, such as versioning and active objects.>

We present an analytical formula for estimating the cardinality of the projection on certain attributes of a subset of a relation in a relational database. This formula takes into account a priori knowledge of the semantics of the real-world objects and relationships that the database is intended to represent. Experimental testing of the formula shows that it has an acceptably low percentage error, and that its worst-case error is smaller than the best-known formula. Furthermore, the formula presented here has the advantage that it does not require a scan of the relation.

The authors focus on one class of schema revisions necessitated by a very basic phenomenon: a given individual object evolves into a family of objects which are similar to it in many ways. This is commonly called the version problem. In theoretical terms, one can handle the above schema change in the standard, object-oriented database models by the interposition of suitable abstractions into the existing type lattice. There are practical and engineering difficulties with such schema changes. The authors propose an incremental mechanism called instance inheritance which is well suited to handling the schema changes without the attendant practical costs. The authors formally characterize this augmentation to the standard database models, and show examples of its applications.>