Coalescing in Temporal

Coalescing is a unary operator applicable to temporal databases; it is similar to duplicate elimination in conventional databases. Tu-ples in a temporal relation that agree on the explicit attribute values and that have adjacent or overlapping time periods are candidates for coalescing. Uncoalesced relations can arise in many ways, e.g., via a projection or union operator, or by not enforcing coa-lescing on update or insertion. In this paper we show how semantically superruous coalesc-ing can be eliminated. We then turn to ee-ciently performing coalescing. We sketch a variety of iterative and non-iterative approaches, via SQL and embedded SQL, demonstrating that coalescing can be formulated in SQL-89. Detailed performance studies show that all such approaches are quite expensive. We propose a spectrum of coalescing algorithms within a DBMS, based on nested-loop, explicit partitioning, explicit sorting, temporal sorting, temporal partitioning, and combined explicit/temporal sorting, and summarize a performance study involving a subset of these algorithms. The study shows that coalesc-ing can be implemented with reasonable ef-ciency, and with modest development cost. Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the VLDB copyright notice and the title of the publication and its date appear, and notice is given that copying is by permission of the Very Large Data Base Endowment. To copy otherwise, or to republish, requires a fee and/or special permission from the Endowment.