ON DATA REPRESENTATION AND USE IN A TEMPORAL RELATIONAL DBMS

Numerous proposals for extending the relational data model to incorpor ate the temporal dimension of data have appeared over the past-decade. It has long been known that these proposals have adopted one of two b asic approaches to the incorporation of time into the extended relatio nal model. Recent work formally contrasted the expressive power of the se two approaches, termed temporally ungrouped and temporally grouped, and demonstrated that the temporally grouped models are more expressi ve. In the temporally ungrouped models, the temporal dimension is adde d through the addition of some number of distinguished attributes to t he schema of each relation, and each tuple is ''stamped'' with tempora l values for these attributes. By contrast, in temporally grouped mode ls the temporal dimension is added to the types of values that serve a s the domain of each ordinary attribute, and the application's schema is left intact. The recent appearance of TSQL2, a temporal extension t o the SQL-92 standard based upon the temporally ungrouped paradigm, me ans that it is Likely that commercial DBMS's will be extended to suppo rt time in this weaker way. Thus the distinction between these two app roaches-and its impact on the day-to-day user of a DBMS-is of increasi ng relevance to the database practitioner and the database user commun ity. In this paper we address this issue from the practical perspectiv e of such a user. Through a series of example queries and updates, we illustrate the differences between these two approaches and demonstrat e that the temporally grouped approach more adequately captures the se mantics of historical data.