Semantics of Time-Varying Information

This paper provides a systematic and comprehensive study of the underlying semantics of temporal databases, summarizing selected results of an intensive collaboration between the two authors over the five-years period from 1991 to 1995. We first examine how facts may be associated with time, most prominently with one or more dimensions of valid time and transaction time. One common case is that of a bitemporal relation, in which facts are associated with timestamps from exactly one valid-time and one transaction-time dimension. These two times may be related in various ways, yielding temporal specialization. Multiple transaction times arise when a fact is stored in one database, then later replicated or transferred to another database. By retaining the transaction times, termed temporal generalization, the original relation can be effectively queried by referencing only the final relation. We attempt to capture the essence of time-varying information via a very simple data model, the bitemporal conceptual data model. Emphasis is placed on the notion of snapshot equivalence of the information content of relations of different data models.