A TEMPORAL DATABASE MODEL SUPPORTING RELATIVE AND ABSOLUTE-TIME

This paper presents a temporal database model which allows the express ion of relative temporal knowledge of data transaction and data validi ty times. The system is founded on an extension to Alien's axiomitizat ion of time, given previously by the authors, which takes both interva ls and points as primitive time elements. A general retrieval mechanis m is presented for a database with a purely relative temporal knowledg e which allows queries with temporal constraints in terms of any logic al combination of Allen's temporal predicates. When absolute temporal duration knowledge is added, the consistency checking algorithm upon w hich the inference mechanism is based reduces to a linear programming problem. A class of databases, termed time-limited databases, is intro duced as a practical solution to the problem of computational complexi ty of retrieval. This class allows absolute and relative time knowledg e in a form which is suitable for many practical applications, where r elative temporal information is only occasionally needed. The architec ture of such a system is given, and it is shown that the efficient ret rieval mechanisms for absolute-time-stamped databases may be adapted t o time-limited databases.