Supporting Valid Time: An Historical Algebra.

Abstract : The authors define an historical algebra for historical relations. This historical algebra, a straightforward extension of the conventional relational algebra, supports valid time, the time when an object or relationship in the enterprise being modeled is valid. Historical versions of the five relational operators union, difference, cartesian product, selection, and projection are defined and a new operator, historical derivation, is introduced. The algebra includes aggregates and is shown to have the expressive power of the temporal query language TQuel. The algebra is consistent with the user-oriented model of historical relations as space-filling objects and satisfies all but one of the associative, commutative, and distributive tautologies involving union, difference, and cartesian product. Keywords: Data base management systems.

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