A Relational Algebra as a Query Language for Temporal DATALOG

This paper introduces a temporal relational algebra as a query language for temporal deductive databases, i.e., Temporal Datalog programs. In Temporal Datalog programs, temporal relationships among data are formalized through temporal operators, not by an explicit reference to time. The minimum model of a given Temporal Datalog program is regarded as the temporal database the program models intensionally. Users query temporal deductive databases using a temporal relational algebra (TRA), which is a pointwise extension of the relational algebra. During the evaluation of TRA expressions, portions of temporal relations are retrieved from a given temporal deductive database when needed. Bottom-up evaluation strategies such as the fixed-point computation can be used to compute portions of temporal relations over intervals. An extension of Temporal Datalog with generic modules is also proposed. Through modules, temporal relations created during the evaluation of TRA expressions may be fed back to the deductive part for further manipulation.