Non-deterministic modelling of logical queries in deductive databases

Abstract

We propose a technique based on Petri Nets formalism to model logic queries in deductive databases. The model is called PNLP (Petri Net model for Logic Programs), and it has a simple formal description and a graphical representation. The PNLP model explicitly represents the relationships between rules and predicates. It is general and flexible enough to demonstrate the flow of control in different algorithms used to evaluate recursive logic queries. In fact the model unifies the level of description of these algorithms, and facilitates identifying similarities and differences between them. The inherent non-determinism in the PNLP model may also be useful in recognizing the parallelism within Horn-clause logic programs. In this paper, the PNLP model is described, and its functionality is demonstrated by modeling several existing algorithms for recursive query evaluation.