Some Undecidable Implication Problems for Path Constraints
We present a class of path constraints of interest in connection with both structured and semi-structured databases, and investigate their associated implication problems. These path constraints are capable of expressing natural integrity constraints that are not only a fundamental part of the semantics of the data, but are also important in query optimization. We show that, despite the simple syntax of the constraints, the implication problem for the constraints is r.e. complete and the finite implication problem for the constraints is co-r.e. complete. Indeed, we establish the existence of a conservative reduction of the set of all first-order sentences to the path constraint language.