We establish the equivalence of: (1) the logical implication problem for a description logic dialect called DLClass that includes a concept constructor for expressing uniqueness constraints, (2) the logical implication problem for path functional dependencies (PFDs), and (3) the problem of answering queries in deductive databases with limited use of successor functions. As a consequence, we settle an open problem concerning lower bounds for the PFD logical implication problem and show that a regularity condition for DLClass that ensures low order polynomial time decidability for its logical implication problem is tight.