On Cyclic Covers and Perfect Models

The relationship between cyclic covers and perfect models of indefinite stratified deductive databases is discussed with respect to query answering, view updates and partial evaluation. The notion of a conjunctive answer to a database query is introduced, and such answers are shown to have a number of advantages over (the more commonly studied) “disjunctive” answers. The problem of computing conjunctive answers is shown to be a special case of the view update problem. Cyclic covers are shown to characterise perfect models, conjunctive answers and view updates, and using this characterisation a method is presented for solving the view update problem, and hence for computing conjunctive answers. In addition, cyclic covers are shown to yield methods of partially evaluating (ie., pre-processing) the intensional database, for instance to facilitate query compilation, the removal of recursion and the removal of positive or negative intensional sub-goals. Such pre-processing is immune to future changes in the extensional database.