Distinguishing distinguished variables

Distinguished variables are variables present in the heads of the rules of a DATALOG (or PRO-LOG) program. Except for their relevance to a query, they are as undistinguished by previous works as others, introduced by resolution steps. What if the selection rule rst selects literals with at least one distinguished variable ? This paper is an attempt to answer this question. To do this, we rst deene generating goals, which are the bodies of the resultants of a partial evaluation of a rule head in the program, the unfolding rule being \Unfold, as much as possible, atoms containing distinguished variables". Next the properties of generating goals are discussed, and an adaptation of SLD-resolution taking into account these properties is presented. The question of the niteness of the set of generating goals is then raised. Although decidability of this property is still an open question in the general case, decidability is shown for signiicant classes of Datalog programs, including bounded programs and linear programs. Moreover, we show that every binary chained single rule programs (sirups) (under the additional commonly accepted assumption that databases contain only Extensional DataBase (EDB) predicates) is equivalent to a program with nite sets of generating goals. Details on work in progress are given which suggest that the property might be decidable for the much more general class of binary chained programs. A scheme for compiling these classes of programs into a set of asynchronously communicating processes is presented as an extension to a previous proposition by Van Gelder.