SLDMagic | An Improved Magic Set Technique

We present a rewriting method for Datalog-programs which simulates SLD-resolution more closely than the ordinary \magic set" method does. This is especially advantageous in the case of tail-recursive programs, but already in non-recursive programs we can often save a number of joins. In contrast to the method of Ross 7], we do not only solve the problem of tail-recursion, but try to simulate SLD-resolution as fully as possible. Based on an idea of Bry 3], our method can be described in an elegant way by means of a meta-interpreter. An especially nice feature of our approach is that we get many other known optimizations \for free" when we simulate SLD-resolution in this way. In contrast to our earlier method 2], we can now handle the full range from a direct simulation of SLD-resolution to the standard \magic set" technique (selectable by annotations within the program). We compare the eeciency of SLD-resolution and magic sets within a common framework, explain their main diierences, and give hints for choosing between them.