Space optimization in the bottom-up evaluation of logic programs
S. Sudarshan, Divesh Srivastava, Raghu Ramakrishnan, Jeffrey F. Naughton · 1991
In the bottom-up evaluation of a logic program, all generated facts are usually assumed to be stored until the end of the evaluation. Considerable gains can be achieved by instead discarding facts that are no longer required: the space needed to evaluate the program is reduced, I/O costs may be reduced, and the costs of maintaining and accessing indices, eliminating duplicates etc. are reduced. Thus, discarding facts early could achieve time as well as space improvements. Given an evaluation method that is sound, complete and does not repeat derivation steps, we consider how facts can be discarded during the evaluation without compromising these properties. Our first contribution is to show that such a space optimization technique has three distinct components. Informally, we must make all derivations that we can with each fact, detect all duplicate derivations of facts and try to order the computation so as to minimize the "lifespan " of each fact. This separation enables us to use di...