Tractable Cover Compilations

Tractable covers are introduced as a new ap­ proach to equivalence-preserving compilation of propositional knowledge bases. First, a gen­ eral framework is presented. Then, two specific cases are considered. In the first one, partial interpretations are used to shape the knowl­ edge base into tractable formulas from several possible classes. In the second case, they are used to derive renamable Horn formulas. This last case is proved less space-consuming than prime implicants cover compilations for every knowledge base. Finally, experimental results show that the new approaches can prove effi­ cient w.r.t. direct query answering and offer significant time and space savings w.r.t. prime implicants covers.