First, the possibilistic extension, a possible manifestation of the extention rule, is proposed. A reasoning mechanic in possibilistic logic, using the possibilistic extension rule, is then introduced, using the concept of the complementary factor to estimate its complexity. The definitions of entailment tractable and satisfiability tractable are extended to introduce the concepts of possibilistic entailment tractable and consistent-computing tractable. Based on possibilistic extension rules, EPPCCCL (each pair of possibilistic clauses contains complementary literals) theory is introduced, which is proved to be possibilistic entailment tractable and consistent-computing tractable; therefore, the theory can be regarded as a target language of possibilistic knowledge compilation.