Plan Modification versus Plan Generation: A Complexity-Theoretic Perspective

The ability of a planner to modify a plan is considered as a valuable tool for improving ee-ciency of planning by avoiding the repetition of the same planning eeort. From a computational complexity point of view, however, it is by no means obvious that modifying a plan is com-putationally as easy as planning from scratch if the modiication has to follow the principle of \conservatism," i.e., to reuse as much of the old plan as possible. Indeed, considering proposi-tional STRIPS planning, it turns out that conservative plan modiication is as hard as planning and can sometimes be harder than plan generation. Furthermore, this holds even if we consider modiication problems where the old and the new goal speciication are similar. We put these results into perspective and discuss the relationship to existing plan modiication systems.