Plan Reuse versus Plan Generation :

The ability of a planner to reuse parts of old plans is hypothesized to be a valuable tool for improving eeciency of planning by avoiding the repetition of the same planning eeort. We test this hypothesis from an analytical and empirical point of view. A comparative worst-case complexity analysis of generation and reuse under diierent assumptions reveals that it is not possible to achieve a provable eeciency gain of reuse over generation. Further, assuming \conservative" plan modiication, plan reuse can actually be strictly more diicult than plan generation. While these results do not imply that there won't be an eeciency gain in some situations, retrieval of a good plan may present a serious bottleneck for plan reuse systems, as we will show. Finally, we present the results of an empirical study of two diierent plan reuse systems, pointing out possible pitfalls one should be aware of when attempting to employ reuse methods.