Executing Parallel Plans Faster by Adding Actions

If considering only sequential execution of plans, the execution time of a plan will increase if adding actions to it. However, if also considering parallel execution of plans, then the addition of actions to a plan may sometimes decrease the execution time. It is further shown that plans which are sub-optimal in this way are likely to be frequently generated by standard partial-order planners. Unfortunately, optimizing the execution time of a plan by adding actions is NP-hard and cannot even be approximated within a constant ratio.