@Override
public PhysicalContext resolve(PhysicalContext pctx) throws SemanticException {
// create dispatcher and graph walker
Dispatcher disp = new LocalMapJoinTaskDispatcher(pctx);
TaskGraphWalker ogw = new TaskGraphWalker(disp);
// get all the tasks nodes from root task
ArrayList<Node> topNodes = new ArrayList<Node>();
topNodes.addAll(pctx.getRootTasks());
// begin to walk through the task tree.
ogw.startWalking(topNodes, null);
return pctx;
}