GraphWalker ogw = new DefaultGraphWalker(disp);
// Create a list of topOp nodes
List<Node> topNodes = new ArrayList<Node>();
topNodes.addAll(pCtx.getTopOps().values());
ogw.startWalking(topNodes, null);
// We have finished tree walking (correlation detection).
// We will first see if we need to abort (the operator tree has not been changed).
// If not, we will start to transform the operator tree.
abort = corrCtx.isAbort();
if (abort) {