@Override
  public PhysicalContext resolve(PhysicalContext pctx) throws SemanticException {
    SemanticDispatcher disp = new SkewJoinTaskDispatcher(pctx);
    SemanticGraphWalker ogw = new DefaultGraphWalker(disp);
    ArrayList<Node> topNodes = new ArrayList<Node>();
    topNodes.addAll(pctx.getRootTasks());
    ogw.startWalking(topNodes, null);
    return pctx;
  }
An implementation of PhysicalPlanResolver. It iterator each task with a rule
dispatcher for its reducer operator tree, for task with join op in reducer,
it will try to add a conditional task associated a list of skew join tasks.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/physical/SkewJoinResolver.java