@Override
    public Object process(Node nd, Stack<Node> stack, NodeProcessorCtx procCtx,
        Object... nodeOutputs) throws SemanticException {
      return process(nd, stack, procCtx, false, nodeOutputs);
    }
Determines the push down predicates in its where expression and then
combines it with the push down predicates that are passed from its children.
ql/src/java/org/apache/hadoop/hive/ql/ppd/OpProcFactory.java