private static ExprNodeColumnDesc extractColumn(ExprNodeDesc expr) {
    if (expr instanceof ExprNodeColumnDesc) {
      return (ExprNodeColumnDesc)expr;
    }
    if (expr instanceof ExprNodeFieldDesc) {
      return extractColumn(((ExprNodeFieldDesc)expr).getDesc());
    }
    return null;
  }
Extract fields from the given {@link ExprNodeFieldDesc node descriptor}
ql/src/java/org/apache/hadoop/hive/ql/plan/ExprNodeDescUtils.java