private static ExprNodeDesc extractConstant(ExprNodeDesc expr) {
    if (!(expr instanceof ExprNodeGenericFuncDesc)) {
      return expr;
    }
    ExprNodeConstantDesc folded = foldConstant(((ExprNodeGenericFuncDesc) expr));
    return folded == null ? expr : folded;
  }
Extract fields from the given {@link ExprNodeFieldDesc node descriptor}
ql/src/java/org/apache/hadoop/hive/ql/plan/ExprNodeDescUtils.java