public boolean isRuntimeConstant() {
    if(body != null) {
      return body.isRuntimeConstant();
    }
    return false;
  }
GenericUDFMacro wraps a user-defined macro expression into a GenericUDF
interface.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFMacro.java