@Override
    public String toString() {
      return String.format("Window Spec=[%s%s%s]",
          sourceId == null ? "" : "Name='" + sourceId + "'",
          partitioning == null ? "" : partitioning,
          windowFrame == null ? "" : windowFrame);
    }
Add default order spec if there is no order and validate order spec for valued based
windowing since only one sort key is allowed.
@param wFn Window function spec
@throws SemanticException
ql/src/java/org/apache/hadoop/hive/ql/parse/WindowingSpec.java