@Override
  public void writeStructBegin(TStruct struct) throws TException {
    stackLevel++;
    if (stackLevel == 1) {
      topLevelStructFieldID = 0;
      ascending = (topLevelStructFieldID >= sortOrder.length() || sortOrder
          .charAt(topLevelStructFieldID) != '-');
    } else {
      writeRawBytes(nonNullByte, 0, 1);
      // If the struct is null and level > 1, SerDes will call
      // writeNull();
    }
  }
Whether the current field is sorted ascendingly. Always equals to
sortOrder.charAt(topLevelStructFieldID) != '-'
serde/src/java/org/apache/hadoop/hive/serde2/thrift/TBinarySortableProtocol.java