code: func (b *builtinOctIntSig) evalString(row chunk.Row) (string, bool, error) {
  	val, isNull, err := b.args[0].EvalInt(b.ctx, row)
  	if isNull || err != nil {
  		return "", isNull, err
  	}
  
  	return strconv.FormatUint(uint64(val), 8), false, nil
  }
docs: // evalString evals OCT(N).
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_oct