code: func (b *builtinDayNameSig) evalString(row chunk.Row) (string, bool, error) {
  	idx, isNull, err := b.evalIndex(row)
  	if isNull || err != nil {
  		return "", isNull, err
  	}
  	return types.WeekdayNames[idx], false, nil
  }
docs: // evalString evals a builtinDayNameSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_dayname