code: func (b *builtinFromUnixTime2ArgSig) evalString(row chunk.Row) (res string, isNull bool, err error) {
  	format, isNull, err := b.args[1].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return "", true, err
  	}
  	t, isNull, err := evalFromUnixTime(b.ctx, b.tp.Decimal, row, b.args[0])
  	if isNull || err != nil {
  		return "", isNull, err
  	}
  	res, err = t.DateFormat(format)
  	return res, err != nil, err
  }
docs: // evalString evals a builtinFromUnixTime2ArgSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_from-unixtime