code: func (b *builtinOrdSig) evalInt(row chunk.Row) (int64, bool, error) {
  	str, isNull, err := b.args[0].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return 0, isNull, err
  	}
  	if len(str) == 0 {
  		return 0, false, nil
  	}
  
  	_, size := utf8.DecodeRuneInString(str)
  	leftMost := str[:size]
  	var result int64
  	var factor int64 = 1
  	for i := len(leftMost) - 1; i >= 0; i-- {
  		result += int64(leftMost[i]) * factor
  		factor *= 256
  	}
  
  	return result, false, nil
  }
docs: // evalInt evals a builtinOrdSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_ord