code: func (b *builtinConcatSig) evalString(row chunk.Row) (d string, isNull bool, err error) {
  	var s []byte
  	for _, a := range b.getArgs() {
  		d, isNull, err = a.EvalString(b.ctx, row)
  		if isNull || err != nil {
  			return d, isNull, err
  		}
  		s = append(s, []byte(d)...)
  	}
  	return string(s), false, nil
  }
docs: // evalString evals a builtinConcatSig
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_concat