code: func builtinConcat(args []types.Datum, _ context.Context) (d types.Datum, err error) {
  	var s []byte
  	for _, a := range args {
  		if a.IsNull() {
  			return d, nil
  		}
  		var ss string
  		ss, err = a.ToString()
  		if err != nil {
  			return d, errors.Trace(err)
  		}
  		s = append(s, []byte(ss)...)
  	}
  	d.SetBytesAsString(s)
  	return d, nil
  }
docs: // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_concat