code: func composeGlobalPrivUpdate(priv mysql.PrivilegeType, value string) (string, error) {
  	if priv == mysql.AllPriv {
  		strs := make([]string, 0, len(mysql.Priv2UserCol))
  		for _, v := range mysql.Priv2UserCol {
  			strs = append(strs, fmt.Sprintf(`%s='%s'`, v, value))
  		}
  		return strings.Join(strs, ", "), nil
  	}
  	col, ok := mysql.Priv2UserCol[priv]
  	if !ok {
  		return "", errors.Errorf("Unknown priv: %v", priv)
  	}
  	return fmt.Sprintf(`%s='%s'`, col, value), nil
  }
docs: // composeGlobalPrivUpdate composes update stmt assignment list string for global scope privilege update.