Using filesort

CREATE TABLE `buttons` (
  `id_button` int(10) unsigned NOT NULL auto_increment,
  `id_user` int(10) unsigned NOT NULL,
  `title` varchar(60) collate utf8_czech_ci NOT NULL,
  `url` text collate utf8_czech_ci NOT NULL,
  `show` binary(1) NOT NULL default '1',
  `datetime` datetime NOT NULL,
  `hide_datetime` datetime default NULL,
  PRIMARY KEY  (`id_button`),
  KEY `id_user+show+id_button` (`id_user`,`show`,`id_button`),
  KEY `id_user+show` (`id_user`,`show`)
)

SELECT `url` FROM `buttons` WHERE `id_user` = 2 AND `show` = 1 ORDER BY `id_button` ASC LIMIT 7

id : 1
select_type : SIMPLE
table : buttons
type : ref
possible_keys : id_user+show+id_button,id_user+show
key : id_user+show+id_button
key_len : 4
ref : const
rows : 5
extra : Using where; Using filesort