SELECT tid FROM bl_topics USE INDEX (forum_id) WHERE forum_id IN (.....)

CREATE TABLE `bl_topics` (
 `tid` int(10) NOT NULL AUTO_INCREMENT,
 `tpage` varchar(250) NOT NULL,
 `tpagen` varchar(250) NOT NULL,
 `taccess` tinyint(3) unsigned NOT NULL DEFAULT '0',
 `title` varchar(250) NOT NULL DEFAULT '',
 `description` varchar(70) DEFAULT NULL,
 `state` varchar(8) DEFAULT NULL,
 `posts` int(10) DEFAULT NULL,
 `starter_id` mediumint(8) NOT NULL DEFAULT '0',
 `start_date` int(10) DEFAULT NULL,
 `last_poster_id` mediumint(8) NOT NULL DEFAULT '0',
 `last_post` int(10) DEFAULT NULL,
 `starter_name` varchar(32) DEFAULT NULL,
 `last_poster_name` varchar(32) DEFAULT NULL,
 `poll_state` varchar(8) DEFAULT NULL,
 `last_vote` int(10) DEFAULT NULL,
 `views` int(10) DEFAULT NULL,
 `forum_id` int(10) unsigned NOT NULL DEFAULT '0',
 `pinned` tinyint(1) DEFAULT NULL,
 `total_votes` int(5) NOT NULL DEFAULT '0',
 `awards_status` tinyint(1) DEFAULT '0',
 `f_pid` int(10) unsigned NOT NULL DEFAULT '0',
 `f_post` text NOT NULL,
 `bl_image` varchar(64) NOT NULL ,
 `user_like` mediumint(8) unsigned NOT NULL DEFAULT '0',
 `user_unlike` mediumint(8) unsigned NOT NULL DEFAULT '0',
 `liveline` tinyint(3) unsigned NOT NULL DEFAULT '0',
 `on_wall` tinyint(3) unsigned NOT NULL DEFAULT '0',
 `tmp_topic` mediumint(8) unsigned NOT NULL DEFAULT '0',
 `get_cup` tinyint(1) unsigned NOT NULL DEFAULT '0',
 `topic_plus` mediumint(8) unsigned NOT NULL DEFAULT '0',
 `topic_minus` mediumint(8) unsigned NOT NULL DEFAULT '0',
 PRIMARY KEY (`tid`),
 KEY `last_post` (`last_post`),
 KEY `tpage` (`tpage`),
 KEY `taccess` (`taccess`),
 KEY `starter_id` (`starter_id`),
 KEY `blogs` (`forum_id`,`taccess`,`starter_id`),
 KEY `f_pid` (`f_pid`),
 KEY `liveline` (`liveline`),
 KEY `on_wall` (`on_wall`),
 KEY `start_date` (`start_date`),
 KEY `liveline_idx` (`taccess`,`liveline`,`tid`),
 KEY `profile_idx` (`forum_id`,`taccess`),
 KEY `forum_id` (`forum_id`),
 KEY `reputation` (`tid`,`forum_id`),
 KEY `friendsline_idx_2` (`forum_id`,`taccess`,`on_wall`,`pinned`,`last_post`),
 KEY `friendsline_idx_1` (`forum_id`,`on_wall`,`tid`),
 KEY `friendsline_idx` (`forum_id`,`on_wall`,`taccess`,`tid`),
 KEY `tpagen` (`tpagen`)
) ENGINE=InnoDB AUTO_INCREMENT=1387076 DEFAULT CHARSET=cp1251