function deleteCommentsFromTheEndOfTopics() {
    global $ident, $debug;
    // Select topics from wich comments will be deleted
    $result = doquery("SELECT id, maxhsz FROM topicok WHERE maxhsz != 0");
    if (mysqli_num_rows($result) > 0) {
        while(list($topicId, $maxHsz) = mysqli_fetch_row($result)) {
            getLock("topiclock_$topicId");
            // Select comments to be protected
            doquery("DROP TEMPORARY TABLE IF EXISTS `protected_$topicId`");
            doquery("CREATE TEMPORARY TABLE `protected_$topicId` ( `h_uniqid` CHAR(13) NOT NULL) ENGINE=MEMORY");
            doquery("INSERT INTO `protected_$topicId` SELECT h_uniqid FROM `comments_$topicId` ORDER BY h_uniqid DESC LIMIT $maxHsz");
            // Select comments to be deleted one by one
            doquery("DROP TEMPORARY TABLE IF EXISTS `onebyone_$topicId`");
            doquery("CREATE TEMPORARY TABLE `onebyone_$topicId` ( `h_uniqid` CHAR(13) NOT NULL) ENGINE=MEMORY");
            // Search comments which answers or antecedents can be found in another topic
            doquery("INSERT INTO `onebyone_$topicId` SELECT h_uniqid FROM `comments_$topicId` WHERE ((antecendent_topicid != '$topicId' AND antecendent_topicid != 0) OR (answers NOT LIKE '%-topic_$topicId-%' AND answers != '')) AND h_uniqid NOT IN (SELECT h_uniqid FROM `protected_$topicId`)");
            $res = doquery("SELECT h_uniqid FROM `onebyone_$topicId`");
            $deleteOneByOneStr = '';
            if (mysqli_num_rows($res) > 0) {
                // There are comments in the table, put the deletion string together
                $commentArray = array();
                while(list($h_uniqid) = mysqli_fetch_row($res)) {
                    $deleteOneByOneStr .= "$topicId-$h_uniqid|";
                }
            }
            // Lets find comments which are external links containing, and not elements of selected comments before
            doquery("DROP TEMPORARY TABLE IF EXISTS `commentlink_$topicId`");
            doquery("CREATE TEMPORARY TABLE `commentlink_$topicId` ( `h_uniqid` CHAR(13) NOT NULL) ENGINE=MEMORY");
            doquery("INSERT INTO `commentlink_$topicId` SELECT h_uniqid FROM `comments_$topicId` WHERE h_uniqid NOT IN (SELECT h_uniqid FROM `onebyone_$topicId`) AND h_uniqid < (SELECT MIN(h_uniqid) FROM `protected_$topicId`) AND (commenttext LIKE '%<a href=%' OR commenttext LIKE '%<img src=%' OR commenttext LIKE '%[flash src=%')");

            // Okay, we have the comments, delete from links table
            doquery("DELETE FROM links WHERE h_uniqid IN (SELECT h_uniqid FROM `commentlink_$topicId`)");

            // Search comments which are answers to some comments to be deleted
            doquery("DROP TEMPORARY TABLE IF EXISTS `answers_$topicId`");
            doquery("CREATE TEMPORARY TABLE `answers_$topicId` ( `h_uniqid` CHAR(13) NOT NULL) ENGINE=MEMORY");
            doquery("INSERT INTO `answers_$topicId` SELECT h_uniqid FROM `comments_$topicId` WHERE answers != '' AND h_uniqid < (SELECT MIN(h_uniqid) FROM `protected_$topicId`) AND h_uniqid NOT IN (SELECT h_uniqid FROM `onebyone_$topicId`)");
            // Update reply comment to have no antecedent, where the antecedent is in this topic
            doquery("UPDATE `comments_$topicId` SET antecedent_id = 0, antecendent_tid = 0, antecendent_topicid = 0, antecendent_uniqid = '' WHERE h_uniqid >= (SELECT MIN(h_uniqid) FROM `protected_$topicId`) AND antecendent_uniqid IN (SELECT h_uniqid FROM `answers_$topicId`) AND antecendent_topicid = '$topicId'");

            // Delete comments which can be deleted in one query
            doquery("DELETE FROM `comments_$topicId` WHERE h_uniqid < (SELECT MIN(h_uniqid) FROM `protected_$topicId`) AND h_uniqid NOT IN (SELECT h_uniqid FROM `onebyone_$topicId`)");
            doquery("DELETE FROM `lastcomments` WHERE topicid = '$topicId' AND h_uniqid < (SELECT MIN(h_uniqid) FROM `protected_$topicId`) AND h_uniqid NOT IN (SELECT h_uniqid FROM `onebyone_$topicId`)");

            doquery("DROP TEMPORARY TABLE IF EXISTS `commentlink_$topicId`");
            doquery("DROP TEMPORARY TABLE IF EXISTS `protected_$topicId`");
            doquery("DROP TEMPORARY TABLE IF EXISTS `answers_$topicId`");
            doquery("DROP TEMPORARY TABLE IF EXISTS `onebyone_$topicId`");
            removeLock("topiclock_$topicId");

            if ($deleteOneByOneStr != '') {
                // Delete comments which must be deleted one by one (False is for telling the function not to send mails from the deletion)
                commenttext_torol($deleteOneByOneStr, False);
            }

            // Update bookmarks
            list($lastCommentUniqId, $lastCommentId) = mysqli_fetch_row(doquery("SELECT h_uniqid, id FROM `comments_$topicId` ORDER BY h_uniqid LIMIT 1"));
            doquery("UPDATE bookmarks SET commentid = '$lastCommentId', h_uniqid = '$lastCommentUniqId' WHERE uidtid LIKE '%-$topicId' AND h_uniqid < '$lastCommentUniqId'");
        }
    }
}
Let's see the table structure:

First, a table holding comments for one topic (in this table the topicid is 1)

CREATE TABLE `lasthsz` (
  `id` int(11) unsigned NOT NULL default '0',
  `hid` int(11) unsigned NOT NULL default '0',
  `ido` int(11) unsigned NOT NULL default '0',
  `hostnev` varchar(100) collate latin2_hungarian_ci NOT NULL default '',
  `topicid` int(11) unsigned NOT NULL default '0',
  `antecedent` int(11) unsigned NOT NULL default '0',
  `antecendent_tid` int(11) unsigned NOT NULL default '0',
  `antecendent_topicid` int(11) unsigned NOT NULL default '0',
  `antecendent_uniqid` char(13) collate latin2_hungarian_ci NOT NULL default '',
  `h_uniqid` varchar(13) collate latin2_hungarian_ci NOT NULL default '',
  `commenttext` text collate latin2_hungarian_ci NOT NULL,
  `parsedhsz` text collate latin2_hungarian_ci NOT NULL,
  `edits` varchar(255) collate latin2_hungarian_ci NOT NULL default '',
  `answers` text collate latin2_hungarian_ci NOT NULL,
  PRIMARY KEY  (`h_uniqid`),
  KEY `topicid` (`topicid`),
  KEY `hid` (`hid`),
  KEY `antecedent` (`antecedent`),
  KEY `antecendent_uniqid` (`antecendent_uniqid`),
  FULLTEXT KEY `answers` (`answers`)
) ENGINE=MyISAM DEFAULT CHARSET=latin2 COLLATE=latin2_hungarian_ci;

Bookmarks:

CREATE TABLE `bookmarks` (
  `uidtid` char(12) collate latin2_hungarian_ci NOT NULL default '',
  `hszid` int(10) unsigned NOT NULL default '0',
  `h_uniqid` char(13) collate latin2_hungarian_ci NOT NULL,
  `datum` datetime NOT NULL default '0000-00-00 00:00:00',
  UNIQUE KEY `uidtid` (`uidtid`)
) ENGINE=MyISAM DEFAULT CHARSET=latin2 COLLATE=latin2_hungarian_ci COMMENT='This table holds bookmarks';

Last comments table

CREATE TABLE `lasthsz` (
  `id` int(11) unsigned NOT NULL default '0',
  `hid` int(11) unsigned NOT NULL default '0',
  `ido` int(11) unsigned NOT NULL default '0',
  `hostnev` varchar(100) collate latin2_hungarian_ci NOT NULL default '',
  `topicid` int(11) unsigned NOT NULL default '0',
  `antecedent` int(11) unsigned NOT NULL default '0',
  `antecendent_tid` int(11) unsigned NOT NULL default '0',
  `antecendent_topicid` int(11) unsigned NOT NULL default '0',
  `antecendent_uniqid` char(13) collate latin2_hungarian_ci NOT NULL default '',
  `h_uniqid` varchar(13) collate latin2_hungarian_ci NOT NULL default '',
  `commenttext` text collate latin2_hungarian_ci NOT NULL,
  `parsedhsz` text collate latin2_hungarian_ci NOT NULL,
  `edits` varchar(255) collate latin2_hungarian_ci NOT NULL default '',
  `answers` text collate latin2_hungarian_ci NOT NULL,
  PRIMARY KEY  (`h_uniqid`),
  KEY `topicid` (`topicid`),
  KEY `hid` (`hid`),
  KEY `antecedent` (`antecedent`),
  KEY `antecendent_uniqid` (`antecendent_uniqid`),
  FULLTEXT KEY `answers` (`answers`)
) ENGINE=MyISAM DEFAULT CHARSET=latin2 COLLATE=latin2_hungarian_ci;

Links:

CREATE TABLE `links` (
  `lid` int(10) unsigned NOT NULL auto_increment COMMENT 'link id',
  `mikor` int(10) unsigned NOT NULL COMMENT 'comment time',
  `h_uniqid` char(13) collate latin2_hungarian_ci NOT NULL COMMENT 'comment uniqid (used at deletion)',
  `linkurl` char(255) collate latin2_hungarian_ci NOT NULL COMMENT 'link url',
  PRIMARY KEY  (`lid`),
  KEY `linkurl` (`linkurl`),
  KEY `h_uniqid` (`h_uniqid`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin2 COLLATE=latin2_hungarian_ci COMMENT='This table contains links';

And finally, the topic information table:

CREATE TABLE `topicok` (
  `nev` varchar(255) collate latin2_hungarian_ci NOT NULL default '',
  `pure_nev` varchar(100) collate latin2_hungarian_ci NOT NULL default '',
  `id` int(11) unsigned NOT NULL auto_increment,
  `velemenyszam` int(11) unsigned NOT NULL default '0',
  `tulajid` int(11) unsigned NOT NULL default '0',
  `letiltva` tinyint(3) unsigned NOT NULL default '0',
  `adminonly` tinyint(3) unsigned NOT NULL default '0',
  `status` tinyint(3) unsigned NOT NULL default '0',
  `maxhsz` int(10) unsigned NOT NULL default '0' COMMENT 'delete comments after that amount',
  `ido` int(11) unsigned NOT NULL default '0',
  `maxid` int(11) unsigned NOT NULL default '0',
  `utolsohid` int(11) unsigned NOT NULL default '0',
  `utolsohszid` int(11) unsigned NOT NULL default '0',
  `utolsouniqid` char(13) collate latin2_hungarian_ci NOT NULL,
  `utolsohsz` text collate latin2_hungarian_ci NOT NULL,
  `leiras` text collate latin2_hungarian_ci NOT NULL,
  PRIMARY KEY  (`id`),
  UNIQUE KEY `utolsouniqid` (`utolsouniqid`),
  KEY `ido` (`ido`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin2 COLLATE=latin2_hungarian_ci;