SELECT distinct(forum_topic.id) AS distinct_1
FROM forum_topic, forum_post
WHERE forum_post.author_id = 3134
AND forum_post.topic_id = forum_topic.id
AND forum_topic.forum_id IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
ORDER BY forum_topic.last_post_id DESC  LIMIT 0, 30;

mysql> explain SELECT distinct(forum_topic.id) AS distinct_1  FROM forum_topic, forum_post  WHERE forum_post.author_id = 3134 AND forum_post.topic_id = forum_topic.id AND forum_topic.forum_id IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12) ORDER BY forum_topic.last_post_id DESC  LIMIT 0, 30;
+----+-------------+-------------+--------+-------------------------------------------------------------------------+----------------------+---------+--------------------------------------+------+---------------------------------+
| id | select_type | table       | type   | possible_keys                                                           | key                  | key_len | ref                                  | rows | Extra                           |
+----+-------------+-------------+--------+-------------------------------------------------------------------------+----------------------+---------+--------------------------------------+------+---------------------------------+
|  1 | SIMPLE      | forum_post  | ref    | forum_post_author_id,forum_post_topic_id,viewtopic,forum_post_egosearch | forum_post_egosearch | 4       | const                                | 3076 | Using temporary; Using filesort | 
|  1 | SIMPLE      | forum_topic | eq_ref | PRIMARY,viewforum                                                       | PRIMARY              | 4       | ubuntu_de_inyoka.forum_post.topic_id |    1 | Using where                     | 
+----+-------------+-------------+--------+-------------------------------------------------------------------------+----------------------+---------+--------------------------------------+------+---------------------------------+
2 rows in set (0.00 sec)

mysql> show indexes from forum_post;
+------------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table      | Non_unique | Key_name             | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+------------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| forum_post |          0 | PRIMARY              |            1 | id          | A         |     1640642 |     NULL | NULL   |      | BTREE      | NULL    | 
| forum_post |          1 | forum_post_author_id |            1 | author_id   | A         |      205080 |     NULL | NULL   |      | BTREE      | NULL    | 
| forum_post |          1 | forum_post_topic_id  |            1 | topic_id    | A         |      273440 |     NULL | NULL   |      | BTREE      | NULL    | 
| forum_post |          1 | viewtopic            |            1 | topic_id    | A         |      546880 |     NULL | NULL   |      | BTREE      | NULL    | 
| forum_post |          1 | viewtopic            |            2 | position    | A         |     1640642 |     NULL | NULL   |      | BTREE      | NULL    | 
| forum_post |          1 | forum_post_pub_date  |            1 | pub_date    | A         |     1640642 |     NULL | NULL   |      | BTREE      | NULL    | 
| forum_post |          1 | forum_post_egosearch |            1 | author_id   | A         |        6459 |     NULL | NULL   |      | BTREE      | NULL    | 
| forum_post |          1 | forum_post_egosearch |            2 | id          | A         |     1640642 |     NULL | NULL   |      | BTREE      | NULL    | 
+------------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
8 rows in set (1.04 sec)

AND forum_post.topic_id = forum_topic.id

AND forum_topic.forum_id IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)

--
-- Table structure for table `forum_post`
--

DROP TABLE IF EXISTS `forum_post`;
SET @saved_cs_client     = @@character_set_client;
SET character_set_client = utf8;
CREATE TABLE `forum_post` (
  `id` int(11) NOT NULL auto_increment,
  `author_id` int(11) NOT NULL,
  `topic_id` int(11) NOT NULL,
  `text` longtext collate utf8_unicode_ci,
  PRIMARY KEY  (`id`),
  KEY `forum_post_author_id` (`author_id`),
  KEY `forum_post_topic_id` (`topic_id`),
  KEY `forum_post_egosearch` (`author_id`,`id`)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
SET character_set_client = @saved_cs_client;

--
-- Dumping data for table `forum_post`
--

LOCK TABLES `forum_post` WRITE;
/*!40000 ALTER TABLE `forum_post` DISABLE KEYS */;
INSERT INTO `forum_post` VALUES (1,2,1,'Ad. Et quisque pellentesque penatibus;\n\nRhoncus ridiculus vulputate non nonummy malesuada lectus nisl nascetur proin nibh gravida est laoreet felis donec eros?\n\nmorbi metus odio dolor natoque mus est pellentesque penatibus dictum elit turpis ullamcorper ligula ullamcorper habitant est eu curabitur bibendum.'),(2,3,1,'Amet tincidunt sit sociis sodales facilisi porta aliquet dapibus diam pellentesque penatibus litora bibendum quisque. interdum rutrum sodales hac hendrerit aliquet litora aliquet dignissim convallis mauris et urna nunc laoreet libero;\n\nId sed posuere potenti pellentesque penatibus aptent lorem luctus dapibus diam sit sociis lacinia feugiat habitasse tincidunt tincidunt netus. tellus quam dui vulputate sit sociis nunc orci commodo tortor quisque sit sociis ligula quis eleifend elementum inceptos curae urna! Condimentum leo suscipit quisque nec curae nascetur quis imperdiet platea.'),(3,1,1,'eros rhoncus ridiculus lorem luctus curabitur accumsan sem aliquet aenean enim at auctor massa risus dis tempor tempus elit venenatis eu quis. Eleifend elementum facilisi platea eros curabitur ligula commodo?'),(4,2,1,'Cras per rutrum consectetuer consequat maecenas lectus eros quisque at auctor dictumst tortor amet lacinia lacinia?'),(5,2,2,'Porttitor pharetra blandit pretium condimentum placerat bibendum magnis dictumst venenatis aptent proin torquent accumsan? Rutrum rutrum;'),(6,3,2,'Tellus eget dolor et ligula nullam mauris?'),(7,1,3,'Maecenas ante duis magna maecenas sem eu pellentesque penatibus fames faucibus urna duis aenean porttitor elit id curabitur; quis egestas rhoncus ridiculus accumsan sodales posuere potenti vel porttitor commodo nulla fringilla duis sit sociis curae duis purus amet tristique velit; '),(8,2,4,'Torquent duis nascetur! Eleifend elementum condimentum iaculis natoque facilisis etiam vel quis eu amet montes augue magna. Congue senectus fames faucibus varius condimentum feugiat duis curabitur vel? Curae rhoncus ridiculus.'),(9,1,5,' Pretium etiam aliquet placerat urna accumsan ac metus?');
/*!40000 ALTER TABLE `forum_post` ENABLE KEYS */;
UNLOCK TABLES;

--
-- Table structure for table `forum_topic`
--

DROP TABLE IF EXISTS `forum_topic`;
SET @saved_cs_client     = @@character_set_client;
SET character_set_client = utf8;
CREATE TABLE `forum_topic` (
  `id` int(11) NOT NULL auto_increment,
  `forum_id` int(11) NOT NULL,
  `title` varchar(100) collate utf8_unicode_ci NOT NULL,
  `author_id` int(11) NOT NULL,
  `first_post_id` int(11) default NULL,
  `last_post_id` int(11) default NULL,
  PRIMARY KEY  (`id`),
  KEY `forum_topic_first_post_id` (`first_post_id`),
  KEY `forum_topic_last_post_id` (`last_post_id`),
  CONSTRAINT `first_post_id_refs_id_7ced225d` FOREIGN KEY (`first_post_id`) REFERENCES `forum_post` (`id`),
  CONSTRAINT `last_post_id_refs_id_7ced225d` FOREIGN KEY (`last_post_id`) REFERENCES `forum_post` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
SET character_set_client = @saved_cs_client;

--
-- Dumping data for table `forum_topic`
--

LOCK TABLES `forum_topic` WRITE;
/*!40000 ALTER TABLE `forum_topic` DISABLE KEYS */;
INSERT INTO `forum_topic` VALUES (1,1,'Interdum porta pellentesquepenatibus!',2,1,4),(2,1,'Laoreet nam?',2,5,6),(3,1,'Egestas id!',1,7,9);
/*!40000 ALTER TABLE `forum_topic` ENABLE KEYS */;
UNLOCK TABLES;

--
-- Table structure for table `portal_user`
--

DROP TABLE IF EXISTS `portal_user`;
SET @saved_cs_client     = @@character_set_client;
SET character_set_client = utf8;
CREATE TABLE `portal_user` (
  `id` int(11) NOT NULL auto_increment,
  `username` varchar(30) collate utf8_unicode_ci NOT NULL,
  PRIMARY KEY  (`id`),
  UNIQUE KEY `username` (`username`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
SET character_set_client = @saved_cs_client;

--
-- Dumping data for table `portal_user`
--

LOCK TABLES `portal_user` WRITE;
/*!40000 ALTER TABLE `portal_user` DISABLE KEYS */;
INSERT INTO `portal_user` VALUES (1,'dolor'),(2,'ipsum'),(3,'lorem');
/*!40000 ALTER TABLE `portal_user` ENABLE KEYS */;
UNLOCK TABLES;

alter table forum_post add index xyz (topic_id,author_id)

Explain SELECT
  forum_topic.id as tid
FROM
  forum_topic
WHERE
  EXISTS(
    SELECT
      topic_id
    FROM
      forum_post
    WHERE
      author_id = 3134
      AND tid=topic_id
  )  
ORDER BY
  forum_topic.last_post_id DESC
LIMIT
  0, 30

AND forum_topic.forum_id IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)

+----+--------------------+-------------+-------+-----------------------------------------------------------------------------------+--------------------------+---------+------------+--------+--------------------------+
| id | select_type        | table       | type  | possible_keys                                                                     | key                      | key_len | ref        | rows   | Extra                    |
+----+--------------------+-------------+-------+-----------------------------------------------------------------------------------+--------------------------+---------+------------+--------+--------------------------+
|  1 | PRIMARY            | forum_topic | index | viewforum                                                                         | forum_topic_last_post_id | 5       | NULL       | 188200 | Using where              | 
|  2 | DEPENDENT SUBQUERY | forum_post  | ref   | forum_post_author_id,forum_post_topic_id,viewtopic,forum_post_egosearch,egosearch | egosearch                | 8       | func,const |      1 | Using where; Using index | 
+----+--------------------+-------------+-------+-----------------------------------------------------------------------------------+--------------------------+---------+------------+--------+--------------------------+
2 rows in set (0.00 sec)