SHOW PROFILE for the following query:

    SELECT `topic`.* FROM `topics` AS `topic` 
    LEFT OUTER  JOIN `topic` AS `topic_helper` ON (`topic`.`id` = `topic_helper`.`id` AND `topic_helper`.`created_on` < `topic`.`created_on`) 
    GROUP BY `topic`.`id` HAVING COUNT(*) < 3 
    ORDER BY `topic`.`created_on` DESC;

+--------------------------------+----------+
| Status                         | Duration |
+--------------------------------+----------+
| starting                       | 0.000021 |
| Waiting for query cache lock   | 0.000004 |
| Waiting on query cache mutex   | 0.000004 |
| checking query cache for query | 0.000082 |
| checking permissions           | 0.000006 |
| checking permissions           | 0.000008 |
| Opening tables                 | 0.000359 |
| System lock                    | 0.000018 |
| Waiting for query cache lock   | 0.000004 |
| Waiting on query cache mutex   | 0.000025 |
| init                           | 0.000009 |
| checking permissions           | 0.000004 |
| checking permissions           | 0.000004 |
| checking permissions           | 0.000003 |
| checking permissions           | 0.000004 |
| checking permissions           | 0.000003 |
| checking permissions           | 0.000003 |
| checking permissions           | 0.000091 |
| optimizing                     | 0.000022 |
| statistics                     | 0.000081 |
| preparing                      | 0.000035 |
| Creating tmp table             | 0.000448 |
| Sorting for group              | 0.000009 |
| executing                      | 0.000004 |
| Copying to tmp table           | 0.428565 |
| Sorting result                 | 0.005275 |
| Sending data                   | 0.000772 |
| end                            | 0.000008 |
| removing tmp table             | 0.002753 |
| end                            | 0.000009 |
| query end                      | 0.000010 |
| closing tables                 | 0.000032 |
| freeing items                  | 0.000318 |
| logging slow query             | 0.000004 |
| cleaning up                    | 0.000009 |
+--------------------------------+----------+
35 rows in set (0.00 sec)