MariaDB [teeworlds]> EXPLAIN SELECT * FROM (SELECT *, dense_rank() OVER (PARTITION BY Map ORDER BY TIME) as rank FROM record_race) AS a WHERE a.rank=1;
+------+-------------+-------------+------+---------------+------+---------+------+---------+-----------------+
| id   | select_type | table       | type | possible_keys | key  | key_len | ref  | rows    | Extra           |
+------+-------------+-------------+------+---------------+------+---------+------+---------+-----------------+
|    1 | PRIMARY     | <derived2>  | ALL  | NULL          | NULL | NULL    | NULL | 4298976 | Using where     |
|    2 | DERIVED     | record_race | ALL  | NULL          | NULL | NULL    | NULL | 4298976 | Using temporary |
+------+-------------+-------------+------+---------------+------+---------+------+---------+-----------------+
2 rows in set (0.073 sec)

MariaDB [teeworlds]> EXPLAIN SELECT r1s.map, name, timestamp, time FROM record_race AS ranks INNER JOIN (SELECT map, MIN(time) AS mintime FROM record_race GROUP BY map) AS r1s ON ranks.map = r1s.map AND ranks.time = r1s.mintime ORDER BY timestamp LIMIT 10;
+------+-------------+-------------+-------+------------------------+-----------+---------+------------------------------------------+---------+----------------------------------------------+
| id   | select_type | table       | type  | possible_keys          | key       | key_len | ref                                      | rows    | Extra                                        |
+------+-------------+-------------+-------+------------------------+-----------+---------+------------------------------------------+---------+----------------------------------------------+
|    1 | PRIMARY     | ranks       | index | Map,Map_2,MapTimestamp | Timestamp | 8       | NULL                                     |       1 | Using index                                  |
|    1 | PRIMARY     | <derived2>  | ref   | key0                   | key0      | 519     | teeworlds.ranks.Map,teeworlds.ranks.Time |      10 |                                              |
|    2 | DERIVED     | record_race | index | Map,Map_2,MapTimestamp | Timestamp | 8       | NULL                                     | 4298976 | Using index; Using temporary; Using filesort |
+------+-------------+-------------+-------+------------------------+-----------+---------+------------------------------------------+---------+----------------------------------------------+
3 rows in set (0.000 sec)