gistfile1.txt

BAD: 
MariaDB [csc435]> explain extended  SELECT      (   SELECT t2.start           FROM t2          WHERE t1.t1_id = t2.t1_id        ORDER BY t2.t2_id           LIMIT 1     ) AS first_start        FROM t1;
+------+--------------------+-------+-------+---------------+---------+---------+------+------+----------+-------------+
| id   | select_type        | table | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+------+--------------------+-------+-------+---------------+---------+---------+------+------+----------+-------------+
|    1 | PRIMARY            | t1    | index | NULL          | PRIMARY | 4       | NULL | 9999 |   100.00 | Using index |
|    2 | DEPENDENT SUBQUERY | t2    | index | t1_id         | PRIMARY | 4       | NULL |    1 |   100.00 | Using where |
+------+--------------------+-------+-------+---------------+---------+---------+------+------+----------+-------------+
2 rows in set, 2 warnings (0.01 sec)


GOOD:
MariaDB [csc435]> explain extended  SELECT      (   SELECT t2.start           FROM t2          WHERE t1.t1_id = t2.t1_id        ORDER BY t2.t2_id           LIMIT 1     ) AS first_start        FROM t1;
+------+--------------------+-------+-------+---------------+---------+---------+-----------------+------+----------+-------------+
| id   | select_type        | table | type  | possible_keys | key     | key_len | ref             | rows | filtered | Extra       |
+------+--------------------+-------+-------+---------------+---------+---------+-----------------+------+----------+-------------+
|    1 | PRIMARY            | t1    | index | NULL          | PRIMARY | 4       | NULL            | 9999 |   100.00 | Using index |
|    2 | DEPENDENT SUBQUERY | t2    | ref   | t1_id         | t1_id   | 5       | csc435.t1.t1_id |    1 |   100.00 | Using where |
+------+--------------------+-------+-------+---------------+---------+---------+-----------------+------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec)


GOOD is: rows[2].key=t1_id. 
BAD is: rows[2].key=PRIMARY