I finally remember about index after running the rollback which was pretty fast - due to index on job_no.
mysql> EXPLAIN SELECT * FROM comments, jobs WHERE target_type = 'Job' AND target_id = old_id;
+----+-------------+----------+------+---------------+------+---------+------+-------+------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+------+---------------+------+---------+------+-------+------------------------------------------------+
| 1 | SIMPLE | jobs | ALL | NULL | NULL | NULL | NULL | 13498 | |