mysql> select * from i;
+---------+---------+
| item_id | feed_id |
+---------+---------+
|       1 |       1 | 
|       2 |       1 | 
|       3 |       1 | 
|       4 |       1 | 
|       5 |       1 | 
|       6 |       2 | 
|       7 |       2 | 
|       8 |       2 | 
|       9 |       2 | 
|      10 |       2 | 
|      11 |       2 | 
+---------+---------+
11 rows in set (0.00 sec)

mysql> select * from f;
+---------+------------+---------------+
| feed_id | url_md5_fk | first_item_id |
+---------+------------+---------------+
|       1 | aaa        |             3 | 
|       2 | bbb        |             9 | 
+---------+------------+---------------+
2 rows in set (0.00 sec)


mysql> delete i from i,f where i.feed_id = f.feed_id and item_id < first_item_id;
Query OK, 5 rows affected (0.01 sec)

mysql> select * from f;
+---------+------------+---------------+
| feed_id | url_md5_fk | first_item_id |
+---------+------------+---------------+
|       1 | aaa        |             3 | 
|       2 | bbb        |             9 | 
+---------+------------+---------------+
2 rows in set (0.00 sec)

mysql> select * from i;
+---------+---------+
| item_id | feed_id |
+---------+---------+
|       3 |       1 | 
|       4 |       1 | 
|       5 |       1 | 
|       9 |       2 | 
|      10 |       2 | 
|      11 |       2 | 
+---------+---------+
6 rows in set (0.00 sec)