Someone

EXPLAIN SELECT manufacturer_pn FROM product_tags, cnet WHERE product_tags.product_id = cnet.manufacturer_pn;
+----+-------------+--------------+------+---------------+------+---------+------+--------+------------------------------------------------+
| id | select_type | table        | type | possible_keys | key  | key_len | ref  | rows   | Extra                                |
+----+-------------+--------------+------+---------------+------+---------+------+--------+------------------------------------------------+
|  1 | SIMPLE      | product_tags | ALL  | NULL          | NULL |    NULL | NULL |  23924 |                                |
|  1 | SIMPLE      | cnet         | ALL  | PRIMARY       | NULL |    NULL | NULL | 290635 | Range checked for each record (index map: 0x1) |
+----+-------------+--------------+------+---------------+------+---------+------+--------+------------------------------------------------+
2 rows in set (0.00 sec)