Something

mysql> EXPLAIN EXTENDED SELECT *  FROM invoices i, customers c WHERE i.customers_id = c.customers_id ORDER BY invoices_id DESC;
+----+-------------+-------+--------+---------------+---------+---------+-----------------------------+-------+----------+----------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref                         | rows  | filtered | Extra          |
+----+-------------+-------+--------+---------------+---------+---------+-----------------------------+-------+----------+----------------+
|  1 | SIMPLE      | i     | ALL    | customers_id  | NULL    | NULL    | NULL                        | 25517 |   100.00 | Using filesort |
|  1 | SIMPLE      | c     | eq_ref | PRIMARY       | PRIMARY | 4       | revgrp_admin.i.customers_id |     1 |   100.00 |                |
+----+-------------+-------+--------+---------------+---------+---------+-----------------------------+-------+----------+----------------+
2 rows in set, 1 warning (0.00 sec)