Stuff

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      | c     | ALL  | customers_id  | NULL         | NULL    | NULL                        | 1003 |   100.00 | Using temporary; Using filesort |
|  1 | SIMPLE      | i     | ref  | customers_id  | customers_id | 4       | revgrp_admin.c.customers_id |   30 |   101.76 |                                 |
+----+-------------+-------+------+---------------+--------------+---------+-----------------------------+------+----------+---------------------------------+