[dinke]

nisi u pravu jer:

mysql> explain SELECT m.*, c.description FROM master m
    -> LEFT JOIN categories c ON m.category_id = c.category_id;
+----+-------------+-------+--------+---------------+---------+---------+-----------------------+-------+-------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref                | rows  | Extra |
+----+-------------+-------+--------+---------------+---------+---------+-----------------------+-------+-------+
|  1 | SIMPLE      | m     | ALL    | NULL          | NULL    |    NULL | NULL                  | 40427 |       |
|  1 | SIMPLE      | c     | eq_ref | PRIMARY       | PRIMARY |       4 | measure.m.category_id |     1 |       |
+----+-------------+-------+--------+---------------+---------+---------+-----------------------+-------+-------+
2 rows in set (0.00 sec)

mysql> alter table master add index category_id(category_id);
Query OK, 40000 rows affected (1.06 sec)
Records: 40000  Duplicates: 0  Warnings: 0

mysql> explain SELECT m.*, c.description FROM master m
    ->  LEFT JOIN categories c ON m.category_id = c.category_id;
+----+-------------+-------+--------+---------------+-------------+---------+-----------------------+-------+-------------+
| id | select_type | table | type   | possible_keys | key         | key_len | ref                   | rows  | Extra       |
+----+-------------+-------+--------+---------------+-------------+---------+-----------------------+-------+-------------+
|  1 | SIMPLE      | m     | index  | NULL          | category_id |       5 | NULL                  | 40715 | Using index |
|  1 | SIMPLE      | c     | eq_ref | PRIMARY       | PRIMARY     |       4 | measure.m.category_id |     1 |             |
+----+-------------+-------+--------+---------------+-------------+---------+-----------------------+-------+-------------+
2 rows in set (0.00 sec)

Dakle, nakon dodavanja index on se koristi (type kolona u explain outputu).