mysql> SELECT count(DISTINCT `posts`.id) AS count_all FROM `posts`  LEFT OUTER JOIN `categories` ON `categorie_id`;
+-----------+
| count_all |
+-----------+
|     45618 |
+-----------+
1 row in set (4.12 sec)

mysql> explain SELECT count(DISTINCT `posts`.id) AS count_all FROM `posts`  LEFT OUTER JOIN `categories` ON `categorie_id`;
+----+-------------+------------+-------+---------------+------------------------------+---------+------+-------+-------------+
| id | select_type | table      | type  | possible_keys | key                          | key_len | ref  | rows  | Extra       |
+----+-------------+------------+-------+---------------+------------------------------+---------+------+-------+-------------+
|  1 | SIMPLE      | posts      | index | NULL          | index_posts_on_categorie_id  | 5       | NULL | 45861 | Using index |
|  1 | SIMPLE      | categories | index | NULL          | PRIMARY                      | 4       | NULL |   199 | Using index |
+----+-------------+------------+-------+---------------+------------------------------+---------+------+-------+-------------+