Essayez ceci :
SELECT post_id
FROM posts
ORDER BY post_id DESC
LIMIT 0, 10;
Pagination via LIMIT n'a pas beaucoup de sens sans commander de toute façon, et cela devrait résoudre votre problème.
mysql> explain select * from foo;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | foo | index | NULL | PRIMARY | 4 | NULL | 20 | Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain select * from foo limit 0, 10;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | foo | index | NULL | PRIMARY | 4 | NULL | 20 | Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain select * from foo order by id desc limit 0, 10;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | foo | index | NULL | PRIMARY | 4 | NULL | 10 | Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)
Concernant vos derniers commentaires sur la jointure de commentaire. Avez-vous un index sur comment(post_id) ? avec mes données de test, j'obtiens les résultats suivants :
mysql> alter table comments add index pi (post_id);
Query OK, 0 rows affected (0.15 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> explain select c.id from comments c inner join (select id from posts o order by id limit 0, 10) p on c.post_id = p.id;
+----+-------------+------------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+-------+---------------+---------+---------+------+------+--------------------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 10 | |
| 1 | PRIMARY | c | ref | pi | pi | 5 | p.id | 4 | Using where; Using index |
| 2 | DERIVED | o | index | NULL | PRIMARY | 4 | NULL | 10 | Using index |
+----+-------------+------------+-------+---------------+---------+---------+------+------+--------------------------+
et pour la référence de taille de table :
mysql> select count(*) from posts;
+----------+
| count(*) |
+----------+
| 15021 |
+----------+
1 row in set (0.01 sec)
mysql> select count(*) from comments;
+----------+
| count(*) |
+----------+
| 1000 |
+----------+
1 row in set (0.00 sec)
