Prova questo:
SELECT post_id
FROM posts
ORDER BY post_id DESC
LIMIT 0, 10;
Impaginazione tramite LIMIT non ha molto senso senza ordinare comunque e dovrebbe risolvere il tuo problema.
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)
Per quanto riguarda i tuoi ultimi commenti sul commento, unisciti. Hai un indice su comment(post_id) ? con i miei dati di test ottengo i seguenti risultati:
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 |
+----+-------------+------------+-------+---------------+---------+---------+------+------+--------------------------+
e per riferimento alle dimensioni della tabella:
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)
