MySQLでLimit句を改善する方法

これを試してください：

SELECT post_id
    FROM posts
    ORDER BY post_id DESC
    LIMIT 0, 10;

LIMITによるページネーション とにかく注文しないとあまり意味がなく、問題が解決するはずです。

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)

コメント参加についての最後のコメントについて。 comment(post_id)にインデックスがありますか ？テストデータを使用すると、次の結果が得られます。

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              |
+----+-------------+------------+-------+---------------+---------+---------+------+------+--------------------------+

およびテーブルサイズの参照用：

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)