如何提高MySQL中的限制條款

Question

我有posts表中有10k行，我想通過它創建分頁。所以，我對於這一目的的一個查詢：

SELECT post_id 
    FROM posts 
    LIMIT 0, 10; 

當我Explain該查詢，我得到了一個結果：

所以我不明白爲什麼MySQL的需要遍歷通9976用於查找10個第一行的行？如果有人幫助我優化此查詢，我將非常感激。

我也知道這話題MySQL ORDER BY/LIMIT performance: late row lookups，但即使我修改查詢到下一個問題仍然存在：

SELECT t.post_id 
FROM (
     SELECT post_id 
     FROM posts 
     ORDER BY 
       post_id 
     LIMIT 0, 10 
     ) q 
JOIN posts t 
ON  q.post_id = t.post_id 

更新

@ pala_的解決方案適用於上述簡單情況，但現在正在測試更復雜的查詢時使用inner join。我的目的是加入評論表和帖子表，不幸的是，當我解釋新的查詢仍然遍歷9976行。

Select comm.comment_id 
from comments as comm 
    inner join (
     SELECT post_id 
     FROM posts 
     ORDER BY post_id 
     LIMIT 0, 10 
    ) as paged_post on comm.post_id = paged_post.post_id; 

你有一些想法是什麼原因導致這種MySQL行爲？

Answer 1

試試這個：

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

分頁通過LIMIT沒有多大意義，反正沒有訂貨，它應該解決您的問題。

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) 

關於您最近對評論加入的評論。你有comment(post_id)的索引嗎？我的測試數據，我發現了以下結果：

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

和檯面尺寸參考：

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)