php - MySQL 语句执行时间超过一分钟

Question

我在一个数据库很大的网站上工作。当时表中有 100 万条记录。当我执行查询时，执行需要花费太多时间。下面给出了一个示例查询:

select * from `ratings` order by id limit 499500, 500

每个查询都需要一分钟多的时间，但是当我将表减少到 10,000 条记录时，这个查询执行得很快。

据我所知，一个表中有 100 万条记录没有问题，因为在数据库表中，没有大记录的问题。

我在 Stack Overflow 问题的帮助下在表中使用了 id 索引 How do I add indices to MySQL tables? ，但我还是遇到了同样的问题。

*** 我正在使用 CodeIgniter为项目。

Answer 1

请注意，这并不是建议使用 MyISAM。我只用它来让我的 ids、min、max 和 count 排列起来。所以请忽略引擎。

create table ratings
(   id int auto_increment primary key,
    thing int null
)engine=MyISAM;
insert ratings (thing) values (null),(null),(null),(null),(null),(null),(null),(null),(null);
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;

insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;

insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;

我现在有 470 万行

select count(*),min(id),max(id) from ratings;
+----------+---------+---------+
| count(*) | min(id) | max(id) |
+----------+---------+---------+
|  4718592 |       1 | 4718592 |
+----------+---------+---------+
select * from `ratings` order by id limit 499500, 500;
-- 1 second on a dumpy laptop

.

explain select * from `ratings` order by id limit 499500, 500;
+----+-------------+---------+------+---------------+------+---------+------+---------+----------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows    | Extra          |
+----+-------------+---------+------+---------------+------+---------+------+---------+----------------+
|  1 | SIMPLE      | ratings | ALL  | NULL          | NULL | NULL    | NULL | 4718592 | Using filesort |
+----+-------------+---------+------+---------------+------+---------+------+---------+----------------+

.

explain select * from `ratings` where id>=499501 limit 500;
+----+-------------+---------+-------+---------------+---------+---------+------+---------+-----------------------+
| id | select_type | table   | type  | possible_keys | key     | key_len | ref  | rows    | Extra                 |
+----+-------------+---------+-------+---------------+---------+---------+------+---------+-----------------------+
|  1 | SIMPLE      | ratings | range | PRIMARY       | PRIMARY | 4       | NULL | 4198581 | Using index condition |
+----+-------------+---------+-------+---------------+---------+---------+------+---------+-----------------------+

故事的寓意可能是使用 where 子句。

不能排除死锁的可能性。