我正在一个数据库庞大的网站上工作.当时表中有100万条记录.当我执行查询时,执行需要太多时间.下面给出一个示例查询:
select * from `ratings` order by id limit 499500, 500
每个查询都需要超过一分钟,但是当我将表删除到1万条记录时,此查询会快速执行.
正如我所读到的,表中的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.
我已经在Stack Overflow问题How do I add indices to MySQL tables?的帮助下在表中使用了id的索引,但我仍然遇到了同样的问题.
***我正在使用CodeIgniter进行该项目.
解决方法:
请注意,这并不是建议使用MyISAM一分钟.我只是用它来得到我的id,min,max和count来排队.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4.7M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 |
+----+-------------+---------+-------+---------------+---------+---------+------+---------+-----------------------+
故事的道德可能是使用where子句.
人们不能排除陷入僵局的可能性.
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