MySQL8新特性:降序索引详解
作者:iVictor 时间:2024-01-14 10:03:43
前言
MySQL 8.0终于支持降序索引了。其实,从语法上,MySQL 4就支持了,但正如官方文档所言,"they are parsed but ignored",实际创建的还是升序索引。
无图无真相,同一个建表语句,看看MySQL 5.7和8.0的区别。
create table slowtech.t1(c1 int,c2 int,index idx_c1_c2(c1,c2 desc));
MySQL 5.7
mysql> show create table slowtech.t1\G
*************************** 1. row ***************************
Table: t1
Create Table: CREATE TABLE `t1` (
`c1` int(11) DEFAULT NULL,
`c2` int(11) DEFAULT NULL,
KEY `idx_c1_c2` (`c1`,`c2`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
row in set (0.00 sec)
虽然c2列指定了desc,但在实际的建表语句中还是将其忽略了。再来看看MySQL 8.0的结果。
mysql> show create table slowtech.t1\G
*************************** 1. row ***************************
Table: t1
Create Table: CREATE TABLE `t1` (
`c1` int(11) DEFAULT NULL,
`c2` int(11) DEFAULT NULL,
KEY `idx_c1_c2` (`c1`,`c2` DESC)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
row in set (0.00 sec)
c2列还是保留了desc子句。
降序索引的意义
如果一个查询,需要对多个列进行排序,且顺序要求不一致。在这种场景下,要想避免数据库额外的排序-“filesort”,只能使用降序索引。还是上面这张表,来看看有降序索引和没有的区别。
MySQL 5.7
mysql> explain select * from slowtech.t1 order by c1,c2 desc;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------------+
| 1 | SIMPLE | t1 | NULL | index | NULL | idx_c1_c2 | 10 | NULL | 1 | 100.00 | Using index; Using filesort |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------------+
row in set, 1 warning (0.00 sec)
MySQL 8.0
mysql> explain select * from slowtech.t1 order by c1,c2 desc;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| 1 | SIMPLE | t1 | NULL | index | NULL | idx_c1_c2 | 10 | NULL | 1 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
row in set, 1 warning (0.00 sec)
两者的对比可以看出,MySQL 8.0因为降序索引的存在,避免了“filesort”。
这其实是降序索引的主要应用场景。如果只对单个列进行排序,降序索引的意义不是太大,无论是升序还是降序,升序索引完全可以应付。还是同样的表,看看下面的查询。
MySQL 5.7
mysql> explain select * from slowtech.t1 order by c1;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| 1 | SIMPLE | t1 | NULL | index | NULL | idx_c1_c2 | 10 | NULL | 1 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
row in set, 1 warning (0.00 sec)
mysql> explain select * from slowtech.t1 order by c1 desc;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| 1 | SIMPLE | t1 | NULL | index | NULL | idx_c1_c2 | 10 | NULL | 1 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
row in set, 1 warning (0.00 sec)
虽然c1是升序索引,但在第二个查询中,对其进行降序排列时,并没有进行额外的排序,使用的还是索引。在这里,大家容易产生误区,以为升序索引就不能用于降序排列,实际上,对于索引,MySQL不仅支持正向扫描,还可以反向扫描。反向扫描的性能同样不差。以下是官方对于降序索引的压测结果,测试表也只有两列(a,b),建了一个联合索引(a desc,b asc),感兴趣的童鞋可以看看,http://mysqlserverteam.com/mysql-8-0-labs-descending-indexes-in-mysql/
而在8.0中,对于反向扫描,有一个专门的词进行描述“Backward index scan”。
mysql> explain select * from slowtech.t1 order by c1;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| 1 | SIMPLE | t1 | NULL | index | NULL | idx_c1_c2 | 10 | NULL | 1 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
row in set, 1 warning (0.00 sec)
mysql> explain select * from slowtech.t1 order by c1 desc;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+----------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+----------------------------------+
| 1 | SIMPLE | t1 | NULL | index | NULL | idx_c1_c2 | 10 | NULL | 1 | 100.00 | Backward index scan; Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+----------------------------------+
row in set, 1 warning (0.00 sec)
终于不再对group by进行隐式排序
由于降序索引的引入,MySQL 8.0再也不会对group by操作进行隐式排序。
下面看看MySQL 5.7和8中的测试情况
create table slowtech.t1(id int);
insert into slowtech.t1 values(2);
insert into slowtech.t1 values(3);
insert into slowtech.t1 values(1);
MySQL 5.7
mysql> select * from slowtech.t1 group by id;
+------+
| id |
+------+
| 1 |
| 2 |
| 3 |
+------+
rows in set (0.00 sec)
mysql> explain select * from slowtech.t1 group by id;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
| 1 | SIMPLE | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100.00 | Using temporary; Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
row in set, 1 warning (0.00 sec)
“Using filesort”,代表查询中有排序操作,从结果上看,id列确实也是升序输出。
MySQL 8.0
mysql> select * from slowtech.t1 group by id;
+------+
| id |
+------+
| 2 |
| 3 |
| 1 |
+------+
rows in set (0.00 sec)
mysql> explain select * from slowtech.t1 group by id;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 1 | SIMPLE | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100.00 | Using temporary |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
row in set, 1 warning (0.01 sec)
不仅结果没有升序输出,执行计划中也没有“Using filesort”。
可见,MySQL 8.0对于group by操作确实不再进行隐式排序。
从5.7升级到8.0,依赖group by隐式排序的业务可要小心咯。
参考文档
http://mysqlserverteam.com/mysql-8-0-labs-descending-indexes-in-mysql/
来源:https://www.cnblogs.com/ivictor/p/9072361.html
![](/images/zang.png)
![](/images/jiucuo.png)
猜你喜欢
利用Python中xlwt模块操作excel的示例详解
pytorch使用 to 进行类型转换方式
![](https://img.aspxhome.com/file/2023/6/101576_0s.jpg)
想学画画?python满足你!
![](https://img.aspxhome.com/file/2023/1/114601_0s.png)
ASP 支持中文的len(),left(),right()的函数代码
JavaScript table的排序类
js实现网页加载loading的显示
![](https://img.aspxhome.com/file/UploadPic/200711/6/2007116122626422s.jpg)
Python OpenCV Hough直线检测算法的原理实现
![](https://img.aspxhome.com/file/2023/9/121119_0s.png)
MySQL非空约束(not null)案例讲解
Python基类函数的重载与调用实例分析
Python 中的函数装饰器和闭包详解
![](https://img.aspxhome.com/file/2023/8/131628_0s.png)
python实现12306火车票查询器
![](https://img.aspxhome.com/file/2023/8/126968_0s.jpg)
PyQt4 treewidget 选择改变颜色,并设置可编辑的方法
![](https://img.aspxhome.com/file/2023/1/84101_0s.jpg)
仅用几行Python代码就能复制她的U盘文件?
![](https://img.aspxhome.com/file/2023/5/114545_0s.jpg)
Python 用户登录验证的小例子
各种页面定时跳转(倒计时跳转)代码总结
Python生成随机数详解流程
![](https://img.aspxhome.com/file/2023/4/100954_0s.gif)
python爬虫Scrapy框架:媒体管道原理学习分析
![](https://img.aspxhome.com/file/2023/8/87738_0s.jpg)
vue3获取当前路由地址
![](https://img.aspxhome.com/file/2023/6/126546_0s.jpg)