MySQL8新特性：降序索引详解

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/

总结

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