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MySQL 8.0 新特性之降序索引

时间:2018-06-04  来源:未知  作者:admin666

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
1 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
1 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 |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------------+
1 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 |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
1 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 |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
1 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 |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
1 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 8.0

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 |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
1 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 |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+----------------------------------+
1 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 |
+------+
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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
1 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 |
+------+
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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
1 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/

友情链接