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。

createtableslowtech.t1(c1int,c2int,indexidx_c1_c2(c1,c2desc));

MySQL 5.7

mysql>showcreatetableslowtech.t1\G***************************1.row***************************Table:t1CreateTable:CREATETABLE`t1`(`c1`int(11)DEFAULTNULL,`c2`int(11)DEFAULTNULL,KEY`idx_c1_c2`(`c1`,`c2`))ENGINE=InnoDBDEFAULTCHARSET=latin1rowinset(0.00sec)

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>showcreatetableslowtech.t1\G***************************1.row***************************Table:t1CreateTable:CREATETABLE`t1`(`c1`int(11)DEFAULTNULL,`c2`int(11)DEFAULTNULL,KEY`idx_c1_c2`(`c1`,`c2`DESC))ENGINE=InnoDBDEFAULTCHARSET=utf8mb4COLLATE=utf8mb4_0900_ai_cirowinset(0.00sec)

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>explainselect*fromslowtech.t1orderbyc1,c2desc;+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------------+|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|Usingindex;Usingfilesort|+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------------+rowinset,1warning(0.00sec)

MySQL 8.0

mysql>explainselect*fromslowtech.t1orderbyc1,c2desc;+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+|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|Usingindex|+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+rowinset,1warning(0.00sec)

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>explainselect*fromslowtech.t1orderbyc1;+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+|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|Usingindex|+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+rowinset,1warning(0.00sec)mysql>explainselect*fromslowtech.t1orderbyc1desc;+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+|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|Usingindex|+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+rowinset,1warning(0.00sec)

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>explainselect*fromslowtech.t1orderbyc1;+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+|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|Usingindex|+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+rowinset,1warning(0.00sec)mysql>explainselect*fromslowtech.t1orderbyc1desc;+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+----------------------------------+|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|Backwardindexscan;Usingindex|+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+----------------------------------+rowinset,1warning(0.00sec)

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

createtableslowtech.t1(idint);insertintoslowtech.t1values(2);insertintoslowtech.t1values(3);insertintoslowtech.t1values(1);

MySQL 5.7

mysql>select*fromslowtech.t1groupbyid;+------+|id|+------+|1||2||3|+------+rowsinset(0.00sec)mysql>explainselect*fromslowtech.t1groupbyid;+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+|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|Usingtemporary;Usingfilesort|+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+rowinset,1warning(0.00sec)

“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*fromslowtech.t1groupbyid;+------+|id|+------+|2||3||1|+------+rowsinset(0.00sec)mysql>explainselect*fromslowtech.t1groupbyid;+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+|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|Usingtemporary|+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+rowinset,1warning(0.01sec)

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。