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/