前言
MySQL 8.0終于支持降序索引了。其實(shí)，從語法上，MySQL 4就支持了，但正如官方文檔所言，"they are parsed but ignored"，實(shí)際創(chuàng)建的還是升序索引。
無圖無真相，同一個建表語句，看看MySQL 5.7和8.0的區(qū)別。
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: t1Create Table: CREATE TABLE `t1` ( `c1` int(11) DEFAULT NULL, `c2` int(11) DEFAULT NULL, KEY `idx_c1_c2` (`c1`,`c2`)) ENGINE=InnoDB DEFAULT CHARSET=latin1row in set (0.00 sec)
雖然c2列指定了desc，但在實(shí)際的建表語句中還是將其忽略了。再來看看MySQL 8.0的結(jié)果。
mysql> show create table slowtech.t1/G*************************** 1. row *************************** Table: t1Create 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_cirow in set (0.00 sec)
c2列還是保留了desc子句。
降序索引的意義
如果一個查詢，需要對多個列進(jìn)行排序，且順序要求不一致。在這種場景下，要想避免數(shù)據(jù)庫額外的排序-“filesort”，只能使用降序索引。還是上面這張表，來看看有降序索引和沒有的區(qū)別。
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)
兩者的對比可以看出，MySQL 8.0因?yàn)榻敌蛩饕拇嬖?，避免?ldquo;filesort”。
這其實(shí)是降序索引的主要應(yīng)用場景。如果只對單個列進(jìn)行排序，降序索引的意義不是太大，無論是升序還是降序，升序索引完全可以應(yīng)付。還是同樣的表，看看下面的查詢。
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)
雖然c1是升序索引，但在第二個查詢中，對其進(jìn)行降序排列時(shí)，并沒有進(jìn)行額外的排序，使用的還是索引。在這里，大家容易產(chǎn)生誤區(qū)，以為升序索引就不能用于降序排列，實(shí)際上，對于索引，MySQL不僅支持正向掃描，還可以反向掃描。反向掃描的性能同樣不差。以下是官方對于降序索引的壓測結(jié)果，測試表也只有兩列（a，b），建了一個聯(lián)合索引(a desc,b asc)，感興趣的童鞋可以看看，http://mysqlserverteam.com/mysql-8-0-labs-descending-indexes-in-mysql/
而在8.0中，對于反向掃描，有一個專門的詞進(jìn)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)
終于不再對group by進(jìn)行隱式排序
由于降序索引的引入，MySQL 8.0再也不會對group by操作進(jìn)行隱式排序。
下面看看MySQL 5.7和8中的測試情況
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”，代表查詢中有排序操作，從結(jié)果上看，id列確實(shí)也是升序輸出。
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)
不僅結(jié)果沒有升序輸出，執(zhí)行計(jì)劃中也沒有“Using filesort”。
可見，MySQL 8.0對于group by操作確實(shí)不再進(jìn)行隱式排序。
從5.7升級到8.0，依賴group by隱式排序的業(yè)務(wù)可要小心咯。
參考文檔
http://mysqlserverteam.com/mysql-8-0-labs-descending-indexes-in-mysql/
總結(jié)
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