前言
相信大家都知道，索引是有序的；不过，在mysql之前版本中，只支持升序索引，不支持降序索引，这会带来一些问题；在最新的mysql 8.0版本中，终于引入了降序索引，接下来我们就来看一看。
降序索引
单列索引
（1）查看测试表结构
mysql> show create table sbtest1\g
*************************** 1. row ***************************
table: sbtest1
create table: create table `sbtest1` (
`id` int unsigned not null auto_increment,
`k` int unsigned not null default '0',
`c` char(120) not null default '',
`pad` char(60) not null default '',
primary key (`id`),
key `k_1` (`k`)
) engine=innodb auto_increment=1000001 default charset=utf8mb4 collate=utf8mb4_0900_ai_ci max_rows=1000000
1 row in set (0.00 sec)
（2）执行sql语句order by ... limit n，默认是升序，可以使用到索引
mysql> explain select * from sbtest1 order by k limit 10; +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | extra | +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+-------+ | 1 | simple | sbtest1 | null | index | null | k_1 | 4 | null | 10 | 100.00 | null | +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec)
（3）执行sql语句order by ... desc limit n，如果是降序的话，无法使用索引，虽然可以相反顺序扫描，但性能会受到影响
mysql> explain select * from sbtest1 order by k desc limit 10; +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+---------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | extra | +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+---------------------+ | 1 | simple | sbtest1 | null | index | null | k_1 | 4 | null | 10 | 100.00 | backward index scan | +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+---------------------+ 1 row in set, 1 warning (0.00 sec)
（4）创建降序索引
mysql> alter table sbtest1 add index k_2(k desc); query ok, 0 rows affected (6.45 sec) records: 0 duplicates: 0 warnings: 0
（5）再次执行sql语句order by ... desc limit n，可以使用到降序索引
mysql> explain select * from sbtest1 order by k desc limit 10; +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | extra | +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+-------+ | 1 | simple | sbtest1 | null | index | null | k_2 | 4 | null | 10 | 100.00 | null | +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec)
多列索引
（1）查看测试表结构
mysql> show create table sbtest1\g
*************************** 1. row ***************************
table: sbtest1
create table: create table `sbtest1` (
`id` int unsigned not null auto_increment,
`k` int unsigned not null default '0',
`c` char(120) not null default '',
`pad` char(60) not null default '',
primary key (`id`),
key `k_1` (`k`),
key `idx_c_pad_1` (`c`,`pad`)
) engine=innodb auto_increment=1000001 default charset=utf8mb4 collate=utf8mb4_0900_ai_ci max_rows=1000000
1 row in set (0.00 sec)
（2）对于多列索引来说，如果没有降序索引的话，那么只有sql 1才能用到索引，sql 4能用相反顺序扫描，其他两条sql语句只能走全表扫描，效率非常低
sql 1：select * from sbtest1 order by c,pad limit 10;
sql 2：select * from sbtest1 order by c,pad desc limit 10;
sql 3：select * from sbtest1 order by c desc,pad limit 10;
sql 4：explain select * from sbtest1 order by c desc,pad desc limit 10;
mysql> explain select * from sbtest1 order by c,pad limit 10; +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | extra | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ | 1 | simple | sbtest1 | null | index | null | idx_c_pad_1 | 720 | null | 10 | 100.00 | null | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec) mysql> explain select * from sbtest1 order by c,pad desc limit 10; +----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+----------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | extra | +----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+----------------+ | 1 | simple | sbtest1 | null | all | null | null | null | null | 950738 | 100.00 | using filesort | +----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+----------------+ 1 row in set, 1 warning (0.00 sec) mysql> explain select * from sbtest1 order by c desc,pad limit 10; +----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+----------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | extra | +----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+----------------+ | 1 | simple | sbtest1 | null | all | null | null | null | null | 950738 | 100.00 | using filesort | +----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+----------------+ 1 row in set, 1 warning (0.01 sec) mysql> explain select * from sbtest1 order by c desc,pad desc limit 10; +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+---------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | extra | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+---------------------+ | 1 | simple | sbtest1 | null | index | null | idx_c_pad_1 | 720 | null | 10 | 100.00 | backward index scan | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+---------------------+ 1 row in set, 1 warning (0.00 sec)
（3）创建相应的降序索引
mysql> alter table sbtest1 add index idx_c_pad_2(c,pad desc); query ok, 0 rows affected (1 min 11.27 sec) records: 0 duplicates: 0 warnings: 0 mysql> alter table sbtest1 add index idx_c_pad_3(c desc,pad); query ok, 0 rows affected (1 min 14.22 sec) records: 0 duplicates: 0 warnings: 0 mysql> alter table sbtest1 add index idx_c_pad_4(c desc,pad desc); query ok, 0 rows affected (1 min 8.70 sec) records: 0 duplicates: 0 warnings: 0
（4）再次执行sql，均能使用到降序索引，效率大大提升
mysql> explain select * from sbtest1 order by c,pad desc limit 10; +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | extra | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ | 1 | simple | sbtest1 | null | index | null | idx_c_pad_2 | 720 | null | 10 | 100.00 | null | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec) mysql> explain select * from sbtest1 order by c desc,pad limit 10; +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | extra | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ | 1 | simple | sbtest1 | null | index | null | idx_c_pad_3 | 720 | null | 10 | 100.00 | null | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec) mysql> explain select * from sbtest1 order by c desc,pad desc limit 10; +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | extra | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ | 1 | simple | sbtest1 | null | index | null | idx_c_pad_4 | 720 | null | 10 | 100.00 | null | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec)
总结
mysql 8.0引入的降序索引，最重要的作用是，解决了多列排序可能无法使用索引的问题，从而可以覆盖更多的应用场景。
以上就是mysql8.0中的降序索引的详细内容，更多关于mysql 降序索引的资料请关注其它相关文章！
