前言
相信大家都知道，索引是有序的；不過，在MySQL之前版本中，只支持升序索引，不支持降序索引，這會帶來一些問題；在最新的MySQL 8.0版本中，終于引入了降序索引，接下來我們就來看一看。
降序索引
單列索引
（1）查看測試表結(jié)構(gòu)
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）執(zhí)行SQL語句order by ... limit n，默認(rèn)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）執(zhí)行SQL語句order by ... desc limit n，如果是降序的話，無法使用索引，雖然可以相反順序掃描，但性能會受到影響
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）創(chuàng)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）再次執(zhí)行SQL語句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）查看測試表結(jié)構(gòu)
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）對于多列索引來說，如果沒有降序索引的話，那么只有SQL 1才能用到索引，SQL 4能用相反順序掃描，其他兩條SQL語句只能走全表掃描，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）創(chuàng)建相應(yīng)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）再次執(zhí)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)
總結(jié)
MySQL 8.0引入的降序索引，最重要的作用是，解決了多列排序可能無法使用索引的問題，從而可以覆蓋更多的應(yīng)用場景。
以上就是MySQL8.0中的降序索引的詳細(xì)內(nèi)容，更多關(guān)于MySQL 降序索引的資料請關(guān)注腳本之家其它相關(guān)文章！
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