今天聊一下MySQL 8.0新特性 — 降序索引

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。

正文完
2022-05-15