MySQL倒序索引测试2

测试环境

MySQL Community Server 8.0.17 

准备测试数据

DROP TABLE TB001;
CREATE TABLE TB001(ID INT PRIMARY KEY AUTO_INCREMENT,C1 INT);
INSERT INTO TB001(C1) SELECT 1 FROM information_schema.columns LIMIT 1000;

DROP TABLE TB002;
CREATE TABLE TB002(ID INT PRIMARY KEY AUTO_INCREMENT,C1 INT,C2 INT,C3 CHAR(100));


INSERT INTO TB002(C1,C2,C3)
SELECT T1.ID,T2.ID,REPEAT('您',100) FROM TB001 AS T1,TB001 AS T2 WHERE T1.ID<300 AND T2.ID<3000;

测试SQL:

SELECT * 
FROM TB002 
ORDER BY c1,c2 DESC 
LIMIT 10;

使用普通索引

ALTER TABLE TB002 ADD INDEX IDX_C1_C2(C1,C2);

执行计划为：

+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra          |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------+
|  1 | SIMPLE      | TB002 | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 292380 |   100.00 | Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------+

消耗资源情况：

+--------------------------------+----------+----------+------------+--------------+---------------+-------+
| Status                         | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out | Swaps |
+--------------------------------+----------+----------+------------+--------------+---------------+-------+
| starting                       | 0.000046 | 0.000036 |   0.000006 |            0 |             0 |     0 |
| Executing hook on transaction  | 0.000004 | 0.000003 |   0.000001 |            0 |             0 |     0 |
| starting                       | 0.000007 | 0.000006 |   0.000001 |            0 |             0 |     0 |
| checking permissions           | 0.000005 | 0.000004 |   0.000001 |            0 |             0 |     0 |
| Opening tables                 | 0.000024 | 0.000021 |   0.000004 |            0 |             0 |     0 |
| init                           | 0.000005 | 0.000004 |   0.000001 |            0 |             0 |     0 |
| System lock                    | 0.000007 | 0.000005 |   0.000001 |            0 |             0 |     0 |
| optimizing                     | 0.000004 | 0.000004 |   0.000000 |            0 |             0 |     0 |
| statistics                     | 0.000012 | 0.000009 |   0.000002 |            0 |             0 |     0 |
| preparing                      | 0.000013 | 0.000011 |   0.000002 |            0 |             0 |     0 |
| executing                      | 0.313603 | 0.313474 |   0.000000 |            0 |             0 |     0 |
| end                            | 0.000011 | 0.000006 |   0.000000 |            0 |             0 |     0 |
| query end                      | 0.000004 | 0.000004 |   0.000000 |            0 |             0 |     0 |
| waiting for handler commit     | 0.000008 | 0.000008 |   0.000000 |            0 |             0 |     0 |
| closing tables                 | 0.000006 | 0.000006 |   0.000000 |            0 |             0 |     0 |
| freeing items                  | 0.000013 | 0.000013 |   0.000000 |            0 |             0 |     0 |
| logging slow query             | 0.000033 | 0.000033 |   0.000000 |            0 |             8 |     0 |
| cleaning up                    | 0.000014 | 0.000013 |   0.000000 |            0 |             0 |     0 |
+--------------------------------+----------+----------+------------+--------------+---------------+-------+
总执行时间：0.31381650

使用倒序索引

ALTER TABLE TB002 ADD INDEX IDX_C1_C2_DESC(C1,C2 DESC);

执行计划为：

+----+-------------+-------+------------+-------+---------------+----------------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key            | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------------+---------+------+------+----------+-------+
|  1 | SIMPLE      | TB002 | NULL       | index | NULL          | IDX_C1_C2_DESC | 10      | NULL |   10 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+----------------+---------+------+------+----------+-------+

资源消耗情况：

+--------------------------------+----------+----------+------------+--------------+---------------+-------+
| Status                         | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out | Swaps |
+--------------------------------+----------+----------+------------+--------------+---------------+-------+
| starting                       | 0.000046 | 0.000036 |   0.000007 |            0 |             0 |     0 |
| Executing hook on transaction  | 0.000004 | 0.000002 |   0.000000 |            0 |             0 |     0 |
| starting                       | 0.000006 | 0.000006 |   0.000001 |            0 |             0 |     0 |
| checking permissions           | 0.000005 | 0.000004 |   0.000001 |            0 |             0 |     0 |
| Opening tables                 | 0.000024 | 0.000020 |   0.000004 |            0 |             0 |     0 |
| init                           | 0.000005 | 0.000004 |   0.000000 |            0 |             0 |     0 |
| System lock                    | 0.000006 | 0.000005 |   0.000001 |            0 |             0 |     0 |
| optimizing                     | 0.000004 | 0.000004 |   0.000001 |            0 |             0 |     0 |
| statistics                     | 0.000011 | 0.000009 |   0.000002 |            0 |             0 |     0 |
| preparing                      | 0.000022 | 0.000019 |   0.000003 |            0 |             0 |     0 |
| explaining                     | 0.000033 | 0.000027 |   0.000005 |            0 |             0 |     0 |
| end                            | 0.000004 | 0.000003 |   0.000001 |            0 |             0 |     0 |
| query end                      | 0.000003 | 0.000003 |   0.000000 |            0 |             0 |     0 |
| waiting for handler commit     | 0.000007 | 0.000006 |   0.000002 |            0 |             0 |     0 |
| closing tables                 | 0.000006 | 0.000005 |   0.000001 |            0 |             0 |     0 |
| freeing items                  | 0.000009 | 0.000007 |   0.000001 |            0 |             0 |     0 |
| cleaning up                    | 0.000125 | 0.000107 |   0.000020 |            0 |             0 |     0 |
+--------------------------------+----------+----------+------------+--------------+---------------+-------+
总执行时间：0.00032000

总结

对于查询：

SELECT * 
FROM TB002 
ORDER BY c1,c2 DESC 
LIMIT 10;

分别使用普通索引和倒序索引：

ALTER TABLE TB002 ADD INDEX IDX_C1_C2(C1,C2);
ALTER TABLE TB002 ADD INDEX IDX_C1_C2_DESC(C1,C2 DESC);

对于普通索引，查询无法使用索引且使用filesort，导致严重的性能问题。

对于倒序索引，查询使用倒序索引，能快速返回数据，性能较好。

MySQL从"最开始不支持倒序索引"到"支持倒序索引"，功能在不断完善，但相对于商用数据库来说，这还是很"辣鸡"啊，相同的数据结构下，SQL Server能完美使用"普通索引"来优化查询，MySQL的查询优化器还有很长的路要走！！！