MySQL倒序索引测试1

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
WHERE C1=20
ORDER BY C2 DESC 
LIMIT 10;

使用普通索引

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

执行计划：

+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+---------------------+
| id | select_type | table | partitions | type | possible_keys | key       | key_len | ref   | rows | filtered | Extra               |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+---------------------+
|  1 | SIMPLE      | TB002 | NULL       | ref  | IDX_C1_C2     | IDX_C1_C2 | 5       | const | 1000 |   100.00 | Backward index scan |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+---------------------+

资源消耗情况：

+--------------------------------+----------+----------+------------+--------------+---------------+-------+
| Status                         | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out | Swaps |
+--------------------------------+----------+----------+------------+--------------+---------------+-------+
| starting                       | 0.000048 | 0.000000 |   0.000043 |            0 |             0 |     0 |
| Executing hook on transaction  | 0.000004 | 0.000000 |   0.000002 |            0 |             0 |     0 |
| starting                       | 0.000006 | 0.000000 |   0.000006 |            0 |             0 |     0 |
| checking permissions           | 0.000005 | 0.000000 |   0.000005 |            0 |             0 |     0 |
| Opening tables                 | 0.000025 | 0.000000 |   0.000026 |            0 |             0 |     0 |
| init                           | 0.000005 | 0.000000 |   0.000004 |            0 |             0 |     0 |
| System lock                    | 0.000006 | 0.000000 |   0.000006 |            0 |             0 |     0 |
| optimizing                     | 0.000007 | 0.000000 |   0.000007 |            0 |             0 |     0 |
| statistics                     | 0.000042 | 0.000000 |   0.000043 |            0 |             0 |     0 |
| preparing                      | 0.000011 | 0.000000 |   0.000010 |            0 |             0 |     0 |
| executing                      | 0.000066 | 0.000000 |   0.000066 |            0 |             0 |     0 |
| end                            | 0.000003 | 0.000000 |   0.000002 |            0 |             0 |     0 |
| query end                      | 0.000003 | 0.000000 |   0.000003 |            0 |             0 |     0 |
| waiting for handler commit     | 0.000006 | 0.000000 |   0.000006 |            0 |             0 |     0 |
| closing tables                 | 0.000004 | 0.000000 |   0.000004 |            0 |             0 |     0 |
| freeing items                  | 0.000008 | 0.000000 |   0.000008 |            0 |             0 |     0 |
| cleaning up                    | 0.000007 | 0.000000 |   0.000007 |            0 |             0 |     0 |
+--------------------------------+----------+----------+------------+--------------+---------------+-------+

总执行时间：0.00025375

使用倒序索引

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

执行计划：

+----+-------------+-------+------------+------+----------------+----------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys  | key            | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+----------------+----------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | TB002 | NULL       | ref  | IDX_C1_C2_DESC | IDX_C1_C2_DESC | 5       | const | 1000 |   100.00 | NULL  |
+----+-------------+-------+------------+------+----------------+----------------+---------+-------+------+----------+-------+

资源消耗情况：

+--------------------------------+----------+----------+------------+--------------+---------------+-------+
| Status                         | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out | Swaps |
+--------------------------------+----------+----------+------------+--------------+---------------+-------+
| starting                       | 0.000051 | 0.000040 |   0.000008 |            0 |             0 |     0 |
| Executing hook on transaction  | 0.000004 | 0.000002 |   0.000000 |            0 |             0 |     0 |
| starting                       | 0.000006 | 0.000005 |   0.000001 |            0 |             0 |     0 |
| checking permissions           | 0.000005 | 0.000005 |   0.000001 |            0 |             0 |     0 |
| Opening tables                 | 0.000026 | 0.000021 |   0.000004 |            0 |             0 |     0 |
| init                           | 0.000005 | 0.000004 |   0.000001 |            0 |             0 |     0 |
| System lock                    | 0.000006 | 0.000006 |   0.000001 |            0 |             0 |     0 |
| optimizing                     | 0.000008 | 0.000006 |   0.000002 |            0 |             0 |     0 |
| statistics                     | 0.000045 | 0.000038 |   0.000007 |            0 |             0 |     0 |
| preparing                      | 0.000012 | 0.000010 |   0.000002 |            0 |             0 |     0 |
| executing                      | 0.000071 | 0.000059 |   0.000012 |            0 |             0 |     0 |
| end                            | 0.000004 | 0.000004 |   0.000001 |            0 |             0 |     0 |
| query end                      | 0.000004 | 0.000003 |   0.000000 |            0 |             0 |     0 |
| waiting for handler commit     | 0.000007 | 0.000006 |   0.000002 |            0 |             0 |     0 |
| closing tables                 | 0.000021 | 0.000018 |   0.000003 |            0 |             0 |     0 |
| freeing items                  | 0.000013 | 0.000010 |   0.000002 |            0 |             0 |     0 |
| cleaning up                    | 0.000009 | 0.000007 |   0.000002 |            0 |             0 |     0 |
+--------------------------------+----------+----------+------------+--------------+---------------+-------+

总执行时间：0.00029675

总结

对于查询：

SELECT *
FROM TB002
WHERE C1=20
ORDER BY C2 DESC 
LIMIT 10;

对下面的普通索引还是倒序索引，对性能影响较小。

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