MySQL 降序索引
简介：在本教程中，您将了解MySQL降序索引以及如何利用它来提高查询性能。
MySQL降序索引简介
降序索引是以降序存储键值的索引。在MySQL 8.0之前，您可以DESC在索引定义中指定。但是，MySQL忽略了它。与此同时，MySQL可以以相反的顺序扫描索引，但成本很高。
以下语句创建一个带索引的新表：
CREATE TABLE t(
a INT NOT NULL,
b INT NOT NULL,
INDEX a_asc_b_desc (a ASC, b DESC)
);
当您使用SHOW CREATE TABLE在MySQL 5.7，你会发现，DESC如下图所示被忽略：
mysql> SHOW CREATE TABLE t\G;
*************************** 1. row ***************************
Table: t
Create Table: CREATE TABLE `t` (
`a` int(11) NOT NULL,
`b` int(11) NOT NULL,
KEY `a_asc_b_desc` (`a`,`b`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)
从MySQL 8.0开始，如果DESC在索引定义中使用关键字，则键值将按降序存储。在查询中请求降序时，查询优化器可以利用降序索引。
以下显示了MySQL 8.0中的表结构：
mysql> SHOW CREATE TABLE t\G;
*************************** 1. row ***************************
Table: t
Create Table: CREATE TABLE `t` (
`a` int(11) NOT NULL,
`b` int(11) NOT NULL,
KEY `a_asc_b_desc` (`a`,`b` DESC)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.00 sec)
MySQL降序索引示例
首先，使用不同顺序的四个索引重新创建表t：
DROP TABLE t;
CREATE TABLE t (
a INT,
b INT,
INDEX a_asc_b_asc (a ASC , b ASC),
INDEX a_asc_b_desc (a ASC , b DESC),
INDEX a_desc_b_asc (a DESC , b ASC),
INDEX a_desc_b_desc (a DESC , b DESC)
);
其次，使用下面的存储过程来插入行到t表：
DELIMITER $$
CREATE PROCEDURE insertSampleData(
IN rowCount INT,
IN low INT,
IN high INT
)
BEGIN
DECLARE counter INT DEFAULT 0;
REPEAT
SET counter := counter + 1;
-- insert data
INSERT INTO t(a,b)
VALUES(
ROUND((RAND() * (high-low))+high),
ROUND((RAND() * (high-low))+high)
);
UNTIL counter >= rowCount
END REPEAT;
END$$
DELIMITER ;
存储的过程中插入的行数(rowCount)与之间的值low和high到a和b所述列t表。
让我们10,000在t表中插入行，其中随机值介于1和1000之间：
CALL insertSampleData(10000,1,1000);
第三，从t表中查询具有不同排序顺序的数据：
按升序排列a和b列中的值：
EXPLAIN SELECT
*
FROM
t
ORDER BY a , b; -- use index a_asc_b_asc
这是输出：
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | t | NULL | index | NULL | a_asc_b_asc | 10 | NULL | 10192 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.03 sec)
按升序对a列中的值进行排序，按降序对列 b 中的值进行排序：
EXPLAIN SELECT
*
FROM
t
ORDER BY a , b DESC; -- use index a_asc_b_desc
输出是：
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | t | NULL | index | NULL | a_asc_b_desc | 10 | NULL | 10192 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.01 sec)
按降序对a列中的值进行排序，按升序对列 b 中的值进行排序：
EXPLAIN SELECT
*
FROM
t
ORDER BY a DESC , b; -- use index a_desc_b_asc
以下说明输出：
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | t | NULL | index | NULL | a_desc_b_asc | 10 | NULL | 10192 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.42 sec)
按列a和b降序对值进行排序：
EXPLAIN SELECT
*
FROM
t
ORDER BY a DESC , b DESC; -- use index a_desc_b_desc
以下显示输出：
+----+-------------+-------+------------+-------+---------------+---------------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | t | NULL | index | NULL | a_desc_b_desc | 10 | NULL | 10192 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+---------------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.01 sec)
在本教程中，您学习了如何使用MySQL降序索引来提高查询性能。
转播
