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关于mysql:3联合索引覆盖索引及最左匹配原则MySQL索引学习

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导语

在数据检索的过程中，常常会有多个列的匹配需要，明天介绍下联结索引的应用以及最左匹配准则的案例。

最左匹配准则作用在联结索引中，如果表中有一个联结索引 （tcol01,tcol02,tcol03），只有当 SQL 应用到tcol01、tcol02 索引的前提下，tcol03的索引才会被应用；同理只有 tcol01 的索引被应用的前提下，tcol02的索引才会被应用。

上面咱们来列举几个例子来阐明。

步骤

应用 mysql_random_data_load 创立测试数据

建库和建表

CREATE DATABASE IF NOT EXISTS test;
CREATE TABLE `test`.`t3` (`id` int(11) NOT NULL AUTO_INCREMENT,
  `tcol01` tinyint(4) DEFAULT NULL,
  `tcol02` smallint(6) DEFAULT NULL,
  `tcol03` mediumint(9) DEFAULT NULL,
  `tcol04` int(11) DEFAULT NULL,
  `tcol05` bigint(20) DEFAULT NULL,
  `tcol06` float DEFAULT NULL,
  `tcol07` double DEFAULT NULL,
  `tcol08` decimal(10,2) DEFAULT NULL,
  `tcol09` date DEFAULT NULL,
  `tcol10` datetime DEFAULT NULL,
  `tcol11` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `tcol12` time DEFAULT NULL,
  `tcol13` year(4) DEFAULT NULL,
  `tcol14` varchar(100) DEFAULT NULL,
  `tcol15` char(2) DEFAULT NULL,
  `tcol16` blob,
  `tcol17` text,
  `tcol18` mediumtext,
  `tcol19` mediumblob,
  `tcol20` longblob,
  `tcol21` longtext,
  `tcol22` mediumtext,
  `tcol23` varchar(3) DEFAULT NULL,
  `tcol24` varbinary(10) DEFAULT NULL,
  `tcol25` enum('a','b','c') DEFAULT NULL,
  `tcol26` set('red','green','blue') DEFAULT NULL,
  `tcol27` float(5,3) DEFAULT NULL,
  `tcol28` double(4,2) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB;

建联结索引

CREATE INDEX idx_tcol123 ON t1(`tcol01`,`tcol02`,`tcol03`);

写入 100w 条测试数据

./mysql_random_data_load test t1 1000000 --user=root --password=GreatSQL --config-file=/data/GreatSQL/my.cnf

联结索引数据存储形式

先对索引中第一列的数据进行排序，而后在满足第一列数据排序的前提下，再对第二列数据进行排序，以此类推。

如下图：

索引最左准则案例

状况 1：三个索引都能应用上

试验 1：仅有 where 子句

# 三个条件都应用上，优化器能够本人调整程序满足索引要求

[root@GreatSQL][test]>explain SELECT /* NO_CACHE */ * FROM t1 WHERE tcol02=167 AND tcol03=202019 AND tcol01=1;
+----+-------------+-------+------------+------+---------------+-------------+---------+-------------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key         | key_len | ref               | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------------------+------+----------+-------+
|  1 | SIMPLE      | t1    | NULL       | ref  | idx_tcol123   | idx_tcol123 | 9       | const,const,const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------------------+------+----------+-------+
1 row in set, 1 warning (0.11 sec)

[root@GreatSQL][test]>explain SELECT /* NO_CACHE */ * FROM t1 WHERE tcol02=167 AND tcol01=1 AND tcol03=202019 ;
+----+-------------+-------+------------+------+---------------+-------------+---------+-------------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key         | key_len | ref               | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------------------+------+----------+-------+
|  1 | SIMPLE      | t1    | NULL       | ref  | idx_tcol123   | idx_tcol123 | 9       | const,const,const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------------------+------+----------+-------+

试验 2：WHERE 加 order by 子句

# 解析进去只有用到 tcol01，tcol02 索引，因为 `explain` 不会统计 `order by` 索引的信息，所有看起来 `key_len` 长度只有 5；当 tcol03 倒序的时候就会用到 `Backward index scan` 性能

[test]>explain SELECT /* NO_CACHE */ * FROM t1 WHERE tcol01=1 AND tcol02=167 order by tcol03;
+----+-------------+-------+------------+------+---------------+-------------+---------+-------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key         | key_len | ref         | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------------+------+----------+-------+
|  1 | SIMPLE      | t1    | NULL       | ref  | idx_tcol123   | idx_tcol123 | 5       | const,const |  269 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)

[test]>explain SELECT /* NO_CACHE */ * FROM t1 WHERE tcol01=1 AND tcol02=167 order by tcol03 desc;
+----+-------------+-------+------------+------+---------------+-------------+---------+-------------+------+----------+---------------------+
| id | select_type | table | partitions | type | possible_keys | key         | key_len | ref         | rows | filtered | Extra               |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------------+------+----------+---------------------+
|  1 | SIMPLE      | t1    | NULL       | ref  | idx_tcol123   | idx_tcol123 | 5       | const,const |  269 |   100.00 | Backward index scan |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------------+------+----------+---------------------+
1 row in set, 1 warning (0.00 sec)

# 当 order by 中的字段不蕴含在联结索引中的时候，就会用到 `Using filesort`
[root@GreatSQL][test]>EXPLAIN SELECT /* NO_CACHE */ * FROM t1 WHERE tcol01=1 AND tcol02=167 order by tcol04;
+----+-------------+-------+------------+------+---------------+-------------+---------+-------------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key         | key_len | ref         | rows | filtered | Extra          |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------------+------+----------+----------------+
|  1 | SIMPLE      | t1    | NULL       | ref  | idx_tcol123   | idx_tcol123 | 5       | const,const |  269 |   100.00 | Using filesort |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)

试验 3：仅 order by 子句

# 优化器默认采取全副扫描了，因为是查问出所有数据，所以全表扫描回比索引更快，节俭回表的工夫

[root@GreatSQL][test]>explain SELECT /* NO_CACHE */ * FROM t1 ORDER BY tcol01,tcol02,tcol03;
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra          |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------+
|  1 | SIMPLE      | t1    | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 941900 |   100.00 | Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------+
1 row in set, 1 warning (0.00 sec)

[root@GreatSQL][test]>explain SELECT /* NO_CACHE */ * FROM t1 force index(`idx_tcol123`) ORDER BY tcol01,tcol02,tcol03;
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+--------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key         | key_len | ref  | rows   | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+--------+----------+-------+
|  1 | SIMPLE      | t1    | NULL       | index | NULL          | idx_tcol123 | 9       | NULL | 941900 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+--------+----------+-------+
1 row in set, 1 warning (0.00 sec)


# 只筛选索引列，也会应用到索引，也就是所谓的笼罩索引
[root@GreatSQL][test]>explain  SELECT /* NO_CACHE */ tcol01,tcol02,tcol03 FROM t1  ORDER BY tcol01,tcol02,tcol03;
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key         | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+--------+----------+-------------+
|  1 | SIMPLE      | t1    | NULL       | index | NULL          | idx_tcol123 | 9       | NULL | 941900 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)


# 如果是筛选局部数据，那么就会应用到索引而不会全表扫描
[root@GreatSQL][test]>explain SELECT /* NO_CACHE */ * FROM t1 ORDER BY tcol01,tcol02,tcol03 limit 10000,11110;
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key         | key_len | ref  | rows  | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+-------+
|  1 | SIMPLE      | t1    | NULL       | index | NULL          | idx_tcol123 | 9       | NULL | 21110 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+-------+
1 row in set, 1 warning (0.00 sec)


# 调整字段程序后，就变成 `Using filesort` 且没有用到索引，所以当应用 order by 语句，确保与联结索引的程序要统一
[root@GreatSQL][test]>explain SELECT /* NO_CACHE */ * FROM t1  ORDER BY tcol02,tcol01,tcol03 limit 10000,11110;
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra          |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------+
|  1 | SIMPLE      | t1    | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 941900 |   100.00 | Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------+
1 row in set, 1 warning (0.00 sec)

状况 2：上面的 SQL 最多只能用到索引 tcol1，tcol2 局部

# tcol02 范畴查找后，导致数据乱序，于是 tcol03 索引条件用不上，同时回呈现 `Using index condition` 和 `Using MRR`。[root@GreatSQL][test]>explain SELECT /* NO_CACHE */ * FROM t1  WHERE tcol01=1 AND tcol02>100 AND tcol03=202019;
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+----------------------------------+
| id | select_type | table | partitions | type  | possible_keys | key         | key_len | ref  | rows  | filtered | Extra                            |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+----------------------------------+
|  1 | SIMPLE      | t1    | NULL       | range | idx_tcol123   | idx_tcol123 | 5       | NULL | 77976 |    10.00 | Using index condition; Using MRR |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+----------------------------------+
1 row in set, 1 warning (0.00 sec)

[root@GreatSQL][test]>explain SELECT /* NO_CACHE */ * FROM t1  WHERE tcol01=1 AND tcol02>100;
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+----------------------------------+
| id | select_type | table | partitions | type  | possible_keys | key         | key_len | ref  | rows  | filtered | Extra                            |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+----------------------------------+
|  1 | SIMPLE      | t1    | NULL       | range | idx_tcol123   | idx_tcol123 | 5       | NULL | 77976 |   100.00 | Using index condition; Using MRR |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+----------------------------------+
1 row in set, 1 warning (0.00 sec)

[root@GreatSQL][test]>explain SELECT /* NO_CACHE */ * FROM t1  WHERE tcol01=1 AND tcol03=202019 ORDER BY tcol02;
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+--------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key         | key_len | ref   | rows   | filtered | Extra                 |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+--------+----------+-----------------------+
|  1 | SIMPLE      | t1    | NULL       | ref  | idx_tcol123   | idx_tcol123 | 2       | const | 126670 |    10.00 | Using index condition |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+--------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

# 关掉 `Using index condition` 和 `Using MRR` 后再看一下执行打算，理论测试效率要高很多。# 这是因为 ICP 缩小了引擎层和 server 层之间的数据传输和回表申请，不满足条件的申请，间接过滤无需回表
# 实际上开启 ICP 后下面语句有用到 tcol03 的索引局部。[root@GreatSQL][test]>SET optimizer_switch = 'MRR=off';
[root@GreatSQL][test]>SET optimizer_switch = 'index_condition_pushdown=off';

[root@GreatSQL][test]>explain SELECT /* NO_CACHE */ * FROM t1  WHERE tcol01=1 AND tcol02>100 AND tcol03=202019;
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key         | key_len | ref  | rows  | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
|  1 | SIMPLE      | t1    | NULL       | range | idx_tcol123   | idx_tcol123 | 5       | NULL | 77976 |    10.00 | Using where |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

1 row in set (1.81 sec) /* 敞开 ICP 和 MRR 后执行工夫 */
1 row in set (0.01 sec) /* 开启 ICP 和 MRR 后执行工夫 */


[root@GreatSQL][test]>explain SELECT /* NO_CACHE */ * FROM t1  WHERE tcol01=1 AND tcol02>100;
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key         | key_len | ref  | rows  | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
|  1 | SIMPLE      | t1    | NULL       | range | idx_tcol123   | idx_tcol123 | 5       | NULL | 77976 |   100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

40252 rows in set (2.04 sec) /* 敞开 ICP 和 MRR 后执行工夫 */
40252 rows in set (1.58 sec) /* 开启 ICP 和 MRR 后执行工夫 */


[root@GreatSQL][test]>explain SELECT /* NO_CACHE */ * FROM t1  WHERE tcol01=1 AND tcol03=202019 ORDER BY tcol02;
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key         | key_len | ref   | rows   | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+--------+----------+-------------+
|  1 | SIMPLE      | t1    | NULL       | ref  | idx_tcol123   | idx_tcol123 | 2       | const | 126670 |    10.00 | Using where |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

1 row in set (1.99 sec) /* 敞开 ICP 和后执行工夫 */
1 row in set (0.01 sec) /* 开启 ICP 和后执行工夫 */

Using index condition 请看文章 https://mp.weixin.qq.com/s/pt6mr3Ge1ya2aa6WlrpIvQ

Using MRR 前面再介绍。

状况 3：上面的 SQL 用不到索引

[root@GreatSQL][test]>explain SELECT /* NO_CACHE */ * FROM t1 WHERE tcol02=167;
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
|  1 | SIMPLE      | t1    | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 941900 |    10.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)


[root@GreatSQL][test]>explain SELECT /* NO_CACHE */ * FROM t1 WHERE tcol02=167 AND tcol03 >=1;
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
|  1 | SIMPLE      | t1    | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 941900 |     3.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.01 sec)


[root@GreatSQL][test]>explain SELECT /* NO_CACHE */ * FROM t1 ORDER BY tcol02 limit 10000,11000;
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra          |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------+
|  1 | SIMPLE      | t1    | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 941900 |   100.00 | Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------+
1 row in set, 1 warning (0.00 sec)

联结索引有哪些益处

  • 1. 缩小开销。建一个联结索引 (tcol01, tcol02, tcol03)，相当于建设三个索引(tcol01),(tcol01,tcol02),(tcol01,tcol02,tcol03) 的性能。每个索引都会占用写入开销和磁盘开销，对于大量数据的表，应用联结索引会大大的缩小开销。
  • 2. 笼罩索引。对联结索引(tcol01, tcol02, tcol03)，如果有如下的 SQL: select tcol01,tcol02,tcol03 from t1 where tcol01=? and tcol02=? and tcol03=? 那么就能够应用到笼罩索引的性能，查问数据无需回表，缩小随机 IO。
  • 3. 效率高。多列条件的查问下，索引列越多，通过索引筛选出的数据就越少。

联结索引应用倡议

  • 1. 查问条件中的 where、order by、group by 波及多个字段，个别须要创立多列索引，比方后面的 select * from t1 where tcol01=100 and tcol02=50;
  • 2. 创立联结索引的时候，要将区分度高的字段放在后面，如果有一张学生表蕴含学号和姓名，那么在建设联结索引的时候，学号放在姓名后面，因为学号是唯一性的，能过滤更多的数据。
  • 3. 尽量避免 >、<、between、or、like 首字母为 % 的范畴查找，范畴查问可能导致无奈应用索引。
  • 4. 只筛选须要的数据字段，满足笼罩索引的要求，不要用 select * 筛选所有列数据。

Enjoy GreatSQL :)

文章举荐：

面向金融级利用的 GreatSQL 正式开源
https://mp.weixin.qq.com/s/cI…

Changes in GreatSQL 8.0.25 (2021-8-18)
https://mp.weixin.qq.com/s/qc…

MGR 及 GreatSQL 资源汇总
https://mp.weixin.qq.com/s/qX…

GreatSQL MGR FAQ
https://mp.weixin.qq.com/s/J6…

在 Linux 下源码编译装置 GreatSQL/MySQL
https://mp.weixin.qq.com/s/WZ…

# 对于 GreatSQL

GreatSQL 是由万里数据库保护的 MySQL 分支，专一于晋升 MGR 可靠性及性能，反对 InnoDB 并行查问个性，是实用于金融级利用的 MySQL 分支版本。

Gitee:

https://gitee.com/GreatSQL/Gr…

GitHub:

https://github.com/GreatSQL/G…

Bilibili：

https://space.bilibili.com/13…

微信 &QQ 群：

可搜寻增加 GreatSQL 社区助手微信好友，发送验证信息“加群”退出 GreatSQL/MGR 交换微信群

QQ 群：533341697

微信小助手：wanlidbc

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