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首页 手记 3.联合索引、覆盖索引及最左匹配原则|MySQL索引学习

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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MySQL
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导语

在数据检索的过程中，经常会有多个列的匹配需求，今天介绍下联合索引的使用以及最左匹配原则的案例。

最左匹配原则作用在联合索引中，假如表中有一个联合索引（tcol01,tcol02,tcol03），只有当SQL使用到tcol01、tcol02索引的前提下，tcol03的索引才会被使用；同理只有tcol01的索引被使用的前提下，tcol02的索引才会被使用。

下面我们来列举几个例子来说明。

步骤

使用 mysql_random_data_load 创建测试数据

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;

建联合索引

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

联合索引数据存储方式

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：

索引最左原则案例

情况1：三个索引都能使用上

实验1：仅有where子句

# 三个条件都使用上，优化器可以自己调整顺序满足索引要求

[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  |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------------------+------+----------+-------+

实验2：WHERE 加 order by子句

# 解析出来只有用到tcol01，tcol02索引，由于`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`功能

[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中的字段不包含在联合索引中的时候，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)

实验3：仅order by子句

# 优化器默认采取全部扫描了，因为是查询出所有数据，所以全表扫描回比索引更快，节省回表的时间

[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，也会使用到索引，也就是所谓的覆盖索引
[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)


# 如果是筛选部分数据，那么就会使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)


# 调整字段顺序后，就变成`Using filesort`且没有用到索引，所以当使用order by语句，确保与联合索引的顺序要一致
[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)

情况2：下面的SQL最多只能用到索引tcol1，tcol2部分

# tcol02范围查找后，导致数据乱序，于是tcol03索引条件用不上，同时回出现`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`后再看一下执行计划，实际测试效率要高很多。
# 这是因为ICP减少了引擎层和server层之间的数据传输和回表请求，不满足条件的请求，直接过滤无需回表
# 实际上开启ICP后上面语句有用到tcol03的索引部分。

[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) /* 关闭ICP和MRR后执行时间 */
1 row in set (0.01 sec) /* 开启ICP和MRR后执行时间 */


[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) /* 关闭ICP和MRR后执行时间 */
40252 rows in set (1.58 sec) /* 开启ICP和MRR后执行时间 */


[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) /* 关闭ICP和后执行时间 */
1 row in set (0.01 sec) /* 开启ICP和后执行时间 */

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

Using MRR 后面再介绍。

情况3：下面的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)

联合索引有哪些好处

  • 1.减少开销。建一个联合索引(tcol01, tcol02, tcol03)，相当于建立三个索引(tcol01),(tcol01,tcol02),(tcol01,tcol02,tcol03)的功能。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，使用联合索引会大大的减少开销。
  • 2.覆盖索引。对联合索引(tcol01, tcol02, tcol03)，如果有如下的SQL: select tcol01,tcol02,tcol03 from t1 where tcol01=? and tcol02=? and tcol03=? 那么就可以使用到覆盖索引的功能，查询数据无需回表，减少随机IO。
  • 3.效率高。多列条件的查询下，索引列越多，通过索引筛选出的数据就越少。

联合索引使用建议

  • 1.查询条件中的 where、order by、group by 涉及多个字段，一般需要创建多列索引，比如前面的 select * from t1 where tcol01=100 and tcol02=50;
  • 2.创建联合索引的时候，要将区分度高的字段放在前面，假如有一张学生表包含学号和姓名，那么在建立联合索引的时候，学号放在姓名前面，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%的范围查找，范围查询可能导致无法使用索引。
  • 4.只筛选需要的数据字段，满足覆盖索引的要求，不要用 select * 筛选所有列数据。

Enjoy GreatSQL :)

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发表于 2022.04.18 16:35， 共 1100 人浏览

本文原创发布于慕课网 ，转载请注明出处，谢谢合作

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