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mysql 索引优化法则

建表语句

CREATE TABLE staffs(
id INT PRIMARY KEY AUTO_INCREMENT,
NAME VARCHAR (24) NOT NULL DEFAULT '' COMMENT '姓名',
age INT NOT NULL DEFAULT 0 COMMENT '年龄',
pos VARCHAR (20) NOT NULL DEFAULT 0 COMMENT '职位',
add_time TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '入职时间'
) CHARSET utf8 COMMENT '员工记录表';


INSERT INTO staffs(NAME, age, pos, add_time) VALUES('ronnie', 22, 'manager', NOW());
INSERT INTO staffs(NAME, age, pos, add_time) VALUES('John', 23, 'devloper', NOW());
INSERT INTO staffs(NAME, age, pos, add_time) VALUES('2181', 27, 'devloper', NOW());

-- 建复合索引
alter table staffs add index idx_staffs_nameAgePos(name, age, pos);

mysql 优化法则

全值匹配我最爱(Kappa)

遵循最佳左前缀法则

如果索引了多列, 查询从索引的最左前列开始并且不跳过索引中的列。
其实索引本质是个单向链表, 你要先得到头才能逐渐往后取后面的, 中间断了就走不了后面的索引了。

正确使用 Demo:

mysql> explain select * from staffs where name = 'ronnie';
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
| id | select_type | table  | partitions | type | possible_keys         | key                   | key_len | ref   | rows | filtered | Extra |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | staffs | NULL       | ref  | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74      | const |    1 |   100.00 | NULL  |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from staffs where name = 'ronnie' and age = 22;
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-------+
| id | select_type | table  | partitions | type | possible_keys         | key                   | key_len | ref         | rows | filtered | Extra |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-------+
|  1 | SIMPLE      | staffs | NULL       | ref  | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 78      | const,const |    1 |   100.00 | NULL  |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from staffs where name = 'ronnie' and age = 22 and pos = 'manager';
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
| id | select_type | table  | partitions | type | possible_keys         | key                   | key_len | ref               | rows | filtered | Extra |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
|  1 | SIMPLE      | staffs | NULL       | ref  | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 140     | const,const,const |    1 |   100.00 | NULL  |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

违反使用Demo:(完全失效)[带头大哥死了]:

mysql> explain select * from staffs where age = 22 and pos = 'manager';
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | staffs | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from staffs where age = 22;
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | staffs | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from staffs where pos = 'manager';
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | staffs | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

完全失效, 执行了全表扫描。

违反使用Demo(部分失效)[中间兄弟断了]:

mysql> explain select * from staffs where name = 'ronnie' and pos = 'manager';
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+
| id | select_type | table  | partitions | type | possible_keys         | key                   | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | staffs | NULL       | ref  | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74      | const |    1 |    33.33 | Using index condition |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

使用了复合索引 idx_staffs_nameAgePos, 但 reference 只有一个常量参数, 只用了最左侧的 name, 没用到 pos。

不在索引列上做任何操作(计算、函数、(自动 or 手动) 类型转换), 会导致索引失效而转向全表扫描

索引列上执行函数Demo:

mysql> explain select * from staffs where left(name,4) = 'John';
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | staffs | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

可以看到执行了全表扫描, 索引失效。

索引列上执行计算Demo:


mysql> explain select * from staffs where char_length('name') > 3;
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table  | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | staffs | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | NULL  |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

可以看到执行了全表扫描, 索引失效。

索引列上执行类型转换Demo:

mysql> select * from staffs where name = 2181;
+----+------+-----+----------+---------------------+
| id | NAME | age | pos      | add_time            |
+----+------+-----+----------+---------------------+
|  3 | 2181 |  27 | devloper | 2020-01-10 11:38:08 |
+----+------+-----+----------+---------------------+
1 row in set, 2 warnings (0.00 sec)

mysql> explain select * from staffs where name = 2181;
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys         | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | staffs | NULL       | ALL  | idx_staffs_nameAgePos | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
1 row in set, 3 warnings (0.00 sec)

mysql> explain select * from staffs where name = '2181';
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
| id | select_type | table  | partitions | type | possible_keys         | key                   | key_len | ref   | rows | filtered | Extra |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | staffs | NULL       | ref  | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74      | const |    1 |   100.00 | NULL  |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

这种就是比较坑的, 平时写sql是不注意, varChar类型请务必加上' ', 如果不加, mysql底层会进行类型的隐式转换, 造成索引失效。

存储引擎不能使用索引中范围条件右边的列

范围之后全失效, 所以建索引是一门学问, 需要对常用的业务sql进行综合考量。

Demo:

mysql> explain select * from staffs where name = 'John' and age > 22 and pos = 'devloper';
+----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
| id | select_type | table  | partitions | type  | possible_keys         | key                   | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | staffs | NULL       | range | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 78      | NULL |    1 |    33.33 | Using index condition |
+----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

我们的索引是这样建的: name -> age -> pos
由于中间的age进行了范围查询, 会导致后续的索引全部失效。

尽量使用覆盖索引(只访问索引的查询(索引列和查询列一致)), 减少select *

对比Demo:

mysql> explain select * from staffs where name = 'John' and pos = 'devloper';
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+
| id | select_type | table  | partitions | type | possible_keys         | key                   | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | staffs | NULL       | ref  | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74      | const |    1 |    33.33 | Using index condition |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select name, age, pos from staffs where name = 'John' and pos = 'devloper';
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+--------------------------+
| id | select_type | table  | partitions | type | possible_keys         | key                   | key_len | ref   | rows | filtered | Extra                    |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+--------------------------+
|  1 | SIMPLE      | staffs | NULL       | ref  | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74      | const |    1 |    33.33 | Using where; Using index |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

可以看到extra多了 Using index, 表面select操作中使用了覆盖索引(Covering Index), 避免了访问表的数据行。

mysql 在使用不等于(!= 或者 <>) 的时候无法使用索引会导致全表扫描

Demo:

mysql> explain select * from staffs where name != 'John';
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys         | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | staffs | NULL       | ALL  | idx_staffs_nameAgePos | NULL | NULL    | NULL |    3 |    66.67 | Using where |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from staffs where name <> 'John';
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys         | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | staffs | NULL       | ALL  | idx_staffs_nameAgePos | NULL | NULL    | NULL |    3 |    66.67 | Using where |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

is null, is not null 也无法使用索引

底层原因是: null 不能转换为索引表示的数字来索引

Demo:

mysql> explain select * from staffs where name is null;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra            |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
|  1 | SIMPLE      | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | Impossible WHERE |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from staffs where name is not null;
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys         | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | staffs | NULL       | ALL  | idx_staffs_nameAgePos | NULL | NULL    | NULL |    3 |    66.67 | Using where |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

like 以通配符开头('%abc...') mysql 索引失效会变成全表扫描的操作

Demo:

mysql> explain select * from staffs where name like '%ronnie';
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | staffs | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from staffs where name like '%ronnie%';
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | staffs | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from staffs where name like 'ronnie%';
+----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
| id | select_type | table  | partitions | type  | possible_keys         | key                   | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | staffs | NULL       | range | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74      | NULL |    1 |   100.00 | Using index condition |
+----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

可以看到只有% 在右边的查询使用了索引

这个情况就比较蛋疼, 如果你们业务上就需要百分号在左边, 需要使用覆盖索引来优化。

mysql> explain select id, name from staffs where name like '%ronnie%';
+----+-------------+--------+------------+-------+---------------+-----------------------+---------+------+------+----------+--------------------------+
| id | select_type | table  | partitions | type  | possible_keys | key                   | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+--------+------------+-------+---------------+-----------------------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | staffs | NULL       | index | NULL          | idx_staffs_nameAgePos | 140     | NULL |    3 |    33.33 | Using where; Using index |
+----+-------------+--------+------------+-------+---------------+-----------------------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

做查询如果数据量大的话用 Lucene, Solr, ElasticSearch 不香吗?

字符串不加单引号索引失效(mysql会自己做隐式转换)
- Demo 同3

少用or, 用它来连接时会索引失效

Demo:

mysql> explain select * from staffs where name = 'John' or name = 'ronnie';
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys         | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | staffs | NULL       | ALL  | idx_staffs_nameAgePos | NULL | NULL    | NULL |    3 |    66.67 | Using where |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from staffs where name = 'John' or age = 23;
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys         | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | staffs | NULL       | ALL  | idx_staffs_nameAgePos | NULL | NULL    | NULL |    3 |    55.56 | Using where |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from staffs where name = 'John' or pos = 'manager';
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys         | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | staffs | NULL       | ALL  | idx_staffs_nameAgePos | NULL | NULL    | NULL |    3 |    55.56 | Using where |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)

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原文地址：https://www.cnblogs.com/ronnieyuan/p/12176093.html