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)

mysql 索引优化法则的更多相关文章

MYSQL索引优化法则
目录一首诗送给各位: 全值匹配我最爱,最左前缀要遵守: 带头大哥不能死,中间兄弟不能断: 索引列上少计算,范围之后全失效: Like百分写最右,覆盖索引不写星: 不等空值还有or,索引失效要少用: ...
mysql索引优化
mysql 索引优化 >mysql一次查询只能使用一个索引.如果要对多个字段使用索引,建立复合索引. >越小的数据类型通常更好:越小的数据类型通常在磁盘.内存和CPU缓存中都需要更少的空间 ...
Mysql 索引优化分析
MySQL索引优化分析为什么你写的sql查询慢?为什么你建的索引常失效?通过本章内容,你将学会MySQL性能下降的原因,索引的简介,索引创建的原则,explain命令的使用,以及explain输出字 ...
知识点：Mysql 索引优化实战（3）
知识点:Mysql 索引原理完全手册(1) 知识点:Mysql 索引原理完全手册(2) 知识点:Mysql 索引优化实战(3) 知识点:Mysql 数据库索引优化实战(4) 索引原理知识回顾索引的性 ...
MySQL索引优化步骤总结
在项目使用mysql过程中,随着系统的运行,发现一些慢查询,在这里总结一下mysql索引优化步骤 1.开发过程优化开发过程中对业务表中查询sql分析sql执行计划(尤其是业务流水表),主要是查看sq ...
MySQL索引优化看这篇文章就够了！
阅读本文大概需要 5 分钟. 来源:cnblogs.com/songwenjie/p/9410009.html 本文主要讨论MySQL索引的部分知识.将会从MySQL索引基础.索引优化实战和数据库索引 ...
mysql索引优化比普通查询速度快多少
mysql索引优化比普通查询速度快多少一.总结一句话总结:普通查询全表查询,速度较慢,索引优化的话拿空间换时间,一针见血,所以速度要快很多. 索引优化快很多空间换时间 1.软件层面优化数据库查询 ...
mySql索引优化分析
MySQL索引优化分析为什么你写的sql查询慢?为什么你建的索引常失效?通过本章内容,你将学会MySQL性能下降的原因,索引的简介,索引创建的原则,explain命令的使用,以及explain输出字 ...
【ZZ】MySQL 索引优化全攻略 | 菜鸟教程
MySQL 索引优化全攻略 http://www.runoob.com/w3cnote/mysql-index.html

随机推荐

mqtt.mini.js 使用
html文件里直接调用 <!DOCTYPE html> <html lang="en"> <head> <meta charset=&qu ...
java怎么调用子类中父类被覆盖的方法
public class b { { void show() { System.out.println("b"); } } public class c extends b { v ...
Linux批量装机(PXE)！
一 .PXE 简介PXE:Pre-boot Excution Environment,预启动执行环境PXE 是由 Intel 公司开发的网络引导技术,工作在 Client/Server 模式,允许客户 ...
「JLOI2012」树
「JLOI2012」树传送门不得不说这题的数据是真的水... 我们可以想到很明确的一条思路:枚举每一个点向根节点跳,知道路径和不小于 \(s\),恰好等于 \(s\) 就直接加答案. 跳的过程可以 ...
idea提交项目到码云
1.安装Git 2.在码云上创建项目 3.在IDEA提交项目到码云
02.当构造参数过多时使用builder模式
前言 <Effective Java>中文第三版,是一本关于Java基础的书,这本书不止一次有人推荐我看.其中包括我很喜欢的博客园博主五月的仓颉,他曾在自己的博文<给Java程序猿们 ...
pikachu-字符型注入(get) #手工注入
1.检测注入类型 http://127.0.0.1/pikachu-master/vul/sqli/sqli_str.php?name=1&submit=%E6%9F%A5%E8%AF%A2 ...
固定ip配置
1.打开网络和internet设置 2. 3. 4.查看详细信息 5.记住这几个 6.选择属性开始配置 7. 8.只要把ip地址设置为同一网段下面的你想要的设置的空余ip即可,其他掩码和DNS不要修 ...
python-定时任务-apschelduer
python-定时任务-apschelduer 1. apscheduler 1.1. install pip install apscheduler 1.2. basic co ...
小程序云开发使用where查询遇到的问题
想用小程序云开发的where查询,结果不论输入什么都是不报错,开始没注意,后来发现输入数据库中有的数据时,给打印出来查询成功,输入数据库中没有的数据时,也会得到一个集合,只不过这个集合的长度为0而已. ...

mysql 索引优化法则

建表语句

mysql 优化法则

mysql 索引优化法则的更多相关文章

随机推荐

热门专题