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Mysql Join大表小表到底谁驱动谁

wensonlee

wensonlee

2020-04-11

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1、准备

mysql> create table dept(
    id int unsigned auto_increment not null primary key,
    name varchar(20) default ‘‘ not null,
    key(name)
)engine=innodb default charset=utf8mb4;
CREATE TABLE `userinfo` (
  `id` int NOT NULL AUTO_INCREMENT,
  `name` varchar(255) COLLATE utf8mb4_general_ci NOT NULL,
  `passwd` varchar(255) COLLATE utf8mb4_general_ci DEFAULT NULL,
  `phone` varchar(255) COLLATE utf8mb4_general_ci DEFAULT NULL,
  `dept` int NOT NULL DEFAULT ‘0‘,
  PRIMARY KEY (`id`),
  KEY `name` (`name`),
  KEY `dept` (`dept`)
) ENGINE=InnoDB AUTO_INCREMENT=1001 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci

用 excel 伪造数据，并导入

mysql> load data infile ‘/var/lib/mysql-files/new5.txt‘ into table dept fields terminated by ‘ ‘ lines terminated by ‘\n‘;
Query OK, 1000 rows affected (0.03 sec)
Records: 1000  Deleted: 0  Skipped: 0  Warnings: 0

mysql> load data infile ‘/var/lib/mysql-files/new.txt‘ into table userinfo fields terminated by ‘ ‘ lines terminated by ‘\n‘;
Query OK, 120000 rows affected (0.02 sec)
Records: 120000  Deleted: 0  Skipped: 0  Warnings: 0

dept 1000 条， userinfo 120000 条

一、查找 userinfo 名字是 a25 的 id，name，及 dept name

由于有 userinfo = ‘a25‘ 的限制条件，不管谁 join 谁，或者 left join，结果都是一样的

mysql> select count(1) from userinfo left join dept using(dept_id) where userinfo.name = ‘a25‘;
+----------+
| count(1) |
+----------+
|      187 |
+----------+
1 row in set

mysql> select count(1) from dept left join userinfo using(dept_id) where userinfo.name = ‘a25‘;
+----------+
| count(1) |
+----------+
|      187 |
+----------+
1 row in set

mysql> select count(1) from dept join userinfo using(dept_id) where userinfo.name = ‘a25‘;
+----------+
| count(1) |
+----------+
|      187 |
+----------+
1 row in set

mysql> select count(1) from userinfo join dept using(dept_id) where userinfo.name = ‘a25‘;
+----------+
| count(1) |
+----------+
|      187 |
+----------+
1 row in set

那就比较四种方式的效率：

开启慢查询：

set long_query_time = 0.0001;
set global slow_query_log = ON;
mysql> show variables like "slow%";
+---------------------+--------------------------------------+
| Variable_name       | Value                                |
+---------------------+--------------------------------------+
| slow_launch_time    | 2                                    |
| slow_query_log      | ON                                   |
| slow_query_log_file | /var/lib/mysql/muqing-web-2-slow.log |
+---------------------+--------------------------------------+
tail -f /var/lib/mysql/muqing-web-2-slow.log // 实时查看耗时

1、userinfo left join dept

mysql> explain select userinfo.user_id, userinfo.name, dept.name from userinfo left join dept using(dept_id) where userinfo.name = ‘a25‘;
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+
| id | select_type | table    | partitions | type   | possible_keys | key     | key_len | ref                   | rows | filtered | Extra       |
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+
|  1 | SIMPLE      | userinfo | NULL       | ref    | name,name_2   | name    | 1022    | const                 |  187 |      100 | NULL        |
|  1 | SIMPLE      | dept     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | test.userinfo.dept_id |    1 |      100 | Using where |
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+
2 rows in set

mysql> show status like ‘last_query_cost‘;
+-----------------+------------+
| Variable_name   | Value      |
+-----------------+------------+
| Last_query_cost | 130.899000 |
+-----------------+------------+
1 row in set

mysql> explain select straight_join userinfo.user_id, userinfo.name, dept.name from userinfo left join dept using(dept_id) where userinfo.name = ‘a25‘;
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+
| id | select_type | table    | partitions | type   | possible_keys | key     | key_len | ref                   | rows | filtered | Extra       |
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+
|  1 | SIMPLE      | userinfo | NULL       | ref    | name,name_2   | name    | 1022    | const                 |  187 |      100 | NULL        |
|  1 | SIMPLE      | dept     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | test.userinfo.dept_id |    1 |      100 | Using where |
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+
2 rows in set

mysql> show status like ‘last_query_cost‘;
+-----------------+------------+
| Variable_name   | Value      |
+-----------------+------------+
| Last_query_cost | 130.899000 |
+-----------------+------------+
1 row in set

可以看到优化器选择的是顺序是先 userinfo 表，warnings 信息：

mysql> explain select straight_join userinfo.user_id, userinfo.name, dept.name from userinfo left join dept using(dept_id) where userinfo.name = ‘a25‘;
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+
| id | select_type | table    | partitions | type   | possible_keys | key     | key_len | ref                   | rows | filtered | Extra       |
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+
|  1 | SIMPLE      | userinfo | NULL       | ref    | name,name_2   | name    | 1022    | const                 |  187 |      100 | NULL        |
|  1 | SIMPLE      | dept     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | test.userinfo.dept_id |    1 |      100 | Using where |
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+
2 rows in set

mysql> show warnings;
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message                                                                                                                                                                                                                                                                                            |
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note  | 1003 | /* select#1 */ select straight_join `test`.`userinfo`.`user_id` AS `user_id`,`test`.`userinfo`.`name` AS `name`,`test`.`dept`.`name` AS `name` from `test`.`userinfo` left join `test`.`dept` on((`test`.`userinfo`.`dept_id` = `test`.`dept`.`dept_id`)) where (`test`.`userinfo`.`name` = ‘a25‘) |
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set

2、userinfo join dept

mysql> explain select userinfo.user_id, userinfo.name, dept.name from userinfo join dept using(dept_id) where userinfo.name = ‘a25‘;
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+
| id | select_type | table    | partitions | type   | possible_keys | key     | key_len | ref                   | rows | filtered | Extra       |
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+
|  1 | SIMPLE      | userinfo | NULL       | ref    | name,name_2   | name    | 1022    | const                 |  187 |      100 | NULL        |
|  1 | SIMPLE      | dept     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | test.userinfo.dept_id |    1 |      100 | Using where |
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+
2 rows in set

mysql> show status like ‘last_query_cost‘;
+-----------------+------------+
| Variable_name   | Value      |
+-----------------+------------+
| Last_query_cost | 130.899000 |
+-----------------+------------+
1 row in set

mysql> explain select straight_join userinfo.user_id, userinfo.name, dept.name from userinfo join dept using(dept_id) where userinfo.name = ‘a25‘;
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+
| id | select_type | table    | partitions | type   | possible_keys | key     | key_len | ref                   | rows | filtered | Extra       |
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+
|  1 | SIMPLE      | userinfo | NULL       | ref    | name,name_2   | name    | 1022    | const                 |  187 |      100 | NULL        |
|  1 | SIMPLE      | dept     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | test.userinfo.dept_id |    1 |      100 | Using where |
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+
2 rows in set

mysql> show warnings;
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message                                                                                                                                                                                                                                                                                         |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note  | 1003 | /* select#1 */ select straight_join `test`.`userinfo`.`user_id` AS `user_id`,`test`.`userinfo`.`name` AS `name`,`test`.`dept`.`name` AS `name` from `test`.`userinfo` join `test`.`dept` where ((`test`.`userinfo`.`name` = ‘a25‘) and (`test`.`userinfo`.`dept_id` = `test`.`dept`.`dept_id`)) |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set
# Time: 2020-04-10T17:00:38.452393Z
# User@Host: root[root] @ localhost []  Id:    11
# Query_time: 0.000741  Lock_time: 0.000137 Rows_sent: 187  Rows_examined: 374
SET timestamp=1586538038;
select sql_no_cache userinfo.user_id, userinfo.name, dept.name from userinfo left join dept using(dept_id) where userinfo.name = ‘a25‘;
# Time: 2020-04-10T17:01:49.946904Z
# User@Host: root[root] @ localhost []  Id:    11
# Query_time: 0.000766  Lock_time: 0.000149 Rows_sent: 187  Rows_examined: 374
SET timestamp=1586538109;
select straight_join userinfo.user_id, userinfo.name, dept.name from userinfo join dept using(dept_id) where userinfo.name = ‘a25‘;

可以看出两个时间差不多

3、dept left join userinfo

mysql> explain select sql_no_cache straight_join userinfo.user_id, userinfo.name, dept.name from dept left join userinfo using(dept_id) where userinfo.name = ‘a25‘;

+----+-------------+----------+------------+-------+---------------+------+---------+-------+------+----------+-------------+
| id | select_type | table    | partitions | type  | possible_keys | key  | key_len | ref   | rows | filtered | Extra       |
+----+-------------+----------+------------+-------+---------------+------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | dept     | NULL       | index | PRIMARY       | name | 82      | NULL  | 1000 |      100 | Using index |
|  1 | SIMPLE      | userinfo | NULL       | ref   | name,name_2   | name | 1022    | const |  187 |       10 | Using where |
+----+-------------+----------+------------+-------+---------------+------+---------+-------+------+----------+-------------+
2 rows in set

mysql> show status like ‘last_query_cost‘;
+-----------------+---------------+
| Variable_name   | Value         |
+-----------------+---------------+
| Last_query_cost | 106671.417848 |
+-----------------+---------------+
1 row in set
# Time: 2020-04-10T17:32:02.890446Z
# User@Host: root[root] @  [10.0.0.105]  Id:    13
# Query_time: 0.134147  Lock_time: 0.000128 Rows_sent: 187  Rows_examined: 188000
SET timestamp=1586539922;
select sql_no_cache straight_join userinfo.user_id, userinfo.name, dept.name from dept left join userinfo using(dept_id) where userinfo.name = ‘a25‘;

明显比上面的要慢

4、dept join userinfo

mysql> explain select sql_no_cache straight_join userinfo.user_id, userinfo.name, dept.name from dept join userinfo using(dept_id) where userinfo.name = ‘a25‘;
+----+-------------+----------+------------+-------+---------------+------+---------+-------+------+----------+-------------+
| id | select_type | table    | partitions | type  | possible_keys | key  | key_len | ref   | rows | filtered | Extra       |
+----+-------------+----------+------------+-------+---------------+------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | dept     | NULL       | index | PRIMARY       | name | 82      | NULL  | 1000 |      100 | Using index |
|  1 | SIMPLE      | userinfo | NULL       | ref   | name,name_2   | name | 1022    | const |  187 |       10 | Using where |
+----+-------------+----------+------------+-------+---------------+------+---------+-------+------+----------+-------------+
2 rows in set

mysql> show status like ‘last_query_cost‘;
+-----------------+---------------+
| Variable_name   | Value         |
+-----------------+---------------+
| Last_query_cost | 106671.417848 |
+-----------------+---------------+
1 row in set
# Time: 2020-04-10T17:33:35.458204Z
# User@Host: root[root] @  [10.0.0.105]  Id:    13
# Query_time: 0.133475  Lock_time: 0.000164 Rows_sent: 187  Rows_examined: 188000
SET timestamp=1586540015;
select sql_no_cache straight_join userinfo.user_id, userinfo.name, dept.name from dept join userinfo using(dept_id) where userinfo.name = ‘a25‘;

速度和 3 一样，也远大于前2 

所以是大表驱动小表。而且 show status like ‘last_query_cost‘ 还挺准的

二、查找 dept 名字是 a25 的 id，name，及 dept name

 四种查询等效

mysql> select count(1) from userinfo left join dept using(dept_id) where dept.name = ‘a25‘;
+----------+
| count(1) |
+----------+
|      360 |
+----------+
1 row in set

mysql> select count(1) from userinfo join dept using(dept_id) where dept.name = ‘a25‘;
+----------+
| count(1) |
+----------+
|      360 |
+----------+
1 row in set

mysql> select count(1) from dept join userinfo using(dept_id) where dept.name = ‘a25‘;
+----------+
| count(1) |
+----------+
|      360 |
+----------+
1 row in set

mysql> select count(1) from dept left join userinfo using(dept_id) where dept.name = ‘a25‘;
+----------+
| count(1) |
+----------+
|      360 |
+----------+
1 row in set

1、userinfo left join dept

mysql> explain select sql_no_cache straight_join userinfo.user_id, userinfo.name, dept.name from userinfo left join dept using(dept_id) where dept.name = ‘a25‘;
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+--------+----------+-------------+
| id | select_type | table    | partitions | type   | possible_keys | key     | key_len | ref                   | rows   | filtered | Extra       |
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+--------+----------+-------------+
|  1 | SIMPLE      | userinfo | NULL       | ALL    | NULL          | NULL    | NULL    | NULL                  | 120234 |      100 | NULL        |
|  1 | SIMPLE      | dept     | NULL       | eq_ref | PRIMARY,name  | PRIMARY | 4       | test.userinfo.dept_id |      1 |        5 | Using where |
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+--------+----------+-------------+
2 rows in set

mysql> show status like ‘last_query_cost‘;
+-----------------+--------------+
| Variable_name   | Value        |
+-----------------+--------------+
| Last_query_cost | 54209.549000 |
+-----------------+--------------+
1 row in set
# Time: 2020-04-10T17:40:53.462276Z
# User@Host: root[root] @  [10.0.0.105]  Id:    13
# Query_time: 0.110679  Lock_time: 0.000145 Rows_sent: 360  Rows_examined: 240000
SET timestamp=1586540453;
select sql_no_cache straight_join userinfo.user_id, userinfo.name, dept.name from userinfo left join dept using(dept_id) where dept.name = ‘a25‘;

2、userinfo join dept

mysql> explain select sql_no_cache straight_join userinfo.user_id, userinfo.name, dept.name from userinfo join dept using(dept_id) where dept.name = ‘a25‘;
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+--------+----------+-------------+
| id | select_type | table    | partitions | type   | possible_keys | key     | key_len | ref                   | rows   | filtered | Extra       |
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+--------+----------+-------------+
|  1 | SIMPLE      | userinfo | NULL       | ALL    | NULL          | NULL    | NULL    | NULL                  | 120234 |      100 | NULL        |
|  1 | SIMPLE      | dept     | NULL       | eq_ref | PRIMARY,name  | PRIMARY | 4       | test.userinfo.dept_id |      1 |        5 | Using where |
+----+-------------+----------+------------+--------+---------------+---------+---------+-----------------------+--------+----------+-------------+
2 rows in set

mysql> show status like ‘last_query_cost‘;
+-----------------+--------------+
| Variable_name   | Value        |
+-----------------+--------------+
| Last_query_cost | 54209.549000 |
+-----------------+--------------+
# Time: 2020-04-10T17:44:21.701356Z
# User@Host: root[root] @  [10.0.0.105]  Id:    13
# Query_time: 0.105262  Lock_time: 0.000137 Rows_sent: 360  Rows_examined: 240000
SET timestamp=1586540661;
select sql_no_cache straight_join userinfo.user_id, userinfo.name, dept.name from userinfo join dept using(dept_id) where dept.name = ‘a25‘;

速度比上面的好一点点

3、dept left join userinfo

mysql> explain select sql_no_cache straight_join userinfo.user_id, userinfo.name, dept.name from dept left join userinfo using(dept_id) where dept.name = ‘a25‘;
+----+-------------+----------+------------+------+---------------+------+---------+-------+--------+----------+----------------------------------------------------+
| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref   | rows   | filtered | Extra                                              |
+----+-------------+----------+------------+------+---------------+------+---------+-------+--------+----------+----------------------------------------------------+
|  1 | SIMPLE      | dept     | NULL       | ref  | name          | name | 82      | const |      1 |      100 | Using index                                        |
|  1 | SIMPLE      | userinfo | NULL       | ALL  | NULL          | NULL | NULL    | NULL  | 120234 |      100 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+----------+------------+------+---------------+------+---------+-------+--------+----------+----------------------------------------------------+
2 rows in set

mysql> show status like ‘last_query_cost‘;
+-----------------+--------------+
| Variable_name   | Value        |
+-----------------+--------------+
| Last_query_cost | 12128.032803 |
+-----------------+--------------+
# Time: 2020-04-10T17:47:06.147081Z
# User@Host: root[root] @  [10.0.0.105]  Id:    13
# Query_time: 0.031848  Lock_time: 0.000137 Rows_sent: 360  Rows_examined: 120001
SET timestamp=1586540826;
select sql_no_cache straight_join userinfo.user_id, userinfo.name, dept.name from dept left join userinfo using(dept_id) where dept.name = ‘a25‘;

速度竟然比上面的块，不是大表驱动小表吗，继续看

4、dept join userinfo

mysql> explain select sql_no_cache straight_join userinfo.user_id, userinfo.name, dept.name from dept join userinfo using(dept_id) where dept.name = ‘a25‘;
+----+-------------+----------+------------+------+---------------+------+---------+-------+--------+----------+----------------------------------------------------+
| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref   | rows   | filtered | Extra                                              |
+----+-------------+----------+------------+------+---------------+------+---------+-------+--------+----------+----------------------------------------------------+
|  1 | SIMPLE      | dept     | NULL       | ref  | PRIMARY,name  | name | 82      | const |      1 |      100 | Using index                                        |
|  1 | SIMPLE      | userinfo | NULL       | ALL  | NULL          | NULL | NULL    | NULL  | 120234 |       10 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+----------+------------+------+---------------+------+---------+-------+--------+----------+----------------------------------------------------+
2 rows in set

mysql> show status like ‘last_query_cost‘;
+-----------------+--------------+
| Variable_name   | Value        |
+-----------------+--------------+
| Last_query_cost | 12128.032803 |
+-----------------+--------------+
# Time: 2020-04-10T17:48:44.738339Z
# User@Host: root[root] @  [10.0.0.105]  Id:    13
# Query_time: 0.032005  Lock_time: 0.000143 Rows_sent: 360  Rows_examined: 120001
SET timestamp=1586540924;
select sql_no_cache straight_join userinfo.user_id, userinfo.name, dept.name from dept join userinfo using(dept_id) where dept.name = ‘a25‘;

速度也比 1，2快

left join 和 join 相比 按理说会慢一点，因为：join 的伪代码为

outer_iter = iterator over tb1 where col1 in(5, 6)

outer_row = outer_iter.next

while outer_row

　　inner_iter = iterator over tb2 where col3 = outer_row.col3

　　inner_row = inner_iter.next

　　while inner_row

　　　　out_put [ outer_row.col1, inner_row.col2 ]
　　　　inner_row = inner_iter.next
　　end

　　outer_row = outer_iter.next

end

而 left join 会多一次判断，多一次返回 null 结果集。

猜想是 where 条件 获取的过滤多，就用谁来做外表，所以讲不定 一定是大表驱动小表

mysql join varchar

wensonlee

wensonlee

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MySql索引使用策略分析

(建立索引会占用磁盘空间的索引文件。2.很少数据的列也不应该建立索引,比如 一个性别字段 0或者1,在查询中,结果集的数据占了表中数据行的比例比较大,mysql需要扫描的行数很多,增加索引,并不能提高效率

moyekongling 2020-11-13

mysql 如何动态修改复制过滤器

2、这个rds上有一个本地的ECS只读从库，这个只读从库会实时同步线上的rds数据库中的数据，这个只读从库供业务方查询使用

gloryli 2020-11-12

MySQL ddl语句的使用

数据定义语言create、drop、alter语句 。定义对数据库记录的增、删、改操作。定义对数据库、表、字段、用户的访问权限和安全级别。这小节主要了解下数据定义语言DDL。我们用它对数据库、表进行一些管理操作，比如：建库、删库、建表、修改表、删除表、对字

云中舞步 2020-11-12

MySQL中使用binlog时格式该如何选择

每一条会修改数据的sql都会记录到master的bin-log中。slave在复制的时候sql进程会解析成和原来master端执行过的相同的sql来再次执行。另外就是,由于mysql现在发展比较快，很多的新功能加入，使mysql的复制遇到了不小的挑战,自然

要啥自行车一把梭 2020-11-12

mysql 8.0.22 安装配置图文教程

解压后的目录并没有的my.ini文件，没关系可以自行创建在安装根目录下添加的my.ini ，写入基本配置：。# 允许连接失败的次数。初始化MySQL，在安装时，避免权限问题出错我们尽量使用管理员身份运行CMD，否则在安装时会报错，会导致安装失败的情况，如下

aydh 2020-11-12

解决Navicat Premium 连接 MySQL 8.0 报错\"1251\"的问题分析

人闲太久，努力一下就以为是在拼命。1251 - Client does not support authentication protocol requested by server; consider upgrading MySQL client. My

kuwoyinlehe 2020-11-12

MySQL数据操作-DML语句的使用

DML数据操作语言，是指对数据库进行增删改的操作指令，主要有INSERT、UPDATE、DELETE三种，代表插入、更新与删除，这是学习MySQL必要掌握的基本知识。方语法中 [] 中内容可以省略。值的顺序和表中字段顺序须保持一致。

minerk 2020-11-12

详解 MySQL中count函数的正确使用方法

当搞清楚count函数的运行原理后，相信上面几个问题的答案就会了然于胸。为了解决上述的问题，我创建了一张 user 表，它有两个字段：主键id和name，后者可以为null，建表语句如下。`id` int NOT NULL AUTO_INCREMENT C

vitasfly 2020-11-12

MySQL 基于时间点的快速恢复方案

万幸，这份数据是平台上某些商品的价格，基本上是有限个商品，然后价格值也都是固定的，之前有对这个价格表进行备份，于是给他直接重新导入了一份价格表的数据，这个问题也算是解决了。新建一个实例，全库还原，然后应用备份的binlog，一直去追，追到数据被该坏的时间点

jazywoo在路上 2020-11-11

MySQL外键约束的实例讲解

MySQL的外键约束是用来在两个表之间建立链接的，其中一个表发生变化，另外一个表也发生变化。从这个特点来看，它主要是为了保证表数据的一致性和完整性的。也就是说，只要外键的每个非空值出现在指定的主键中，这个外键的内容就是正确的。

敏敏张 2020-11-11

MySQL用truncate命令快速清空一个数据库中的所有表

用文本编辑器把每条truncate语句前后的“|”替换为空字符，方便后面一次复制多条执行。truncate与drop是DDL语句，执行后无法回滚；delete是DML语句，可回滚。truncate会清空表中的所有行，但表结构及其约束、索引等保持不变；dro

世樹 2020-11-11

修改MySQL8.0 默认的数据目录(快捷操作无配置)

使用场景：我们使用的是阿里云，单独购买了数据盘，MySQL 8.0 数据库默认装在系统盘上，为了考虑安全性和空间问题，我们需要将默认的数据库目录更改到其它位置。操作系统：CentOS 7.6 数据库：MySQL 8.0 查看我本人更多原创文章，请点击

zry 2020-11-11
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