MySQL中update修改数据与原数据相同会再次执行吗？
数据库,mysql,oracle,redis
数据库相关
发布日期:   2021-04-05
作者:   欧阳思海
文章字数:   1.2k
阅读时长:   6 分
阅读次数:  
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原文链接：blog.ouyangsihai.cn >> MySQL中update修改数据与原数据相同会再次执行吗？

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作者：powdba https://yq.aliyun.com/articles/694162

https://yq.aliyun.com/articles/694162

背景

本文主要测试MySQL执行update语句时，针对与原数据（即未修改）相同的update语句会在MySQL内部重新执行吗？

测试环境

  • MySQL5.7.25
  • Centos 7.4

    • binlog_format为ROW

    参数

    
root@localhost : (none) 04:53:15 show variables like 'binlog_row_image';
+------------------+-------+
| Variable_name    | Value |
+------------------+-------+
| binlog_row_image | FULL  |
+------------------+-------+
1 row in set (0.00 sec)

root@localhost : (none) 04:53:49 show variables like 'binlog_format'; 
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| binlog_format | ROW   |
+---------------+-------+
1 row in set (0.00 sec)

root@localhost : test 05:15:14 show variables like 'transaction_isolation';
+-----------------------+-----------------+
| Variable_name         | Value           |
+-----------------------+-----------------+
| transaction_isolation | REPEATABLE-READ |
+-----------------------+-----------------+
1 row in set (0.00 sec)

    测试步骤

    session1

    
root@localhost : test 04:49:48 begin;
Query OK, 0 rows affected (0.00 sec)

root@localhost : test 04:49:52 select * from test where id =1;
+----+------+------+------+
| id | sid  | mid  | name |
+----+------+------+------+
|  1 |  999 |  871 | NW   |
+----+------+------+------+
1 row in set (0.00 sec)

root@localhost : (none) 04:54:03 show engine innodb statusGshow master statusG
...
---
LOG
---
Log sequence number 12090390
Log flushed up to   12090390
Pages flushed up to 12090390
Last checkpoint at  12090381
0 pending log flushes, 0 pending chkp writes
33 log i/o's done, 0.00 log i/o's/second

*************************** 1. row ***************************
             File: mysql-bin.000001
         Position: 154
     Binlog_Do_DB: 
 Binlog_Ignore_DB: 
Executed_Gtid_Set: 
1 row in set (0.00 sec)

    session2

    
root@localhost : test 04:47:45 update test set sid=55 where id =1;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

root@localhost : (none) 04:54:03 show engine innodb statusGshow master statusG
...
---
LOG
---
Log sequence number 12091486
Log flushed up to   12091486
Pages flushed up to 12091486
Last checkpoint at  12091477
0 pending log flushes, 0 pending chkp writes
39 log i/o's done, 0.00 log i/o's/second

*************************** 1. row ***************************
             File: mysql-bin.000001
         Position: 500
     Binlog_Do_DB: 
 Binlog_Ignore_DB: 
Executed_Gtid_Set: 8392d215-4928-11e9-a751-0242ac110002:1
1 row in set (0.00 sec)

    session1

    
root@localhost : test 04:49:57 update test set sid=55 where id =1;   
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1  Changed: 0  Warnings: 0

root@localhost : (none) 04:54:03 show engine innodb statusGshow master statusG
...
---
LOG
---
Log sequence number 12091486
Log flushed up to   12091486
Pages flushed up to 12091486
Last checkpoint at  12091477
0 pending log flushes, 0 pending chkp writes
39 log i/o's done, 0.00 log i/o's/second

*************************** 1. row ***************************
             File: mysql-bin.000001
         Position: 500
     Binlog_Do_DB: 
 Binlog_Ignore_DB: 
Executed_Gtid_Set: 8392d215-4928-11e9-a751-0242ac110002:1
1 row in set (0.00 sec)

root@localhost : test 04:52:05 select * from test where id =1;
+----+------+------+------+
| id | sid  | mid  | name |
+----+------+------+------+
|  1 |  999 |  871 | NW   |
+----+------+------+------+
1 row in set (0.00 sec)

root@localhost : test 04:52:42 commit;
Query OK, 0 rows affected (0.00 sec)

root@localhost : test 04:52:52 select * from test where id =1;
+----+------+------+------+
| id | sid  | mid  | name |
+----+------+------+------+
|  1 |   55 |  871 | NW   |
+----+------+------+------+
1 row in set (0.00 sec)

    总结

    在binlog_format=row和binlog_row_image=FULL时，由于MySQL 需要在 binlog 里面记录所有的字段，所以在读数据的时候就会把所有数据都读出来，那么重复数据的update不会执行。即MySQL 调用了 InnoDB 引擎提供的“修改为 (1,55)”这个接口，但是引擎发现值与原来相同，不更新，直接返回

    binlog_format为STATEMENT

    参数

    
root@localhost : (none) 04:53:15 show variables like 'binlog_row_image';
+------------------+-------+
| Variable_name    | Value |
+------------------+-------+
| binlog_row_image | FULL  |
+------------------+-------+
1 row in set (0.00 sec)

root@localhost : (none) 05:16:08  show variables like 'binlog_format';
+---------------+-----------+
| Variable_name | Value     |
+---------------+-----------+
| binlog_format | STATEMENT |
+---------------+-----------+
1 row in set (0.00 sec)

root@localhost : test 05:15:14 show variables like 'transaction_isolation';
+-----------------------+-----------------+
| Variable_name         | Value           |
+-----------------------+-----------------+
| transaction_isolation | REPEATABLE-READ |
+-----------------------+-----------------+
1 row in set (0.00 sec)

    测试步骤

    session1

    
root@localhost : test 05:16:42 begin;
Query OK, 0 rows affected (0.00 sec)

root@localhost : test 05:16:44 select * from test where id =1;
+----+------+------+------+
| id | sid  | mid  | name |
+----+------+------+------+
|  1 |  111 |  871 | NW   |
+----+------+------+------+
1 row in set (0.00 sec)

root@localhost : (none) 05:16:51 show engine innodb statusGshow master statusG
...
---
LOG
---
Log sequence number 12092582
Log flushed up to   12092582
Pages flushed up to 12092582
Last checkpoint at  12092573
0 pending log flushes, 0 pending chkp writes
45 log i/o's done, 0.00 log i/o's/second

*************************** 1. row ***************************
             File: mysql-bin.000001
         Position: 154
     Binlog_Do_DB: 
 Binlog_Ignore_DB: 
Executed_Gtid_Set: 
1 row in set (0.00 sec)

    session2

    
root@localhost : test 05:18:30 update test set sid=999 where id =1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

root@localhost : (none) 05:18:47 show engine innodb statusGshow master statusG
...
---
LOG
---
Log sequence number 12093678
Log flushed up to   12093678
Pages flushed up to 12093678
Last checkpoint at  12093669
0 pending log flushes, 0 pending chkp writes
51 log i/o's done, 0.14 log i/o's/second

*************************** 1. row ***************************
             File: mysql-bin.000001
         Position: 438
     Binlog_Do_DB: 
 Binlog_Ignore_DB: 
Executed_Gtid_Set: 8392d215-4928-11e9-a751-0242ac110002:1
1 row in set (0.00 sec)

    session1

    
root@localhost : test 05:16:47 update test set sid=999 where id =1;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1  Changed: 0  Warnings: 0

root@localhost : (none) 05:20:03 show engine innodb statusGshow master statusG
...
---
LOG
---
Log sequence number 12094504
Log flushed up to   12094504
Pages flushed up to 12094504
Last checkpoint at  12094495
0 pending log flushes, 0 pending chkp writes
56 log i/o's done, 0.00 log i/o's/second

*************************** 1. row ***************************
             File: mysql-bin.000001
         Position: 438
     Binlog_Do_DB: 
 Binlog_Ignore_DB: 
Executed_Gtid_Set: 8392d215-4928-11e9-a751-0242ac110002:1
1 row in set (0.00 sec)

root@localhost : test 05:19:33 select * from test where id =1;     
+----+------+------+------+
| id | sid  | mid  | name |
+----+------+------+------+
|  1 |  999 |  871 | NW   |
+----+------+------+------+
1 row in set (0.00 sec)

root@localhost : test 05:20:44 commit;
Query OK, 0 rows affected (0.01 sec)

root@localhost : test 05:20:57 select * from test where id =1;
+----+------+------+------+
| id | sid  | mid  | name |
+----+------+------+------+
|  1 |  999 |  871 | NW   |
+----+------+------+------+
1 row in set (0.00 sec)

    总结

    在binlog_format=statement和binlog_row_image=FULL时，InnoDB内部认真执行了update语句，即“把这个值修改成 (1,999)“这个操作，该加锁的加锁，该更新的更新。

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    本文GitHub https://github.com/OUYANGSIHAI/JavaInterview 已收录，这是我花了6个月总结的一线大厂Java面试总结，本人已拿大厂offer，欢迎star

    原文链接：blog.ouyangsihai.cn >> MySQL中update修改数据与原数据相同会再次执行吗？

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