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MySQL执行update语句和原数据相同会再次执行吗

转载 作者：qq735679552 更新时间：2022-09-27 22:32:09 25 4
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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 。

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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 。

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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 status\Gshow master status\G
...
---
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 。

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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 status\Gshow master status\G
...
---
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 。

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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 status\Gshow master status\G
...
---
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 。

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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 。

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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 status\Gshow master status\G
...
---
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 。

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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 status\Gshow master status\G
...
---
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 。

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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 status\Gshow master status\G
...
---
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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原文链接：https://zhuanlan.zhihu.com/p/59717198 。

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