MySQL执行update语句和原数据相同会再次执行吗
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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
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 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
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
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
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 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
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
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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