MySQL执行update语句和原数据相同会再次执行吗

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 12090390Log flushed up to 12090390Pages flushed up to 12090390Last checkpoint at 120903810 pending log flushes, 0 pending chkp writes33 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: 0root@localhost : (none) 04:54:03> show engine innodb status\Gshow master status\G...---LOG---Log sequence number 12091486Log flushed up to 12091486Pages flushed up to 12091486Last checkpoint at 120914770 pending log flushes, 0 pending chkp writes39 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-a751-0242ac110002:11 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: 0root@localhost : (none) 04:54:03> show engine innodb status\Gshow master status\G...---LOG---Log sequence number 12091486Log flushed up to 12091486Pages flushed up to 12091486Last checkpoint at 120914770 pending log flushes, 0 pending chkp writes39 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-a751-0242ac110002:11 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 12092582Log flushed up to 12092582Pages flushed up to 12092582Last checkpoint at 120925730 pending log flushes, 0 pending chkp writes45 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: 0root@localhost : (none) 05:18:47> show engine innodb status\Gshow master status\G...---LOG---Log sequence number 12093678Log flushed up to 12093678Pages flushed up to 12093678Last checkpoint at 120936690 pending log flushes, 0 pending chkp writes51 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-a751-0242ac110002:11 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: 0root@localhost : (none) 05:20:03> show engine innodb status\Gshow master status\G...---LOG---Log sequence number 12094504Log flushed up to 12094504Pages flushed up to 12094504Last checkpoint at 120944950 pending log flushes, 0 pending chkp writes56 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-a751-0242ac110002:11 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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