面试官：MySQL 中 update 更新，数据与原数据相同时会执行吗？大部分人答不上来！

来源：https://developer.aliyun.com/article/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

[email protected] : (none) 04:53:15> show variables like 'binlog_row_image';+------------------+-------+| Variable_name    | Value |+------------------+-------+| binlog_row_image | FULL  |+------------------+-------+1 row in set (0.00 sec)[email protected] : (none) 04:53:49> show variables like 'binlog_format'; +---------------+-------+| Variable_name | Value |+---------------+-------+| binlog_format | ROW   |+---------------+-------+1 row in set (0.00 sec)[email protected] : 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

[email protected] : test 04:49:48> begin;Query OK, 0 rows affected (0.00 sec)[email protected] : 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)[email protected] : (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

[email protected] : 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: [email protected] : (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

[email protected] : 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: [email protected] : (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)[email protected] : 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)[email protected] : test 04:52:42> commit;Query OK, 0 rows affected (0.00 sec)[email protected] : 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

[email protected] : (none) 04:53:15> show variables like 'binlog_row_image';+------------------+-------+| Variable_name    | Value |+------------------+-------+| binlog_row_image | FULL  |+------------------+-------+1 row in set (0.00 sec)[email protected] : (none) 05:16:08>  show variables like 'binlog_format';+---------------+-----------+| Variable_name | Value     |+---------------+-----------+| binlog_format | STATEMENT |+---------------+-----------+1 row in set (0.00 sec)[email protected] : 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

[email protected] : test 05:16:42> begin;Query OK, 0 rows affected (0.00 sec)[email protected] : 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)[email protected] : (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

[email protected] : 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: [email protected] : (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

[email protected] : 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: [email protected] : (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)[email protected] : 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)[email protected] : test 05:20:44> commit;Query OK, 0 rows affected (0.01 sec)[email protected] : 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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