背景

本文主要測試MySQL執行update語句時，針對與原數據（即未修改）相同的update語句會在MySQL內部重新執行嗎？

測試環境

• MySQL5.7.25
• Centos 7.4

binlog_format為ROW

參數

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)

測試步驟

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.000001Position: 154Binlog_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 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.000001Position: 500Binlog_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: 0root@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.000001Position: 500Binlog_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)

總結

• 在binlog_format=row和binlog_row_image=FULL時，由于MySQL 需要在 binlog 里面記錄所有的字段，所以在讀數據的時候就會把所有數據都讀出來，那么重復數據的update不會執行。即MySQL 調用了 InnoDB 引擎提供的“修改為 (1,55)”這個接口，但是引擎發現值與原來相同，不更新，直接返回

binlog_format為STATEMENT

參數

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)

測試步驟

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.000001Position: 154Binlog_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 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.000001Position: 438Binlog_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: 0root@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.000001Position: 438Binlog_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)

總結

• 在binlog_format=statement和binlog_row_image=FULL時，InnoDB內部認真執行了update語句，即“把這個值修改成 (1,999)“這個操作，該加鎖的加鎖，該更新的更新。

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