mysql - 如何在Mysql中同时更新不同的行

Question

我有一个大表，我必须更新其中的几行。我尝试使用多个线程同时更新不同的行，但似乎 MySql 锁定了表中的所有行，而不是仅锁定那些与“where”子句匹配的行。因此更新不是并发的，有时会导致锁超时错误。

有没有办法同时更新不同的行？

我在后端使用 Django 来执行更新操作。我可以切换到 Postgres，但这会有帮助还是会出现锁定问题？

更新:添加代码片段:

def process_calculate_training(base_dept, exch_dept):
    # First initialize training to None, this is not happening concurrently.
    Emp.objects.filter(exch_dept=exch_dept, base_dept=base_dept).update(training=None)
    # Compute training and store result
    pass


def start_calculating_training():
    thread_func_args = [['MECH', 'COE'], ['MECH', 'ECE'], ['MECH', 'ICE'], ['MECH', 'IT']]
    with ThreadPool(4) as p:
        p.starmap(process_calculate_training, thread_func_args)

更新:执行'show engine innodb status;'

发现Django自动设置隔离级别为'READ-COMMITTED'。因此 MySql 只锁定那些需要更新的行。但是更新仍然没有同时发生，因为 2 个线程似乎在等待锁定，即使它们必须更新不同的行也是如此。

我在并发更新操作期间执行了show engine innodb status;。以下是结果:

TRANSACTIONS
------------
Trx id counter 2446659
Purge done for trx's n:o < 2446655 undo n:o < 0 state: running but idle
History list length 32
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 421137001004688, not started
0 lock struct(s), heap size 1136, 0 row lock(s)
---TRANSACTION 421137001002848, not started
0 lock struct(s), heap size 1136, 0 row lock(s)
---TRANSACTION 421137001003768, not started
0 lock struct(s), heap size 1136, 0 row lock(s)
---TRANSACTION 2446658, ACTIVE 11 sec fetching rows
mysql tables in use 1, locked 1
LOCK WAIT 4601 lock struct(s), heap size 1024208, 7281 row lock(s), undo log entries 2165
MySQL thread id 427, OS thread handle 139661736408832, query id 651475 localhost root updating
UPDATE `sdk_emp` SET `training` = NULL WHERE (`sdk_emp`.`base_dept` = 'MECH' AND `sdk_emp`.`exch_dept` = 'COE')
------- TRX HAS BEEN WAITING 9 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 345 page no 5238 n bits 240 index PRIMARY of table `empdb`.`sdk_emp` trx id 2446658 lock_mode X locks rec but not gap waiting
Record lock, heap no 135
------------------
---TRANSACTION 2446657, ACTIVE 11 sec fetching rows
mysql tables in use 1, locked 1
13971 lock struct(s), heap size 2351312, 15525 row lock(s), undo log entries 6307
MySQL thread id 424, OS thread handle 139661737813760, query id 651473 localhost root updating
UPDATE `sdk_emp` SET `training` = NULL WHERE (`sdk_emp`.`base_dept` = 'MECH' AND `sdk_emp`.`exch_dept` = 'ECE')
---TRANSACTION 2446656, ACTIVE 11 sec fetching rows
mysql tables in use 1, locked 1
LOCK WAIT 879 lock struct(s), heap size 270544, 1985 row lock(s), undo log entries 411
MySQL thread id 426, OS thread handle 139661736810240, query id 651471 localhost root updating
UPDATE `sdk_emp` SET `training` = NULL WHERE (`sdk_emp`.`base_dept` = 'MECH' AND `sdk_emp`.`exch_dept` = 'ICE')
------- TRX HAS BEEN WAITING 10 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 345 page no 1078 n bits 240 index PRIMARY of table `empdb`.`sdk_emp` trx id 2446656 lock_mode X locks rec but not gap waiting
Record lock, heap no 125
------------------
---TRANSACTION 2446655, ACTIVE 11 sec fetching rows
mysql tables in use 1, locked 1
13984 lock struct(s), heap size 2318544, 15263 row lock(s), undo log entries 6308
MySQL thread id 425, OS thread handle 139661737010944, query id 651469 localhost root updating
UPDATE `sdk_emp` SET `training` = NULL WHERE (`sdk_emp`.`base_dept` = 'MECH' AND `sdk_emp`.`exch_dept` = 'IT')

一段时间后再次运行命令:

TRANSACTIONS
------------
Trx id counter 2446659
Purge done for trx's n:o < 2446655 undo n:o < 0 state: running but idle
History list length 32
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 421137001004688, not started
0 lock struct(s), heap size 1136, 0 row lock(s)
---TRANSACTION 421137001002848, not started
0 lock struct(s), heap size 1136, 0 row lock(s)
---TRANSACTION 421137001003768, not started
0 lock struct(s), heap size 1136, 0 row lock(s)
---TRANSACTION 2446658, ACTIVE 31 sec fetching rows
mysql tables in use 1, locked 1
LOCK WAIT 4601 lock struct(s), heap size 1024208, 7281 row lock(s), undo log entries 2165
MySQL thread id 427, OS thread handle 139661736408832, query id 651475 localhost root updating
UPDATE `sdk_emp` SET `training` = NULL WHERE (`sdk_emp`.`base_dept` = 'MECH' AND `sdk_emp`.`exch_dept` = 'COE')
------- TRX HAS BEEN WAITING 29 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 345 page no 5238 n bits 240 index PRIMARY of table `empdb`.`sdk_emp` trx id 2446658 lock_mode X locks rec but not gap waiting
Record lock, heap no 135
------------------
---TRANSACTION 2446657, ACTIVE 31 sec updating or deleting
mysql tables in use 1, locked 1
27466 lock struct(s), heap size 4120784, 25388 row lock(s), undo log entries 12691
MySQL thread id 424, OS thread handle 139661737813760, query id 651473 localhost root updating
UPDATE `sdk_emp` SET `training` = NULL WHERE (`sdk_emp`.`base_dept` = 'MECH' AND `sdk_emp`.`exch_dept` = 'ECE')
---TRANSACTION 2446656, ACTIVE 31 sec fetching rows
mysql tables in use 1, locked 1
LOCK WAIT 879 lock struct(s), heap size 270544, 1985 row lock(s), undo log entries 411
MySQL thread id 426, OS thread handle 139661736810240, query id 651471 localhost root updating
UPDATE `sdk_emp` SET `training` = NULL WHERE (`sdk_emp`.`base_dept` = 'MECH' AND `sdk_emp`.`exch_dept` = 'ICE')
------- TRX HAS BEEN WAITING 30 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 345 page no 1078 n bits 240 index PRIMARY of table `empdb`.`sdk_emp` trx id 2446656 lock_mode X locks rec but not gap waiting
Record lock, heap no 125
------------------
---TRANSACTION 2446655, ACTIVE 31 sec fetching rows
mysql tables in use 1, locked 1
27489 lock struct(s), heap size 4169936, 25817 row lock(s), undo log entries 12692
MySQL thread id 425, OS thread handle 139661737010944, query id 651469 localhost root updating
UPDATE `sdk_emp` SET `training` = NULL WHERE (`sdk_emp`.`base_dept` = 'MECH' AND `sdk_emp`.`exch_dept` = 'IT')

我无法理解为什么 2 个线程无法获得锁，因为它们将更新不同的行？解决方案是什么？

Answer 1

1. 确保你的表有良好的索引并且你的更新查询没有进行全表扫描或全索引扫描。通过这种方式，您的语句可能需要更少的锁。如果可能，按主键或唯一键更新。

对语句执行 EXPLAIN 将帮助您了解查询的效率。

1. MySQL 默认隔离级别是REPEATABLE-READ，如果你正在使用事务，它会为它接触的行保持行锁直到事务结束，即使行不匹配。

将隔离级别改为READ-COMMITTED可以解决不匹配时行锁立即释放的问题。