Question

我有一张大桌子，必须在其中更新几行。我试图使用多个线程并发地更新不同的行，但是MySql似乎锁定了表中的所有行，而不是仅锁定那些由'where'子句匹配的行。因此，更新不是并发的，有时会导致锁定超时错误。

有什么办法可以同时更新不同的行？

我在后端使用Django执行更新操作。我可以切换到Postgres，但这会有所帮助还是仍然存在锁定问题？

更新：添加了代码段：

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：执行了“显示引擎的innodb状态；”

发现Django自动将隔离级别设置为“ READ-COMMITTED”。因此，MySql仅锁定那些需要更新的行。但是，由于两个线程似乎正在等待锁，因此即使它们必须更新不同的行，更新仍然不会同时发生。

在并发更新操作期间，我做了show engine innodb status;。结果如下：

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个线程无法获取锁，因为它们将更新不同的行？那么解决方案是什么？

Answer 1

确保您的表已正确建立索引，并且更新查询未执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定。尽可能通过主键或唯一键进行更新。

对语句进行解释将有助于您了解查询的效率。

MySQL的默认隔离级别为REPEATABLE-READ，如果您使用的是事务，它将保持它所接触的行的行锁定，直到事务结束为止，即使该行不匹配也是如此。

将隔离级别更改为READ-COMMITTED可以解决以下问题：如果不匹配，行锁将立即释放。

如何在MySQL中同时更新不同的行

1 个答案: