我们正在与一个死锁作斗争，这个死锁每天在我们的生产环境中发生多次。

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LATEST DETECTED DEADLOCK
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2018-12-27 19:07:34 7fcef1959700

***(1) TRANSACTION:

TRANSACTION 2125001468, ACTIVE 2 sec starting index read
mysql tables in use 1, locked 1

LOCK WAIT 3 lock struct(s), heap size 1184, 2 row lock(s), undo log entries 1

MySQL thread id 42190185, OS thread handle 0x7fcffc0b1700, query id 918842488 --- updating

UPDATE synchronization SET service_synchronized_at = NULL WHERE id = 116212

***(1) WAITING FOR THIS LOCK TO BE GRANTED:

RECORD LOCKS space id 615 page no 288 n bits 528 index `PRIMARY` of table `app`.`synchronization` trx table locks 1 total table locks 2  trx id 2125001468 lock_mode X locks rec but not gap waiting lock hold time 2 wait time before grant 0 

***(2) TRANSACTION:

TRANSACTION 2125001355, ACTIVE 5 sec fetching rows
mysql tables in use 2, locked 2
25216 lock struct(s), heap size 3683880, 5297668 row lock(s), undo log entries 94

MySQL thread id 42189517, OS thread handle 0x7fcef1959700, query id 918842042 --- updating

UPDATE synchronization s SET s.service_synchronized_at = now() WHERE s.service_synchronized_at IS NULL AND s.user_id IN (* time consuming select to determine which users should be updated *)

***(2) HOLDS THE LOCK(S):

RECORD LOCKS space id 615 page no 288 n bits 528 index `PRIMARY` of table `app`.`synchronization` trx table locks 2 total table locks 2  trx id 2125001355 lock_mode X lock hold time 3 wait time before grant 0 

***(2) WAITING FOR THIS LOCK TO BE GRANTED:

RECORD LOCKS space id 615 page no 2313 n bits 472 index `PRIMARY` of table `app`.`synchronization` trx table locks 2 total table locks 2  trx id 2125001355 lock_mode X waiting lock hold time 0 wait time before grant 0 

***WE ROLL BACK TRANSACTION (1)

在我想设置的一个查询中 service_synchronized_at = NULL . 在更新之前的第一次查询中，服务\u synchronized \u at始终不为空。
在第二个查询中，我添加了一个where条件 s.service_synchronized_at IS NULL 认为这将导致非锁定行的非空值。我想我错了。
表上只有主索引 id 唯一的约束 user_id （当然，外键在 user_id ).
欢迎任何帮助。