我们正在努力解决一个死锁，该死锁在我们的生产环境中一天要发生多次。
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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)
在一个查询中，我想设置service_synchronized_at = NULL。在更新之前的第一个查询中，service_synchronized_at始终不为null。
在第二个查询中，我添加了一个where条件s.service_synchronized_at IS NULL，认为它将导致非锁定行的值不为null。猜猜我错了。
该表在id上只有主索引，在user_id上有唯一约束（当然还有user_id上的外键）。
欢迎任何帮助。