我使用MySQL 5.5。我注意到在并发情况下发生了特殊的僵局，我不认为这种僵局应该发生。
重现这样，使用两个同时运行的mysql客户端会话：
mysql session 1：
create table parent (id int(11) primary key);
insert into parent values (1);
create table child (id int(11) primary key, parent_id int(11), foreign key (parent_id) references parent(id));
begin;
insert into child (id, parent_id) values (10, 1);
-- this will create shared lock on parent(1)
mysql session 2：
begin;
-- try and get exclusive lock on parent row
select id from parent where id = 1 for update;
-- this will block because of shared lock in session 1
mysql session 1：
-- try and get exclusive lock on parent row
select id from parent where id = 1 for update;
-- observe that mysql session 2 transaction has been rolled back
mysql会话2：
ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction
从显示引擎innodb状态报告的信息是：
------------------------
LATEST DETECTED DEADLOCK
------------------------
161207 10:48:56
*** (1) TRANSACTION:
TRANSACTION 107E67, ACTIVE 43 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 2 lock struct(s), heap size 376, 1 row lock(s)
MySQL thread id 13074, OS thread handle 0x7f68eccfe700, query id 5530424 localhost root statistics
select id from parent where id = 1 for update
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 3714 n bits 72 index `PRIMARY` of table `foo`.`parent` trx id 107E67 lock_mode X locks rec but not gap waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
0: len 4; hex 80000001; asc ;;
1: len 6; hex 000000107e65; asc ~e;;
2: len 7; hex 8600000130; asc 2 ;;
*** (2) TRANSACTION:
TRANSACTION 107E66, ACTIVE 52 sec starting index read
mysql tables in use 1, locked 1
5 lock struct(s), heap size 1248, 2 row lock(s), undo log entries 1
MySQL thread id 12411, OS thread handle 0x7f68ecfac700, query id 5530425 localhost root statistics
select id from parent where id = 1 for update
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 0 page no 3714 n bits 72 index `PRIMARY` of table `foo`.`parent` trx id 107E66 lock mode S locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
0: len 4; hex 80000001; asc ;;
1: len 6; hex 000000107e65; asc ~e;;
2: len 7; hex 8600000130; asc 2 ;;
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 3714 n bits 72 index `PRIMARY` of table `foo`.`parent` trx id 107E66 lock_mode X locks rec but not gap waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
0: len 4; hex 80000001; asc ;;
1: len 6; hex 000000107e65; asc ~e;;
2: len 7; hex 8600000130; asc 2 ;;
*** WE ROLL BACK TRANSACTION (1)
您可以看到交易(1)没有显示任何已经获得的S或X锁;它只是阻止了尝试获得排他锁。由于没有循环，在这种情况下不应该有僵局，据我所知。
这是一个已知的MySQL bug吗？有其他人遇到过吗？使用了哪些解决方法？
这些是我们可以采取的可能步骤：
>减少我们对外键的使用(在我们的生产场景中，我们只是软删除引用的表中的行，但是是icky)
>获取排他锁，而不是隐式共享锁(将减少我们的并发吞吐量)
>改变我们的逻辑，所以我们不再需要在添加子行的同一个事务中对父进行排他锁(有风险和难度)
>将我们的MySQL版本更改为不显示此行为的版本
还有其他选择我们没有考虑？
