在MySQL中插入意图锁的解决方案

时间：2017-07-06 13:11:22

标签： mysql concurrency transactions locking

我的表非常简单：

CREATE TABLE `d` (
    `id` int(11) DEFAULT NULL,
    UNIQUE KEY `id` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

没有记录：

select * from d;
Empty set (0,01 sec)

然后我尝试在不同的会话中打开两个交易：

会话＃1： 

begin;
Query OK, 0 rows affected (0,00 sec)

select * from d where id = 100 for update;
Empty set (0,00 sec)

会话＃2： 

begin;
Query OK, 0 rows affected (0,00 sec)

select * from d where id = 700 for update;
Empty set (0,00 sec)

现在我尝试在会话＃2 和会话＆＃34;冻结＆＃34;

中插入新记录 
insert into d values (700);

当我尝试在会话＃1 中执行相同操作（使用其他ID字段）时，它会崩溃：

insert into d values (100); --> ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction in Session #1
insert into d values (700); --> Query OK, 1 row affected (4,08 sec) in Session #2

我该如何解决僵局？ InnoDB状态为：

------------------------
LATEST DETECTED DEADLOCK
------------------------
2017-07-06 15:59:25 0x70000350d000
*** (1) TRANSACTION:
TRANSACTION 43567, ACTIVE 15 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 3 lock struct(s), heap size 1136, 2 row lock(s), undo log entries 1
MySQL thread id 4, OS thread handle 123145358217216, query id 89 localhost root update
insert into d values (700)
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 126 page no 4 n bits 72 index id of table `trx`.`d` trx id 43567 lock_mode X insert intention waiting
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
 0: len 8; hex 73757072656d756d; asc supremum;;

*** (2) TRANSACTION:
TRANSACTION 43568, ACTIVE 7 sec inserting
mysql tables in use 1, locked 1
3 lock struct(s), heap size 1136, 2 row lock(s), undo log entries 1
MySQL thread id 3, OS thread handle 123145357938688, query id 90 localhost root update
insert into d values (100)
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 126 page no 4 n bits 72 index id of table `trx`.`d` trx id 43568 lock_mode X
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
 0: len 8; hex 73757072656d756d; asc supremum;;

*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 126 page no 4 n bits 72 index id of table `trx`.`d` trx id 43568 lock_mode X insert intention waiting
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
 0: len 8; hex 73757072656d756d; asc supremum;;

*** WE ROLL BACK TRANSACTION (2)

3 个答案:

答案 0 :(得分：2)

此死锁错误是MySQL InnoDB引擎中的错误，已在12年内未得到修复。 （缺陷号25847：https://bugs.mysql.com/bug.php?id=25847，解决方法：How do I lock on an InnoDB row that doesn't exist yet?）它与唯一密钥无关。运行此查询也将导致相同的死锁错误。

Session #1: CREATE TABLE t (id int) ENGINE=InnoDB;
Session #1: SET AUTOCOMMIT = 0;
Session #1: SELECT id FROM t WHERE id = 1 FOR UPDATE;
Session #2: SET AUTOCOMMIT = 0;
Session #2: SELECT id FROM t WHERE id = 2 FOR UPDATE;
Session #1: INSERT INTO t (id) VALUES (1); -- Hang
Session #2: INSERT INTO t (id) VALUES (2); -- Session #1: OK, Session #2: Deadlock found when trying to get lock; try restarting transaction

InnoDB状态相同：

------------------------
LATEST DETECTED DEADLOCK
------------------------
2019-10-24 00:25:31 0x1638
*** (1) TRANSACTION:
TRANSACTION 1287, ACTIVE 62 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 3 lock struct(s), heap size 1136, 2 row lock(s)
MySQL thread id 7, OS thread handle 9444, query id 143 localhost ::1 root update
INSERT INTO t (id) VALUES (1)
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 23 page no 3 n bits 72 index GEN_CLUST_INDEX of table `test`.`t` trx id 1287 lock_mode X insert intention waiting
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
 0: len 8; hex 73757072656d756d; asc supremum;;

*** (2) TRANSACTION:
TRANSACTION 1288, ACTIVE 19 sec inserting, thread declared inside InnoDB 5000
mysql tables in use 1, locked 1
3 lock struct(s), heap size 1136, 2 row lock(s)
MySQL thread id 9, OS thread handle 5688, query id 145 localhost ::1 root update
INSERT INTO t (id) VALUES (2)
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 23 page no 3 n bits 72 index GEN_CLUST_INDEX of table `test`.`t` trx id 1288 lock_mode X
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
 0: len 8; hex 73757072656d756d; asc supremum;;

*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 23 page no 3 n bits 72 index GEN_CLUST_INDEX of table `test`.`t` trx id 1288 lock_mode X insert intention waiting
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
 0: len 8; hex 73757072656d756d; asc supremum;;

*** WE ROLL BACK TRANSACTION (2)

答案 1 :(得分：1)

请注意，从MySQL 8.0.1开始， 性能模式暴露了innodb数据锁。

请参阅https://dev.mysql.com/doc/refman/8.0/en/data-locks-table.html

请参阅https://dev.mysql.com/doc/refman/8.0/en/data-lock-waits-table.html

在此示例中，仅在会话1中第一次选择后，锁定为：

mysql> select * from performance_schema.data_locks \G
*************************** 1. row ***************************                                                                                                              
               ENGINE: INNODB                                                                                                                                               
       ENGINE_LOCK_ID: 1808:76                                                                                                                                              
ENGINE_TRANSACTION_ID: 1808                                                                                                                                                 
            THREAD_ID: 35                                                                                                                                                   
             EVENT_ID: 13081                                                                                                                                                
        OBJECT_SCHEMA: test                                                                                                                                                 
          OBJECT_NAME: d                                                                                                                                                    
       PARTITION_NAME: NULL                                                                                                                                                 
    SUBPARTITION_NAME: NULL
           INDEX_NAME: NULL
OBJECT_INSTANCE_BEGIN: 139756088373592
            LOCK_TYPE: TABLE
            LOCK_MODE: IX
          LOCK_STATUS: GRANTED
            LOCK_DATA: NULL
*************************** 2. row ***************************
               ENGINE: INNODB
       ENGINE_LOCK_ID: 1808:2:5:1
ENGINE_TRANSACTION_ID: 1808
            THREAD_ID: 35
             EVENT_ID: 13111
        OBJECT_SCHEMA: test
          OBJECT_NAME: d
       PARTITION_NAME: NULL
    SUBPARTITION_NAME: NULL
           INDEX_NAME: id
OBJECT_INSTANCE_BEGIN: 139756088370552
            LOCK_TYPE: RECORD
            LOCK_MODE: X
          LOCK_STATUS: GRANTED
            LOCK_DATA: supremum pseudo-record <--- HERE
2 rows in set (0.00 sec)

这不是死锁本身的解决方案，但是对锁定的可见性对于理解这个问题还有很长的路要走。

这里，SELECT FOR UPDATE都锁定＆＃34; suprenum＆＃34;记录，因为id 100和700大于表中的最大ID（它是空的）。

一旦有更多记录（比如ID = 500），两个查询应该同时执行，因为ID中的不同间隙将被锁定。

答案 2 :(得分：1)

我怀疑发生了死锁，因为InnoDB在处理“差距”时保守。请注意，100和700都在未触及的土地的同一个模糊区域。 InnoDB不能（或至少不）处理“100”和“700”不同的事实。 InnoDB希望标记单个行，但表中没有已包含这些ID的行。

交易2可能会超时（见innodb_lock_wait_timeout）。当你第二次使用＃2进行交易时，＃2仍然想要一个锁定，InnoDB就会受到惩罚并放弃。

底线：与死锁一起生活。当它们发生时，请回到BEGIN。你发现了另一个不明显的情况，即发生了不必要的死锁。

此外，我怀疑修复此案例的代码会减慢大多数其他情况，并导致一系列错误，需要多个版本才能发现并修复。

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