一、實驗復現
MySQL版本8.0，隔離級別RR和RC均能復現。
1.創建表，構造數據
CREATE TABLE `t2` (
`a` int NOT NULL,
`b` int DEFAULT NULL,
`c` int DEFAULT NULL,
PRIMARY KEY (`a`),
UNIQUE KEY `b` (`b`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
insert into t2 values(1,1,1);
2.操作步驟
| time | session1 | session2 | session3 |
|---|---|---|---|
| t1 | begin;update t2 set c=2 where b=1; | ||
| t2 | begin;delete from t2 where a=1; | ||
| t3 | begin;delete from t2 where b=1; | ||
| t4 | rollback; | ||
| t5 | Query OK, 1 row affected | (1213, 'Deadlock found when trying to get lock; try restarting transaction') |
二、死鎖分析
t1時刻
MySQL root@127.0.0.1:performance_schema> select ENGINE_TRANSACTION_ID,THREAD_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_S
-> TATUS,LOCK_DATA from performance_schema.data_locks;
+-----------------------+-----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
| ENGINE_TRANSACTION_ID | THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+-----------------------+-----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
| 6689 | 68 | test | t2 | <null> | TABLE | IX | GRANTED | <null> |
| 6689 | 68 | test | t2 | b | RECORD | X,REC_NOT_GAP | GRANTED | 1, 1 |
| 6689 | 68 | test | t2 | PRIMARY | RECORD | X,REC_NOT_GAP | GRANTED | 1 |
+-----------------------+-----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
常看performance_schema庫下的data_locks表，t1時刻對表t2主鍵和唯一索引b上加了兩個記錄鎖，X,REC_NOT_GAP代表不是gap鎖和next-key lock，GRANTED代表已經被授予，ENGINE_TRANSACTION_ID代表事務id號。
t2時刻
+-----------------------+-----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
| ENGINE_TRANSACTION_ID | THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+-----------------------+-----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
| 6690 | 67 | test | t2 | <null> | TABLE | IX | GRANTED | <null> |
| 6690 | 67 | test | t2 | PRIMARY | RECORD | X,REC_NOT_GAP | WAITING | 1 |
| 6689 | 68 | test | t2 | <null> | TABLE | IX | GRANTED | <null> |
| 6689 | 68 | test | t2 | b | RECORD | X,REC_NOT_GAP | GRANTED | 1, 1 |
| 6689 | 68 | test | t2 | PRIMARY | RECORD | X,REC_NOT_GAP | GRANTED | 1 |
+-----------------------+-----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
t2時刻，對t2主鍵索引為1的記錄也要加x record lock，此時鎖處於等待，session1主鍵為1的記錄上鎖還未釋放。
t3時刻
+-----------------------+-----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
| ENGINE_TRANSACTION_ID | THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+-----------------------+-----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
| 6691 | 66 | test | t2 | <null> | TABLE | IX | GRANTED | <null> |
| 6691 | 66 | test | t2 | b | RECORD | X,REC_NOT_GAP | WAITING | 1, 1 |
| 6690 | 67 | test | t2 | <null> | TABLE | IX | GRANTED | <null> |
| 6690 | 67 | test | t2 | PRIMARY | RECORD | X,REC_NOT_GAP | WAITING | 1 |
| 6689 | 68 | test | t2 | <null> | TABLE | IX | GRANTED | <null> |
| 6689 | 68 | test | t2 | b | RECORD | X,REC_NOT_GAP | GRANTED | 1, 1 |
| 6689 | 68 | test | t2 | PRIMARY | RECORD | X,REC_NOT_GAP | GRANTED | 1 |
+-----------------------+-----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
t3時刻，對表t2唯一索引b為1的記錄也要加x record lock，此時鎖處於等待，session1唯一索引b為1的記錄上鎖還未釋放。
t4時刻
session1事務回滾，主鍵索引和唯一索引b上的鎖釋放，session2獲得了主鍵索引為1的x record lock，等待唯一索引為1上的x record lock；session3獲得了唯一索引為1上的x record lock，主鍵索引為1的x record lock，發生了死鎖
t5時刻
+-----------------------+-----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
| ENGINE_TRANSACTION_ID | THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+-----------------------+-----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
| 6690 | 67 | test | t2 | <null> | TABLE | IX | GRANTED | <null> |
| 6690 | 67 | test | t2 | PRIMARY | RECORD | X,REC_NOT_GAP | GRANTED | 1 |
| 6690 | 67 | test | t2 | b | RECORD | X,REC_NOT_GAP | GRANTED | 1, 1 |
+-----------------------+-----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
session3事務回滾，session2獲得了表t2主鍵和唯一索引b上加了兩個記錄鎖。
show engine innodb status\G查看死鎖信息
------------------------
LATEST DETECTED DEADLOCK
------------------------
2021-06-25 13:18:39 0x7f7dd6fd0700
*** (1) TRANSACTION:
TRANSACTION 6691, ACTIVE 84 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 3 lock struct(s), heap size 1136, 3 row lock(s)
MySQL thread id 27, OS thread handle 140179212433152, query id 147 127.0.0.1 root updating
delete from t2 where b=1
*** (1) HOLDS THE LOCK(S):
RECORD LOCKS space id 4 page no 5 n bits 72 index b of table `test`.`t2` trx id 6691 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 80000001; asc ;;
1: len 4; hex 80000001; asc ;;
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 4 page no 4 n bits 72 index PRIMARY of table `test`.`t2` trx id 6691 lock_mode X locks rec but not gap waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 5; compact format; info bits 32
0: len 4; hex 80000001; asc ;;
1: len 6; hex 000000001a22; asc ";;
2: len 7; hex 020000011e0151; asc Q;;
3: len 4; hex 80000001; asc ;;
4: len 4; hex 80000001; asc ;;
*** (2) TRANSACTION:
TRANSACTION 6690, ACTIVE 111 sec updating or deleting
mysql tables in use 1, locked 1
LOCK WAIT 3 lock struct(s), heap size 1136, 3 row lock(s), undo log entries 1
MySQL thread id 28, OS thread handle 140179212105472, query id 146 127.0.0.1 root updating
delete from t2 where a=1
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 4 page no 4 n bits 72 index PRIMARY of table `test`.`t2` trx id 6690 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 5; compact format; info bits 32
0: len 4; hex 80000001; asc ;;
1: len 6; hex 000000001a22; asc ";;
2: len 7; hex 020000011e0151; asc Q;;
3: len 4; hex 80000001; asc ;;
4: len 4; hex 80000001; asc ;;
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 4 page no 5 n bits 72 index b of table `test`.`t2` trx id 6690 lock_mode X locks rec but not gap waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 80000001; asc ;;
1: len 4; hex 80000001; asc ;;
*** WE ROLL BACK TRANSACTION (1)
4、總結
MySQL8.0通過data_locks這張表，結合show engine innodb status的死鎖輸出，很容易分析出鎖等待和死鎖相關信息。