如果遇到死锁了，怎么解决呢？找到原始的锁ID，然后KILL掉一直持有的那个线程就可以了， 但是众多线程，可怎么找到引起死锁的线程ID呢？ MySQL 发展到现在，已经非常强大了，这个问题很好解决。 直接从数据字典连查找。
我们来演示下。
线程A，我们用来锁定某些记录，假设这个线程一直没提交，或者忘掉提交了。 那么就一直存在，但是数据里面显示的只是SLEEP状态。
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mysql> set @@autocommit=0;
Query OK, 0 rows affected (0.00 sec)
mysql> use test;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> show tables;
+—————-+
| Tables_in_test |
+—————-+
| demo_test |
| t3 |
+—————-+
2 rows in set (0.00 sec)
mysql> select * from t3;
+—-+——–+——–+————+—-+—-+—-+
| id | fname | lname | birthday | c1 | c2 | c3 |
+—-+——–+——–+————+—-+—-+—-+
| 19 | lily19 | lucy19 | 2013-04-18 | 19 | 0 | 0 |
| 20 | lily20 | lucy20 | 2013-03-13 | 20 | 0 | 0 |
+—-+——–+——–+————+—-+—-+—-+
2 rows in set (0.00 sec)
mysql> update t3 set birthday = '2022-02-23' where id = 19;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select connection_id();
+—————–+
| connection_id() |
+—————–+
| 16 |
+—————–+
1 row in set (0.00 sec)
mysql>
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线程B， 我们用来进行普通的更新，但是遇到问题了，此时不知道是哪个线程把这行记录给锁定了？
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mysql> use test;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> select @@autocommit;
+————–+
| @@autocommit |
+————–+
| 1 |
+————–+
1 row in set (0.00 sec)
mysql> update t3 set birthday='2018-01-03' where id = 19;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> select connection_id();
+—————–+
| connection_id() |
+—————–+
| 17 |
+—————–+
1 row in set (0.00 sec)
mysql> show processlist;
+—-+——+———–+——+———+——+——-+——————+
| Id | User | Host | db | Command | Time | State | Info |
+—-+——+———–+——+———+——+——-+——————+
| 10 | root | localhost | NULL | Sleep | 1540 | | NULL |
| 11 | root | localhost | NULL | Sleep | 722 | | NULL |
| 16 | root | localhost | test | Sleep | 424 | | NULL |
| 17 | root | localhost | test | Query | 0 | init | show processlist |
| 18 | root | localhost | NULL | Sleep | 5 | | NULL |
+—-+——+———–+——+———+——+——-+——————+
5 rows in set (0.00 sec)
mysql> show engine innodb status\G
————
TRANSACTIONS
————
Trx id counter 189327
Purge done for trx's n:o < 189323 undo n:o < 0 state: running but idle
History list length 343
LIST OF TRANSACTIONS FOR EACH SESSION:
—TRANSACTION 0, not started
MySQL thread id 11, OS thread handle 0x7f70a0c98700, query id 994 localhost root init
show engine innodb status
—TRANSACTION 189326, ACTIVE 2 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 17, OS thread handle 0x7f70a0bd5700, query id 993 localhost root updating
update t3 set birthday='2018-01-03' where id = 19
——- TRX HAS BEEN WAITING 2 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 529 page no 3 n bits 72 index `PRIMARY` of table `test`.`t3` trx id 189326 lock_mode X waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 9; compact format; info bits 0
0: len 2; hex 3139; asc 19;;
1: len 6; hex 00000002e38c; asc ;;
2: len 7; hex 7e00000d2827c9; asc ~ (' ;;
3: len 6; hex 6c696c793139; asc lily19;;
4: len 6; hex 6c7563793139; asc lucy19;;
5: len 3; hex 8fcc57; asc W;;
6: len 4; hex 80000013; asc ;;
7: len 4; hex 80000000; asc ;;
8: len 4; hex 80000000; asc ;;
——————
—TRANSACTION 189324, ACTIVE 641 sec
2 lock struct(s), heap size 376, 3 row lock(s), undo log entries 1
MySQL thread id 16, OS thread handle 0x7f70a0b94700, query id 985 localhost root cleaning up
Trx read view will not see trx with id >= 189325, sees < 189325
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上面的信息很繁多，也看不清楚到底哪里是哪里。
不过现在，我们只要从数据字典里面拿出来这部分信息就OK了。
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mysql> SELECT * FROM information_schema.INNODB_TRX\G
*************************** 1. row ***************************
trx_id: 189324
trx_state: RUNNING
trx_started: 2013-04-18 17:48:14
trx_requested_lock_id: NULL
trx_wait_started: NULL
trx_weight: 3
trx_mysql_thread_id: 16
trx_query: NULL
trx_operation_state: NULL
trx_tables_in_use: 0
trx_tables_locked: 0
trx_lock_structs: 2
trx_lock_memory_bytes: 376
trx_rows_locked: 3
trx_rows_modified: 1
trx_concurrency_tickets: 0
trx_isolation_level: REPEATABLE READ
trx_unique_checks: 1
trx_foreign_key_checks: 1
trx_last_foreign_key_error: NULL
trx_adaptive_hash_latched: 0
trx_adaptive_hash_timeout: 10000
trx_is_read_only: 0
trx_autocommit_non_locking: 0
1 row in set (0.01 sec)
mysql>
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原来是线程16忘掉COMMIT了。