在mysql死锁问题发生时，一般是找到原始的锁id，然后杀掉那个线程，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。
这里有个演示实例，大家做参考：
线程a，用来锁定某些记录，假设这个线程一直没提交，或忘掉提交了，那么就一直存在，但是数据里面显示的只是sleep状态。
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)
线程b， 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？
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 statusg
------------
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
上面的信息很繁多，也看不清楚到底哪里是哪里。
不过现在，只要从数据字典里面拿出来这部分信息就ok了。
原来是线程16忘掉commit了。