MySQL DeadLock故障排查全过程记录

【作者】

刘博：携程技术保障中心数据库高级经理，主要关注Sql server和Mysql的运维和故障处理。

【环境】

版本号：5.6.21

隔离级别：REPEATABLE READ

【问题描述】

接到监控报警，有一个线上的应用DeadLock报错，每15分钟会准时出现，报错统计如下图：

登录Mysql服务器查看日志：

mysql> show engine innodb status\G*** (1) TRANSACTION:TRANSACTION 102973, ACTIVE 11 sec starting index readmysql tables in use 3, locked 3LOCK WAIT 4 lock struct(s), heap size 1136, 3 row lock(s)MySQL thread id 6, OS thread handle 140024996574976, query id 83 localhost us updatingUPDATE TestTableSET column1 = 1,Column2 = sysdate(),Column3= '026'Column4 = 0AND column5 = 485AND column6 = 'SEK'*** (1) WAITING FOR THIS LOCK TO BE GRANTED:RECORD LOCKS space id 417 page no 1493 n bits 1000 index idx_column6 of table test.TestTable trx id 102973 lock_mode X waitingRecord lock, heap no 859 PHYSICAL RECORD: n_fields 2; compact format; info bits 00: len 3; hex 53454b; asc SEK;;1: len 8; hex 80000000007e1452; asc ~ R;;*** (2) TRANSACTION:TRANSACTION 102972, ACTIVE 26 sec starting index readmysql tables in use 3, locked 3219 lock struct(s), heap size 24784, 2906 row lock(s), undo log entries 7MySQL thread id 5, OS thread handle 140024996841216, query id 84 localhost us updatingUPDATE TestTableSET Column1 = 1,Column2 = sysdate(),Column3 = '026'Column4 = 0AND Column5 = 485AND Column6 = 'SEK'*** (2) HOLDS THE LOCK(S):RECORD LOCKS space id 417 page no 1493 n bits 1000 index idx_Column6 of table test.TestTable trx id 102972 lock_mode XRecord lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 00: len 8; hex 73757072656d756d; asc supremum;;Record lock, heap no 859 PHYSICAL RECORD: n_fields 2; compact format; info bits 00: len 3; hex 53454b; asc SEK;;1: len 8; hex 80000000007e1452; asc ~ R;;

【初步分析】

先分析下(2) TRANSACTION，TRANSACTION 32231892482。

等待的锁信息为：

0: len 3; hex 53454b; asc SEK;;1: len 8; hex 80000000007eea14; asc

0: len 3; hex 53454b; asc SEK;;1: len 8; hex 80000000007eeac4; asc

等待的锁信息为：

0: len 3; hex 53454b; asc SEK;;1: len 8; hex 80000000007eeac4; asc

TRANSACTION	Hold	Wait
32231892617	53454b\80000000007eea14	53454b\80000000007eeac4
32231892482	53454b\80000000007eeac4	53454b\80000000007eea14

让我们再看一下explain结果：

mysql>desc UPDATE TestTable SET Column1=1, Column2 = sysdate(),Column3 = '025' Column4 = 0 AND Column5 = 477 AND Column6 = 'SEK' \G;
*************************** 1. row ***************************
id: 1
select_type: UPDATE
table: TestTable
partitions: NULL
type: index_merge
possible_keys: column5_index,idx_column5_column6_Column1,idxColumn6
key: column5_index,idxColumn6
key_len: 8,9
ref: NULL
rows: 7
filtered: 100.00
Extra: Using intersect(column5_index,idxColumn6); Using where

可以看到 EXTRA 列：

Using intersect(column5_index,idxColumn6)

相关文档：http://dev.mysql.com/doc/refman/5.7/en/index-merge-optimization.html

The Index Merge method is used to retrieve rows with several range scans and to merge their results into one. The merge can produce unions, intersections, or unions-of-intersections of its underlying scans. This access method merges index scans from a single table; it does not merge scans across multiple tables.

【模拟与验证】

根据以上初步分析，猜测应该就是intersect造成的，于是在测试环境模拟验证，开启2个session模拟死锁：

时间序列	Session1	Session2
1	Begin；
2	UPDATE TestTable SET Column2 = sysdate() Column4 = 0 AND Column5 = 47 AND Column6 = 'SEK 执行成功，影响7行
3		Begin；
4		UPDATE TestTable SET Column2 = sysdate(),Column4 = 0 AND Column5 = 485 AND Column6 = 'SEK'；被Blocking
5	UPDATE TestTable SET Column2 = sysdate(),Column4 = 0 AND Column5 = 485 AND Column6 = 'SEK'；执行成功	ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction

依据以上信息可以发现Session2虽然被Block了，但也获取了一些Session1在时间序列5时所需资源的X锁，可以再开启一个查询select count(Column5) from TestTable where Column5 = 485，设置SET TRANSACTION ISOLATION LEVEL SERIALIZABLE，去查询Column5 = 485的行，观察锁等待的信息：
mysql> SELECT r.trx_id waiting_trx_id, r.trx_mysql_thread_id waiting_thread, r.trx_query waiting_query, b.trx_id

mysql> SELECT r.trx_id waiting_trx_id, r.trx_mysql_thread_id waiting_thread, r.trx_query waiting_query, b.trx_id blocking_trx_id, b.trx_mysql_thread_id blocking_thread, b.trx_query blocking_query FROM information_schema.innodb_lock_waits w INNER JOIN information_schema.innodb_trx b ON b.trx_id = w.blocking_trx_id INNER JOIN information_schema.innodb_trx r ON r.trx_id = w.requesting_trx_id \G;*************************** 1. row ***************************waiting_trx_id: 103006waiting_thread: 36waiting_query: UPDATE TestTable SET Column1 = 1, Column2 = sysdate(), Column3 = '026' Column4 = 0 AND Column5 = 485 AND Column6 = 'SEK'blocking_trx_id: 103003blocking_thread: 37blocking_query: NULL*************************** 2. row ***************************waiting_trx_id: 421500433538672waiting_thread: 39waiting_query: select count(Column5) from TestTable where Column5 = 485blocking_trx_id: 103006blocking_thread: 36blocking_query: UPDATE TestTable SET Column1 = 1, Column2 = sysdate(), Column3 = '026' Column4 = 0 AND Column5 = 485 AND Column6 = 'SEK'2 rows in set, 1 warning (0.00 sec)mysql> select * from information_schema.innodb_lock_waits \G;*************************** 1. row ***************************requesting_trx_id: 103006requested_lock_id: 103006:417:1493:859blocking_trx_id: 103003blocking_lock_id: 103003:417:1493:859*************************** 2. row ***************************requesting_trx_id: 421500433538672requested_lock_id: 421500433538672:417:749:2blocking_trx_id: 103006blocking_lock_id: 103006:417:749:22 rows in set, 1 warning (0.00 sec)

时间点	Session1	Session2
1	477 SEK
2		485SEK
3	485SEK	死锁发生

可以看到485 SEK这两个资源形成了一个环状，最终发生死锁。

【解决方法】

最佳的方法是添加column5和Column6的联合索引。
我们环境当时的情况发现Column6的筛选度非常低，就删除了Column6的索引。
10:55左右删除索引后，报错没有再发生：

总结

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