【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 read
mysql tables in use 3, locked 3
LOCK 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 updating
UPDATE TestTable
SET column1 = 1,
Column2 = sysdate(),
Column3= '026’
Column4 = 0
AND column5 = 485
AND 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 waiting
Record lock, heap no 859 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 3; hex 53454b; asc SEK;;
1: len 8; hex 80000000007e1452; asc ~ R;;
*** (2) TRANSACTION:
TRANSACTION 102972, ACTIVE 26 sec starting index read
mysql tables in use 3, locked 3
219 lock struct(s), heap size 24784, 2906 row lock(s), undo log entries 7
MySQL thread id 5, OS thread handle 140024996841216, query id 84 localhost us updating
UPDATE TestTable
SET Column1 = 1,
Column2 = sysdate(),
Column3 = '026'
Column4 = 0
AND Column5 = 485
AND 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 X
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
0: len 8; hex 73757072656d756d; asc supremum;;
Record lock, heap no 859 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 3; hex 53454b; asc SEK;;
1: len 8; hex 80000000007e1452; asc ~ R;;
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 601 page no 89642 n bits 1000 index idx_column6 of table test.TestTable trx id 32231892482 lock_mode X locks rec but not gap waiting
Record lock, heap no 38 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 3; hex 53454b; asc SEK;;
1: len 8; hex 80000000007eea14; asc ~ ;;
大致一看，更新同一索引的同一行，应该是一个Block，报TimeOut的错才对，怎么会报DeadLock?
【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持有的锁信息为：
0: len 3; hex 53454b; asc SEK;;
1: len 8; hex 80000000007eeac4; asc再先分析下(1) TRANSACTION，TRANSACTION 32231892617。
等待的锁信息为：
0: len 3; hex 53454b; asc SEK;;
1: len 8; hex 80000000007eeac4; asc于是可以画出的死锁表，两个资源相互依赖，造成死锁：
| 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)从5.1开始，引入了 index merge 优化技术，对同一个表可以使用多个索引分别进行条件扫描。
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 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: 103006
waiting_thread: 36
waiting_query: UPDATE TestTable SET Column1 = 1, Column2 = sysdate(), Column3 = '026' Column4 = 0 AND Column5 = 485 AND Column6 = 'SEK'
blocking_trx_id: 103003
blocking_thread: 37
blocking_query: NULL
*************************** 2. row ***************************
waiting_trx_id: 421500433538672
waiting_thread: 39
waiting_query: select count(Column5) from TestTable where Column5 = 485
blocking_trx_id: 103006
blocking_thread: 36
blocking_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: 103006
requested_lock_id: 103006:417:1493:859
blocking_trx_id: 103003
blocking_lock_id: 103003:417:1493:859
*************************** 2. row ***************************
requesting_trx_id: 421500433538672
requested_lock_id: 421500433538672:417:749:2
blocking_trx_id: 103006
blocking_lock_id: 103006:417:749:2
2 rows in set, 1 warning (0.00 sec)
mysql> select * from INNODB_LOCKS \G;
*************************** 1. row ***************************
lock_id: 103006:417:1493:859
lock_trx_id: 103006
lock_mode: X
lock_type: RECORD
lock_table: test.TestTable
lock_index: idxColumn6
lock_space: 417
lock_page: 1493
lock_rec: 859
lock_data: 'SEK', 8262738
*************************** 2. row ***************************
lock_id: 103003:417:1493:859
lock_trx_id: 103003
lock_mode: X
lock_type: RECORD
lock_table:test.TestTable
lock_index: idxColumn6
lock_space: 417
lock_page: 1493
lock_rec: 859
lock_data: 'SEK', 8262738
*************************** 3. row ***************************
lock_id: 421500433538672:417:749:2
lock_trx_id: 421500433538672
lock_mode: S
lock_type: RECORD
lock_table: test.TestTable
lock_index: column5_index
lock_space: 417
lock_page: 749
lock_rec: 2
lock_data: 485, 8317620
*************************** 4. row ***************************
lock_id: 103006:417:749:2
lock_trx_id: 103006
lock_mode: X
lock_type: RECORD
lock_table: test.TestTable
lock_index: column5_index
lock_space: 417
lock_page: 749
lock_rec: 2
lock_data: 485, 8317620
4 rows in set, 1 warning (0.00 sec)
可以看到Session2，trx_id 103006阻塞了trx_id 421500433538672，而trx_id 421500433538672 requested_lock也正好是lock_data: 485, 8317620。由此可见Session2虽然别block了，但是还是获取到了Index column5_index相关的锁。被Block是因为intersect的原因，还需要idxColumn6的锁，至此思路已经清晰，对整个分配锁的信息简化一下，如下表格（请求到的锁用青色表示，需获取但未获取到的锁用红色表示）：
| 时间点 | Session1 | Session2 |
|---|---|---|
| 1 | 477 SEK | |
| 2 | 485 SEK | |
| 3 | 485 SEK | 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：