| 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
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】
总结
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