【作者】
劉博：攜程技術保障中心數據庫高級經理，主要關注Sql server和Mysql的運維和故障處理。
【環境】
版本號：5.6.21
隔離級別：REPEATABLE READ
【問題描述】
接到監控報警，有一個線上的應用DeadLock報錯，每15分鍾會准時出現，報錯統計如下圖：
登錄Mysql服務器查看日志：
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，應該是一個Block，報TimeOut的錯才對，怎么會報DeadLock?
【初步分析】
先分析下(2) TRANSACTION，TRANSACTION 32231892482。
等待的鎖信息為：
0: len 3; hex 53454b; asc SEK;;
1: len 8; hex 80000000007eea14; asc持有的鎖信息為：
0: len 3; hex 53454b; asc SEK;;
1: len 8; hex 80000000007eeac4; asc再先分析下(1) TRANSACTION，TRANSACTION 32231892617。
等待的鎖信息為：
0: len 3; hex 53454b; asc SEK;;
1: len 8; hex 80000000007eeac4; asc於是可以畫出的死鎖表，兩個資源相互依賴，造成死鎖：
| TRANSACTION | Hold | Wait |
|---|---|---|
| 32231892617 | 53454b\80000000007eea14 | 53454b\80000000007eeac4 |
| 32231892482 | 53454b\80000000007eeac4 | 53454b\80000000007eea14 |
讓我們再看一下explain結果：
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)從5.1開始，引入了 index merge 優化技術，對同一個表可以使用多個索引分別進行條件掃描。
相關文檔：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.【模擬與驗證】
根據以上初步分析，猜測應該就是intersect造成的，於是在測試環境模擬驗證，開啟2個session模擬死鎖：
| 時間序列 | Session1 | Session2 |
|---|---|---|
| 1 | Begin； | |
| 2 | UPDATE TestTable SET Column2 = sysdate() Column4 = 0 AND Column5 = 47 AND Column6 = 'SEK 執行成功，影響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'； 執行成功 |
ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction |
依據以上信息可以發現Session2雖然被Block了，但也獲取了一些Session1在時間序列5時所需資源的X鎖，可以再開啟一個查詢select count(Column5) from TestTable where Column5 = 485，設置SET TRANSACTION ISOLATION LEVEL SERIALIZABLE，去查詢Column5 = 485的行，觀察鎖等待的信息：
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。由此可見Session2雖然別block了，但是還是獲取到了Index column5_index相關的鎖。被Block是因為intersect的原因，還需要idxColumn6的鎖，至此思路已經清晰，對整個分配鎖的信息簡化一下，如下表格（請求到的鎖用青色表示，需獲取但未獲取到的鎖用紅色表示）：
| 時間點 | Session1 | Session2 |
|---|---|---|
| 1 | 477 SEK | |
| 2 | 485 SEK | |
| 3 | 485 SEK | 死鎖發生 |
可以看到485 SEK這兩個資源形成了一個環狀，最終發生死鎖。
【解決方法】
- 最佳的方法是添加column5和Column6的聯合索引。
- 我們環境當時的情況發現Column6的篩選度非常低，就刪除了Column6的索引。
10:55左右刪除索引后，報錯沒有再發生：
