聊聊MySQL中的死鎖
死鎖是指兩個或者多個事務(wù)互相持有對方所需的資源，從而導(dǎo)致它們都無法繼續(xù)執(zhí)行的情況。下圖是一個死鎖的示例，事務(wù)1鎖住了id=1的數(shù)據(jù)（比如更新id=1的數(shù)據(jù)記錄），同時請求鎖住id=2的數(shù)據(jù)，但事務(wù)2持有id=2的鎖，同時又請求id=1的鎖，這樣就造成了相互等待對方釋放鎖的情況，從而產(chǎn)生了死鎖：
上圖是死鎖產(chǎn)生的示例說明，我們用實際的SQL來演示死鎖的產(chǎn)生，首先創(chuàng)建一個測試表，它只有兩個字段，id和數(shù)量，id為自增類型，然后向表中插入兩條數(shù)據(jù)：
CREATE TABLE `t_test` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`quantity` int(2) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
INSERT INTO `t_test` VALUES ('1', '1');
INSERT INTO `t_test` VALUES ('2', '2');
如果有兩個事務(wù)更新表中id等于1和2的數(shù)據(jù)，但更新的順序相反，像下面這樣，就會出現(xiàn)死鎖：
最后，事務(wù)2提示死鎖的錯誤，而事務(wù)1則執(zhí)行成功，當(dāng)然，在事務(wù)的最后需要加上COMMIT語句。查詢表中的數(shù)據(jù)進(jìn)行確認(rèn)，發(fā)現(xiàn)id=1的數(shù)量更新為了101，而id=2的數(shù)量更新成了102。
另外，由于sql執(zhí)行較快，直接執(zhí)行上面兩個事務(wù)中的sql可能不會產(chǎn)生死鎖的情況，我們可以稍做修改，也就在UPDATE語句后面加上SLEEP函數(shù)，SLEEP會讓當(dāng)前進(jìn)程暫停執(zhí)行指定的時間（單位為秒）。分別在兩個事務(wù)中執(zhí)行下面的語句，稍等幾秒鐘，就可以看到出現(xiàn)死鎖：
# 事務(wù)1
START TRANSACTION;
UPDATE t_test SET quantity=101 WHERE id = 1;
SELECT SLEEP(10) FROM dual;
UPDATE t_test SET quantity=102 WHERE id = 2;
COMMIT;
# 事務(wù)2
START TRANSACTION;
UPDATE t_test SET quantity=201 WHERE id = 2;
SELECT SLEEP(10) FROM dual;
UPDATE t_test SET quantity=202 WHERE id = 1;
COMMIT;
SHOW ENGINE INNODB STATUS; ------------------------
LATEST DETECTED DEADLOCK
------------------------
2023-11-08 15:57:23 0x4df8
*** (1) TRANSACTION:
TRANSACTION 350231, ACTIVE 12 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 3 lock struct(s), heap size 1136, 2 row lock(s), undo log entries 1
MySQL thread id 3, OS thread handle 19044, query id 339 localhost ::1 root updating
UPDATE t_test SET quantity=102 WHERE id = 2
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 743 page no 3 n bits 72 index PRIMARY of table `test`.`t_test` trx id 350231 lock_mode X locks rec but not gap waiting
Record lock, heap no 3 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 4; hex 80000002; asc ;;
1: len 6; hex 000000055818; asc X ;;
2: len 7; hex 2f000001401cb2; asc / @ ;;
3: len 4; hex 800000c9; asc ;;
*** (2) TRANSACTION:
TRANSACTION 350232, ACTIVE 10 sec starting index read, thread declared inside InnoDB 5000
mysql tables in use 1, locked 1
3 lock struct(s), heap size 1136, 2 row lock(s), undo log entries 1
MySQL thread id 5, OS thread handle 19960, query id 340 localhost 127.0.0.1 root updating
UPDATE t_test SET quantity=202 WHERE id = 1
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 743 page no 3 n bits 72 index PRIMARY of table `test`.`t_test` trx id 350232 lock_mode X locks rec but not gap
Record lock, heap no 3 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 4; hex 80000002; asc ;;
1: len 6; hex 000000055818; asc X ;;
2: len 7; hex 2f000001401cb2; asc / @ ;;
3: len 4; hex 800000c9; asc ;;
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 743 page no 3 n bits 72 index PRIMARY of table `test`.`t_test` trx id 350232 lock_mode X locks rec but not gap waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 4; hex 80000001; asc ;;
1: len 6; hex 000000055817; asc X ;;
2: len 7; hex 2e0000018d1edf; asc . ;;
3: len 4; hex 80000065; asc e;;
*** WE ROLL BACK TRANSACTION (2)-
讓事務(wù)盡可能的小且短；
-
合理設(shè)置事務(wù)隔離級別； -
合理設(shè)置鎖等待超時時間； -
確定好事務(wù)操作的順序；
創(chuàng)建合適的索引，減少加鎖的情況。
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