有人可以解释以下InnoDB锁定行为吗？它以READ_COMMITTED模式运行，只有一个表，它具有非重叠的主键和唯一键：
CREATE TABLE test3(
p BIGINT NOT NULL,
u BIGINT NOT NULL,
PRIMARY KEY (p),
UNIQUE KEY(u));
INSERT INTO test3 VALUES(10, 10);
正在运行两个事务（T2稍后启动）。
BEGIN;
INSERT INTO test3 VALUES(20, 20);
T1然后出于无关原因阻止应用程序代码。
BEGIN;
UPDATE test3 SET p=9 WHERE p=10;
此时T2阻止T1尝试获取锁定帮助。
mysql> SELECT * FROM information_schema.innodb_trx \G;
*************************** 1. row ***************************
trx_id: 158ABD
trx_state: LOCK WAIT
trx_started: 2014-04-23 03:26:43
trx_requested_lock_id: 158ABD:0:312:3
trx_wait_started: 2014-04-23 03:26:43
trx_weight: 6
trx_mysql_thread_id: 6749
trx_query: update test3 set p=9 where p=10
trx_operation_state: updating or deleting
trx_tables_in_use: 1
trx_tables_locked: 1
trx_lock_structs: 4
trx_lock_memory_bytes: 1248
trx_rows_locked: 3
trx_rows_modified: 2
trx_concurrency_tickets: 0
trx_isolation_level: READ COMMITTED
trx_unique_checks: 1
trx_foreign_key_checks: 1
trx_last_foreign_key_error: NULL
trx_adaptive_hash_latched: 0
trx_adaptive_hash_timeout: 10000
*************************** 2. row ***************************
trx_id: 158AB8
trx_state: RUNNING
trx_started: 2014-04-23 03:25:28
trx_requested_lock_id: NULL
trx_wait_started: NULL
trx_weight: 3
trx_mysql_thread_id: 6773
trx_query: NULL
trx_operation_state: NULL
trx_tables_in_use: 0
trx_tables_locked: 0
trx_lock_structs: 2
trx_lock_memory_bytes: 376
trx_rows_locked: 1
trx_rows_modified: 1
trx_concurrency_tickets: 0
trx_isolation_level: READ COMMITTED
trx_unique_checks: 1
trx_foreign_key_checks: 1
trx_last_foreign_key_error: NULL
trx_adaptive_hash_latched: 0
trx_adaptive_hash_timeout: 10000
2 rows in set (0.00 sec)
ERROR:
No query specified
mysql> SELECT * FROM information_schema.innodb_locks;
+----------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+-----------+
| lock_id | lock_trx_id | lock_mode | lock_type | lock_table | lock_index | lock_space | lock_page | lock_rec | lock_data |
+----------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+-----------+
| 158ABD:0:312:3 | 158ABD | S | RECORD | `test`.`test3` | `u` | 0 | 312 | 3 | 20 |
| 158AB8:0:312:3 | 158AB8 | X | RECORD | `test`.`test3` | `u` | 0 | 312 | 3 | 20 |
+----------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+-----------+
2 rows in set (0.00 sec)
mysql> SELECT * FROM information_schema.innodb_lock_waits;
+-------------------+-------------------+-----------------+------------------+
| requesting_trx_id | requested_lock_id | blocking_trx_id | blocking_lock_id |
+-------------------+-------------------+-----------------+------------------+
| 158ABD | 158ABD:0:312:3 | 158AB8 | 158AB8:0:312:3 |
+-------------------+-------------------+-----------------+------------------+
1 row in set (0.00 sec)
我启用了InnoDB锁定监视器，这里显示的是：
---TRANSACTION 158BD1, ACTIVE 3 sec updating or deleting
mysql tables in use 1, locked 1
LOCK WAIT 4 lock struct(s), heap size 1248, 3 row lock(s), undo log entries 2
MySQL thread id 6817, OS thread handle 0x7f6cfd8ac700, query id 3255951 localhost 127.0.0.1 root Updating
update test3 set p=9 where p=10
------- TRX HAS BEEN WAITING 3 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 1672 n bits 72 index `u` of table `test`.`test3` trx id 158BD1 lock mode S locks rec but not gap waiting
Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 8; hex 8000000000000014; asc ;;
1: len 8; hex 8000000000000014; asc ;;
------------------
TABLE LOCK table `test`.`test3` trx id 158BD1 lock mode IX
RECORD LOCKS space id 0 page no 1670 n bits 72 index `PRIMARY` of table `test`.`test3` trx id 158BD1 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 32
0: len 8; hex 800000000000000a; asc ;;
1: len 6; hex 000000158bd1; asc ;;
2: len 7; hex 13000005bf27ab; asc ' ;;
3: len 8; hex 800000000000000a; asc ;;
RECORD LOCKS space id 0 page no 1672 n bits 72 index `u` of table `test`.`test3` trx id 158BD1 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
0: len 8; hex 800000000000000a; asc ;;
1: len 8; hex 800000000000000a; asc ;;
RECORD LOCKS space id 0 page no 1672 n bits 72 index `u` of table `test`.`test3` trx id 158BD1 lock mode S locks rec but not gap waiting
Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 8; hex 8000000000000014; asc ;;
1: len 8; hex 8000000000000014; asc ;;
---TRANSACTION 158BCE, ACTIVE 148 sec
2 lock struct(s), heap size 376, 1 row lock(s), undo log entries 1
MySQL thread id 6810, OS thread handle 0x7f6cfd82a700, query id 3255952 localhost 127.0.0.1 root
show engine innodb status
TABLE LOCK table `test`.`test3` trx id 158BCE lock mode IX
RECORD LOCKS space id 0 page no 1672 n bits 72 index `u` of table `test`.`test3` trx id 158BCE lock_mode X locks rec but not gap
Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 8; hex 8000000000000014; asc ;;
1: len 8; hex 8000000000000014; asc ;;
看起来T1没有任何间隙锁，只锁定插入的记录。这在READ-COMMITTED隔离级别下是预期的。另一方面，T2需要三个锁：
所以问题似乎是MySql在唯一索引中的下一条记录上获取额外的共享锁。有什么想法吗？
（以下文字并不完全准确，请参阅上面的更新）
为什么T2尝试获取T1插入的u中的索引记录20的共享锁？似乎T2根本没有尝试触及该记录。我的理解是T1在20上进行了一次独占锁定，并且内部间隙锁定在（10,20）上。 T2应该只对10进行独占锁定。
如果u上的索引不是唯一的，也不会发生这种情况。
MySQL版本为5.5.35-0ubuntu0.12.04.2，但我观察到与5.6相同的行为。
谢谢！
答案 0 :(得分：1)
INSERT语句似乎会影响为[10,20]而不是（10,20）设置的插入意图间隙锁定;而其他INSERT语句仍然允许在没有锁定的情况下发生，UPDATE语句被锁定。
为了说明这一点，在开始任何交易之前，插入另一条记录：
INSERT INTO test3 VALUES (11, 11);
现在，当您同时播放两个事务时，UPDATE语句将不会锁定，因为间隙锁定现在设置为[11,20]，而下一键锁定为10。
类似地，当两个事务都执行INSERT语句时，两个事务都在没有锁定的情况下执行：
A: BEGIN;
A: INSERT INTO test3 VALUES (20, 20);
B: BEGIN;
B: INSERT INTO test3 VALUES (15, 15);
似乎如果唯一的唯一约束是主键，则上述根本不是问题;对我来说有意义的唯一原因是UNIQUE约束依赖于主键，这不知何故使MySQL绊倒了。