InnoDB中的锁

Question

表如下：

Create Table: CREATE TABLE `t` (
  `a` int(11) DEFAULT NULL,
  KEY `idx_a` (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

数据如下：

+------+
| a    |
+------+
|   11 |
|   12 |
|   13 |
|   14 |
+------+

有两个事务同时开启：
事务1：

mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

mysql> delete from t where a = 11;
Query OK, 1 row affected (0.00 sec)

mysql>

事务2

mysql> start transaction;
Query OK, 0 rows affected (0.02 sec)

mysql> insert into t select 9;

ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

mysql> insert into t select 15;
Query OK, 1 row affected (0.02 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> insert into t select 2;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

问题是为什么事务1删除11，会把11和它之前的范围锁定，导致事务2不能再这个区间插入数据。

这是InnoDB的事务状态，看的不是很明白，事务隔离级别为REPEATABLE-READ：

LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 18446744072393214172, not started
0 lock struct(s), heap size 1080, 0 row lock(s)
---TRANSACTION 637972, ACTIVE 28 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 2 lock struct(s), heap size 1080, 1 row lock(s), undo log entries 1
MySQL thread id 4, OS thread handle 2804091712, query id 24 122.205.8.162 root executing
insert into t select 9
------- TRX HAS BEEN WAITING 28 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 6 page no 4 n bits 80 index idx_a of table `test`.`t` trx id 637972 lock_mode X locks gap before rec insert intention waiting
Record lock, heap no 4 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 8000000b; asc     ;;
 1: len 6; hex 000000000414; asc       ;;

------------------
---TRANSACTION 637971, ACTIVE 32 sec
4 lock struct(s), heap size 1080, 3 row lock(s), undo log entries 1
MySQL thread id 5, OS thread handle 2802842432, query id 23 122.205.8.162 root cleaning up

可能优点啰嗦，望解答。

Answer 1

------- TRX HAS BEEN WAITING 28 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 6 page no 4 n bits 80 index idx_a of table `test`.`t` trx id 637972 lock_mode X locks gap before rec insert intention waiting
Record lock, heap no 4 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 8000000b; asc     ;;
 1: len 6; hex 000000000414; asc       ;;

这里说的很清楚啊 lock_mode X意味着是排它锁 gap代表是区间锁
也就是说在insert之前该表加入了区间排他锁，为什么呢？
因为之前执行的这句delete from t where a = 11;会在(negative infinity,11]这个区间加上排他锁，为什么是排他锁而不是Record Lock呢，因为你这里的a并非唯一索引,只是一个普通的索引，具体的看http://dev.mysql.com/doc/refman/5.7/en/innodb-locking.html

Answer 2

mysql默认的会话隔离级别是repeated read,会产生更多的gap锁，如果可以接受幻读，可以考虑降为read commit级别，减少锁冲突的概率。