InnoDB中锁的算法(1)

Ⅰ、InnoDB锁算法的介绍

首先明确一点,锁锁住的是什么？锁锁住的是索引

Record Lock

单个行记录上的锁
Gap Lock

锁定一个范围，但不包含记录本身
Next-key Lock

Gap Lock + Record Lock 锁定一个范围，并且锁定记录本身

Ⅱ、模拟加锁场景

(root@localhost) [test]> desc l;

+-------+---------+------+-----+---------+-------+

| Field | Type    | Null | Key | Default | Extra |

+-------+---------+------+-----+---------+-------+

| a     | int(11) | NO   | PRI | NULL    |       |

| b     | int(11) | YES  | MUL | NULL    |       |

| c     | int(11) | YES  | UNI | NULL    |       |

| d     | int(11) | YES  |     | NULL    |       |

+-------+---------+------+-----+---------+-------+

4 rows in set (0.00 sec)

(root@localhost) [test]> select * from l;

+---+------+------+------+

| a | b    | c    | d    |

+---+------+------+------+

| 2 |    4 |    6 |    8 |

| 4 |    6 |    8 |   10 |

| 6 |    8 |   10 |   12 |

| 8 |   10 |   12 |   14 |

+---+------+------+------+

4 rows in set (0.02 sec)

(root@localhost) [test]> begin;

Query OK, 0 rows affected (0.00 sec)

(root@localhost) [test]> select * from l where a = 2 for update;

+---+------+------+------+

| a | b    | c    | d    |

+---+------+------+------+

| 2 |    4 |    6 |    8 |

+---+------+------+------+

1 row in set (0.03 sec)

对主键为2的这条记录加锁,这里可以表示三个意思

①record lock：对2加X锁

②gap lock：对(负无穷,2)加X锁

    thd1：hold 2 x gap

    thd2：hold 2 x record

    上面两个是兼容的,也就是说,thd2直接操作2这条记录是可以操作的,不需要等待

    thd3：insert 1,这个线程就要wait,因为1在这个范围内

③next-key lock 锁住(负无穷,2] 

oralce中只有record lock，没有别的意思

一般来说,此处我们根据不同事务隔离级别来分析这个加锁情况如下：

rc

所有某条记录的加锁都是record锁,所有insert不用等待,并发度更好

--->lock_mode X locks rec but not gap
rr

所有对某条记录加锁都用的next-key locking,insert 并行性能或许有点差

--->lock_mode X

特殊情况：

会把加锁模式优化为record lock,前提是锁住的那个index是unique的,并且只返回(锁住)一条记录

(a,b)复合索引,查a=? 用的还是next-key locking,查a=?,b=?就会用record lock

Ⅲ、正儿八经的分析几个场景看看

3.1 对主键加锁

(root@localhost) [test]> show variables like 'tx_isolation';

+---------------+-----------------+

| Variable_name | Value           |

+---------------+-----------------+

| tx_isolation  | REPEATABLE-READ |

+---------------+-----------------+

1 row in set (0.01 sec)

(root@localhost) [test]> begin;

Query OK, 0 rows affected (0.00 sec)

(root@localhost) [test]> select * from l where a <=2 for update;

+---+------+------+------+

| a | b    | c    | d    |

+---+------+------+------+

| 2 |    4 |    6 |    8 |

+---+------+------+------+

1 row in set (0.01 sec)

(root@localhost) [test]> show engine innodb status\G

...

LIST OF TRANSACTIONS FOR EACH SESSION:

---TRANSACTION 31220336, ACTIVE 16 sec

2 lock struct(s), heap size 1136, 2 row lock(s)

MySQL thread id 416, OS thread handle 139830453040896, query id 5627 localhost root starting

show engine innodb status

TABLE LOCK table `test`.`l` trx id 31220336 lock mode IX

RECORD LOCKS space id 1358 page no 3 n bits 72 index PRIMARY of table `test`.`l` trx id 31220336 lock_mode X

Record lock, heap no 2 PHYSICAL RECORD: n_fields 6; compact format; info bits 0

 0: len 4; hex 80000002; asc     ;;

 1: len 6; hex 000001c1b939; asc      9;;

 2: len 7; hex e0000001a80110; asc        ;;

 3: len 4; hex 80000004; asc     ;;

 4: len 4; hex 80000006; asc     ;;

 5: len 4; hex 80000008; asc     ;;

Record lock, heap no 3 PHYSICAL RECORD: n_fields 6; compact format; info bits 0

 0: len 4; hex 80000004; asc     ;;

 1: len 6; hex 000001c1b93a; asc      :;;

 2: len 7; hex e1000001a90110; asc        ;;

 3: len 4; hex 80000006; asc     ;;

 4: len 4; hex 80000008; asc     ;;

 5: len 4; hex 8000000a; asc     ;;

...

按道理我们锁住的应该是(负无穷,2],但实际上锁住的范围已经到了4这条记录,此时插入3是插不进去的,为什么？

为了保证解决幻读,要把2到它后面这条记录4这段范围锁住,这时候如果新插入一个2,在原来的2后面是插不进来的,如果4不锁住,新开一个线程可以删除4,又可以新插入一个4

rc的话就是只锁住记录本身,如下：

(root@localhost) [(none)]> show variables like 'tx_isolation';

+---------------+----------------+

| Variable_name | Value          |

+---------------+----------------+

| tx_isolation  | READ-COMMITTED |

+---------------+----------------+

1 row in set (0.00 sec)

(root@localhost) [(none)]> begin;

Query OK, 0 rows affected (0.00 sec)

(root@localhost) [test]> select * from l where a <=2 for update;

+---+------+------+------+

| a | b    | c    | d    |

+---+------+------+------+

| 2 |    4 |    6 |    8 |

+---+------+------+------+

1 row in set (0.00 sec)

(root@localhost) [test]> show engine innodb status\G

...

LIST OF TRANSACTIONS FOR EACH SESSION:

---TRANSACTION 31220337, ACTIVE 6 sec

2 lock struct(s), heap size 1136, 1 row lock(s)

MySQL thread id 443, OS thread handle 139830452774656, query id 5649 localhost root starting

show engine innodb status

TABLE LOCK table `test`.`l` trx id 31220337 lock mode IX

RECORD LOCKS space id 1358 page no 3 n bits 72 index PRIMARY of table `test`.`l` trx id 31220337 lock_mode X locks rec but not gap

Record lock, heap no 2 PHYSICAL RECORD: n_fields 6; compact format; info bits 0

 0: len 4; hex 80000002; asc     ;;

 1: len 6; hex 000001c1b939; asc      9;;

 2: len 7; hex e0000001a80110; asc        ;;

 3: len 4; hex 80000004; asc     ;;

 4: len 4; hex 80000006; asc     ;;

 5: len 4; hex 80000008; asc     ;;

...

唯一索引和主键情况一样

3.2 对二级索引加锁

先看rc事务隔离级别

(root@localhost) [test]> begin;

Query OK, 0 rows affected (0.00 sec)

(root@localhost) [test]> select * from l where b = 6 for update;

+---+------+------+------+

| a | b    | c    | d    |

+---+------+------+------+

| 4 |    6 |    8 |   10 |

+---+------+------+------+

1 row in set (0.02 sec

(root@localhost) [test]> show engine innodb status\G

...

LIST OF TRANSACTIONS FOR EACH SESSION:

---TRANSACTION 31220338, ACTIVE 35 sec

3 lock struct(s), heap size 1136, 2 row lock(s)

MySQL thread id 443, OS thread handle 139830452774656, query id 5653 localhost root starting

show engine innodb status

TABLE LOCK table `test`.`l` trx id 31220338 lock mode IX

RECORD LOCKS space id 1358 page no 5 n bits 72 index b of table `test`.`l` trx id 31220338 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 4; hex 80000006; asc     ;;

 1: len 4; hex 80000004; asc     ;;

RECORD LOCKS space id 1358 page no 3 n bits 72 index PRIMARY of table `test`.`l` trx id 31220338 lock_mode X locks rec but not gap

Record lock, heap no 3 PHYSICAL RECORD: n_fields 6; compact format; info bits 0

 0: len 4; hex 80000004; asc     ;;

 1: len 6; hex 000001c1b93a; asc      :;;

 2: len 7; hex e1000001a90110; asc        ;;

 3: len 4; hex 80000006; asc     ;;

 4: len 4; hex 80000008; asc     ;;

 5: len 4; hex 8000000a; asc     ;;

...

先对二级索引b加record锁：lock_mode X locks rec but not gap锁住了(6,4),6是二级索引,4是主键值

再对聚集索引加锁也是record locks,锁聚集索引index primary,锁住了a=4

再分析rr隔离级别下的情况,如下：

(root@localhost) [test]> show variables like 'tx_isolation';

+---------------+-----------------+

| Variable_name | Value           |

+---------------+-----------------+

| tx_isolation  | REPEATABLE-READ |

+---------------+-----------------+

1 row in set (0.00 sec)

(root@localhost) [test]> begin;

Query OK, 0 rows affected (0.00 sec)

(root@localhost) [test]> select * from l where b = 6 for update;

+---+------+------+------+

| a | b    | c    | d    |

+---+------+------+------+

| 4 |    6 |    8 |   10 |

+---+------+------+------+

1 row in set (0.01 sec)

(root@localhost) [test]> show engine innodb status\G

...

LIST OF TRANSACTIONS FOR EACH SESSION:

---TRANSACTION 31220340, ACTIVE 5 sec

4 lock struct(s), heap size 1136, 3 row lock(s)

MySQL thread id 444, OS thread handle 139830446065408, query id 5673 localhost root starting

show engine innodb status

TABLE LOCK table `test`.`l` trx id 31220340 lock mode IX

RECORD LOCKS space id 1358 page no 5 n bits 72 index b of table `test`.`l` trx id 31220340 lock_mode X

Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0

 0: len 4; hex 80000006; asc     ;;

 1: len 4; hex 80000004; asc     ;;

RECORD LOCKS space id 1358 page no 3 n bits 72 index PRIMARY of table `test`.`l` trx id 31220340 lock_mode X locks rec but not gap

Record lock, heap no 3 PHYSICAL RECORD: n_fields 6; compact format; info bits 0

 0: len 4; hex 80000004; asc     ;;

 1: len 6; hex 000001c1b93a; asc      :;;

 2: len 7; hex e1000001a90110; asc        ;;

 3: len 4; hex 80000006; asc     ;;

 4: len 4; hex 80000008; asc     ;;

 5: len 4; hex 8000000a; asc     ;;

RECORD LOCKS space id 1358 page no 5 n bits 72 index b of table `test`.`l` trx id 31220340 lock_mode X locks gap before rec

Record lock, heap no 4 PHYSICAL RECORD: n_fields 2; compact format; info bits 0

 0: len 4; hex 80000008; asc     ;;

 1: len 4; hex 80000006; asc     ;;

...

这个就稍微有点复杂了,依稀可以看到是加了三个锁,我们挨个分析一波

第一个锁锁住索引b(4,6],next-key lock锁 lock_mode X
第二个锁是对主键a=4这条唯一记录的主键上加一个记录锁（因为唯一）,lock_mode X locks rec but not gap
第三个锁是gap before rec 锁住了b(6,8),也就是对8加了gap

为什么要锁住(6,8)?

假设不锁住这块,一个线程插入了(3,6),只锁住(4,6]那就可以插入了,那原来的线程第一次返回的只有一条b=6的记录,那第二次执行就出现了两条b=6,就幻读了

tips：

新插入的6是在(6,8)这个范围里的，新插入的相同的记录，都在已存在的记录后面 4 6 6(新插) 8