从一个案例深入剖析InnoDB隐式锁和可见性判断(2)-阿里云开发者社区

从一个案例深入剖析InnoDB隐式锁和可见性判断(2)

2022-09-27 69

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简介： 从一个案例深入剖析InnoDB隐式锁和可见性判断

4、关于page的max trx id

我们上面多次提到二级索引page的max trx id，这个max trx id实际就是PAGE_MAX_TRX_ID，它位于page的offset 56后的8个字节，实际上这个值只会存在于二级索引上，主键没有这个值，我们可以看到如下：

表结构和数据
mysql> show create table testimp4 \G
*************************** 1. row ***************************
       Table: testimp4
Create Table: CREATE TABLE `testimp4` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `a` int(11) DEFAULT NULL,
  `b` int(11) DEFAULT NULL,
  `d` varchar(200) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `b` (`b`),
  KEY `d` (`d`)
) ENGINE=InnoDB AUTO_INCREMENT=10000 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql> select *from testimp4;
+------+------+------+------------------------------------+
| id   | a    | b    | d                                  |
+------+------+------+------------------------------------+
|    5 |    5 |  300 | NULL                               |
|    6 | 7000 | 7700 | 1124                               |
|   11 | 7000 | 7700 | 1124                               |
|   12 | 7000 | 7700 | 1124                               |
|   13 | 2900 | 1800 | NULL                               |
|   14 | 2900 | 1800 | NULL                               |
| 1000 |   88 | 1499 | NULL                               |
| 4000 | 6000 | 5904 | iiiafsafasfihhhccccchhhigggofgo111 |
| 4001 | 7000 | 7700 | 1124454555                         |
| 9999 | 9999 | 9999 | a                                  |
+------+------+------+------------------------------------+
10 rows in set (0.00 sec)

每次每行更新后会更新这个值，如果大于则修改，小于则不变。函数page_update_max_trx_id中有如下片段

begin；insert into testimp4 values(10000,10000,10000,'gp');（不提交）

四、关于加锁的阶段

我们一般锁需要加锁的都是DML语句和select for update这样的语句，这里将加锁分为数据查找和数据修改两个阶段。

对于select for update：

主键访问数据：访问主键判断是否存在隐式锁，然后加显示锁。二级索引访问数据（需要回表的情况）：访问二级索引判断是否存在隐式锁，然后加显示锁，接着回表主键判断是否存在隐式锁，然后加显示锁。

对于update/delete：

主键访问修改数据：数据查找阶段主键判断是否存在隐式锁，然后加显示锁。数据修改阶段涉及到了其他二级索引，那么维护相应的二级索引加隐含锁。

二级索引访问修改数据：数据查找阶段二级索引判断是否存在隐式锁（可能需要回表判断），二级索引加显示锁，数据修改阶段回表修改主键数据加显示锁，然后维护各个二级索引（修改字段涉及的二级索引或者修改主键则包含全部二级索引）加隐式锁。

对于insert而言如果没有堵塞（插入印象锁和gap lock堵塞），那么始终为隐式锁。

注意这里我们看到了隐式锁，隐式锁不会占用row的结构体，因此在show engine innodb status里面是看不到的，除非有其他事务显示将其转换为显示锁。我们来做几个例子如下（REPEATABLE READ隔离级别）：

4.1 插入数据

begin；insert into testimp4 values(10000,10000,10000,'gp');（不提交）

# T1时刻S1锁状态：

---TRANSACTION 94487, ACTIVE 5 sec
1 lock struct(s), heap size 1160, 0 row lock(s), undo log entries 1
MySQL thread id 11, OS thread handle 140737089492736, query id 482 localhost root starting
show engine innodb status
TABLE LOCK table `test`.`testimp4` trx id 94487 lock mode IX

# T2时刻S1锁状态：
---TRANSACTION 94487, ACTIVE 271 sec
2 lock struct(s), heap size 1160, 1 row lock(s), undo log entries 1
MySQL thread id 11, OS thread handle 140737089492736, query id 484 localhost root starting
show engine innodb status
TABLE LOCK table `test`.`testimp4` trx id 94487 lock mode IX
RECORD LOCKS space id 501 page no 3 n bits 80 index PRIMARY of table `test`.`testimp4` trx id 94487 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 80002710; asc ' ;;
1: len 6; hex 000000017117; asc q ;;
2: len 7; hex d0000002c40110; asc ;;
3: len 4; hex 80002710; asc ' ;;
4: len 4; hex 80002710; asc ' ;;
5: len 2; hex 6770; asc gp;;

# T3时刻S1锁状态：
---TRANSACTION 94487, ACTIVE 337 sec
3 lock struct(s), heap size 1160, 2 row lock(s), undo log entries 1
MySQL thread id 11, OS thread handle 140737089492736, query id 521 localhost root starting
show engine innodb status
TABLE LOCK table `test`.`testimp4` trx id 94487 lock mode IX
RECORD LOCKS space id 501 page no 3 n bits 80 index PRIMARY of table `test`.`testimp4` trx id 94487 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 80002710; asc ' ;;
1: len 6; hex 000000017117; asc q ;;
2: len 7; hex d0000002c40110; asc ;;
3: len 4; hex 80002710; asc ' ;;
4: len 4; hex 80002710; asc ' ;;
5: len 2; hex 6770; asc gp;;

RECORD LOCKS space id 501 page no 4 n bits 80 index b of table `test`.`testimp4` trx id 94487 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 80002710; asc ' ;;
1: len 4; hex 80002710; asc ' ;;

# T4时刻S1锁状态：
---TRANSACTION 94487, ACTIVE 408 sec
4 lock struct(s), heap size 1160, 3 row lock(s), undo log entries 1
MySQL thread id 11, OS thread handle 140737089492736, query id 559 localhost root starting
show engine innodb status
TABLE LOCK table `test`.`testimp4` trx id 94487 lock mode IX
RECORD LOCKS space id 501 page no 3 n bits 80 index PRIMARY of table `test`.`testimp4` trx id 94487 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 80002710; asc ' ;;
1: len 6; hex 000000017117; asc q ;;
2: len 7; hex d0000002c40110; asc ;;
3: len 4; hex 80002710; asc ' ;;
4: len 4; hex 80002710; asc ' ;;
5: len 2; hex 6770; asc gp;;

RECORD LOCKS space id 501 page no 4 n bits 80 index b of table `test`.`testimp4` trx id 94487 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 80002710; asc ' ;;
1: len 4; hex 80002710; asc ' ;;

RECORD LOCKS space id 501 page no 5 n bits 80 index d of table `test`.`testimp4` trx id 94487 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 2; hex 6770; asc gp;;
1: len 4; hex 80002710; asc ' ;;

实际上我们看到这里insert语句后主键和各个索引都上了隐含锁只是看不到，通过其他S2,S3,S4我们逐步把这些隐式锁转换为了显示锁。

4.2 delete语句通过主键删除数据

# T1时刻S1锁状态：
---TRANSACTION 94493, ACTIVE 3 sec
2 lock struct(s), heap size 1160, 1 row lock(s), undo log entries 1
MySQL thread id 11, OS thread handle 140737089492736, query id 567 localhost root
TABLE LOCK table `test`.`testimp4` trx id 94493 lock mode IX
RECORD LOCKS space id 501 page no 3 n bits 80 index PRIMARY of table `test`.`testimp4` trx id 94493 lock_mode X locks rec but not gap
Record lock, heap no 12 PHYSICAL RECORD: n_fields 6; compact format; info bits 32
 0: len 4; hex 8000270f; asc   ' ;;
 1: len 6; hex 00000001711d; asc     q ;;
 2: len 7; hex 550000003b071b; asc U   ;  ;;
 3: len 4; hex 8000270f; asc   ' ;;
 4: len 4; hex 8000270f; asc   ' ;;
 5: len 1; hex 61; asc a;;

# T2时刻S1锁状态：
---TRANSACTION 94493, ACTIVE 112 sec
4 lock struct(s), heap size 1160, 3 row lock(s), undo log entries 1
MySQL thread id 11, OS thread handle 140737089492736, query id 567 localhost root
TABLE LOCK table `test`.`testimp4` trx id 94493 lock mode IX
RECORD LOCKS space id 501 page no 3 n bits 80 index PRIMARY of table `test`.`testimp4` trx id 94493 lock_mode X locks rec but not gap
Record lock, heap no 12 PHYSICAL RECORD: n_fields 6; compact format; info bits 32
 0: len 4; hex 8000270f; asc   ' ;;
 1: len 6; hex 00000001711d; asc     q ;;
 2: len 7; hex 550000003b071b; asc U   ;  ;;
 3: len 4; hex 8000270f; asc   ' ;;
 4: len 4; hex 8000270f; asc   ' ;;
 5: len 1; hex 61; asc a;;

RECORD LOCKS space id 501 page no 4 n bits 80 index b of table `test`.`testimp4` trx id 94493 lock_mode X locks rec but not gap
Record lock, heap no 12 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 8000270f; asc   ' ;;
 1: len 4; hex 8000270f; asc   ' ;;

# T3时刻S1锁状态：
---TRANSACTION 94493, ACTIVE 133 sec
4 lock struct(s), heap size 1160, 3 row lock(s), undo log entries 1
MySQL thread id 11, OS thread handle 140737089492736, query id 567 localhost root
TABLE LOCK table `test`.`testimp4` trx id 94493 lock mode IX
RECORD LOCKS space id 501 page no 3 n bits 80 index PRIMARY of table `test`.`testimp4` trx id 94493 lock_mode X locks rec but not gap
Record lock, heap no 12 PHYSICAL RECORD: n_fields 6; compact format; info bits 32
 0: len 4; hex 8000270f; asc   ' ;;
 1: len 6; hex 00000001711d; asc     q ;;
 2: len 7; hex 550000003b071b; asc U   ;  ;;
 3: len 4; hex 8000270f; asc   ' ;;
 4: len 4; hex 8000270f; asc   ' ;;
 5: len 1; hex 61; asc a;;

RECORD LOCKS space id 501 page no 4 n bits 80 index b of table `test`.`testimp4` trx id 94493 lock_mode X locks rec but not gap
Record lock, heap no 12 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 8000270f; asc   ' ;;
 1: len 4; hex 8000270f; asc   ' ;;

RECORD LOCKS space id 501 page no 5 n bits 80 index d of table `test`.`testimp4` trx id 94493 lock_mode X locks rec but not gap
Record lock, heap no 12 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 1; hex 61; asc a;;
 1: len 4; hex 8000270f; asc   ' ;;