MySQL锁系列（二）之 锁解读

原文链接

https://keithlan.github.io/2017/06/05/innodb_locks_show_engine/

背景

  1. 锁系列第一期的时候介绍的锁，我们要如何去解读呢？
  2. 在哪里能够看到这些锁？

锁信息解读

工欲善其事必先利其器
show engine innodb status 关于锁的信息是最详细的

案例一（有索引的情况）

  • 前期准备
 
dba:lc_3>
dba:lc_3>
dba:lc_3> show create table a;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------
----------------------------------------------------------------------+
| Table | Create Table
|
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------
----------------------------------------------------------------------+
| a | CREATE TABLE `a` (
`a` int(11) NOT NULL,
`b` int(11) DEFAULT NULL,
`c` int(11) DEFAULT NULL,
`d` int(11) DEFAULT NULL,
PRIMARY KEY (`a`),
UNIQUE KEY `idx_b` (`b`),
KEY `idx_c` (`c`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------
----------------------------------------------------------------------+
1 row in set (0.00 sec)
 
dba:lc_3>
dba:lc_3> select * from a;
+---+------+------+------+
| a | b | c | d |
+---+------+------+------+
| 1 | 3 | 5 | 7 |
| 3 | 5 | 7 | 9 |
| 5 | 7 | 9 | 11 |
| 7 | 9 | 11 | 13 |
+---+------+------+------+
4 rows in set (0.00 sec)
  • 产生锁的语句
dba:lc_3> set tx_isolation = 'repeatable-read'; --事务隔离级别为repeatable-read，以后介绍
Query OK, 0 rows affected (0.00 sec)
 
begin;
select * from a where c=7 for update;
  • show engine innodb status
 
------------
TRANSACTIONS
------------
Trx id counter 133588132
Purge done for trx's n:o < 133588131 undo n:o < 0 state: running but idle
History list length 836
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 421565826149088, not started
0 lock struct(s), heap size 1136, 0 row lock(s)
---TRANSACTION 133588131, ACTIVE 8 sec
4 lock struct(s), heap size 1136, 3 row lock(s)
MySQL thread id 116, OS thread handle 140001238423296, query id 891 localhost dba cleaning up
TABLE LOCK table `lc_3`.`a` trx id 133588131 lock mode IX
RECORD LOCKS space id 281 page no 5 n bits 72 index idx_c of table `lc_3`.`a` trx id 133588131 lock_mode X
Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 80000007; asc ;;
1: len 4; hex 80000003; asc ;;
 
RECORD LOCKS space id 281 page no 3 n bits 72 index PRIMARY of table `lc_3`.`a` trx id 133588131 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 80000003; asc ;;
1: len 6; hex 000007f66444; asc dD;;
2: len 7; hex fc0000271d011d; asc ' ;;
3: len 4; hex 80000005; asc ;;
4: len 4; hex 80000007; asc ;;
5: len 4; hex 80000009; asc ;;
 
RECORD LOCKS space id 281 page no 5 n bits 72 index idx_c of table `lc_3`.`a` trx id 133588131 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 80000009; asc ;;
1: len 4; hex 80000005; asc ;;
  • show engine innodb status 解读
* Trx id counter 133588132
 
描述的是：下一个事务的id为133588132
 
* Purge done for trx's n:o < 133588131 undo n:o < 0 state: running but idle
 
Purge线程已经将trxid小于133588131的事务都purge了，目前purge线程的状态为idle
Purge线程无法控制
 
* History list length 836
 
undo中未被清除的事务数量，如果这个值非常大，说明系统来不及回收undo，需要人工介入了。
 
疑问：上面的purge都已经刷新完了，为什么History list length 不等于0，这是一个有意思的问题
 
* ---TRANSACTION 133588131, ACTIVE 8 sec
 
当前事务id为133588131
 
* 4 lock struct(s), heap size 1136, 3 row lock(s)
 
产生了4个锁对象结构，占用内存大小1136字节，3条记录被锁住(1个表锁，3个记录锁)
 
* TABLE LOCK table `lc_3`.`a` trx id 133588131 lock mode IX
 
在a表上面有一个表锁，这个锁的模式为IX（排他意向锁）
 
* RECORD LOCKS space id 281 page no 5 n bits 72 index idx_c of table `lc_3`.`a` trx id 133588131 lock_mode X
 
在space id=281（a表的表空间），page no=5的页上，对表a上的idx_c索引加了记录锁，锁模式为：next-key 锁（这个在上一节中有告知）
该页上面的位图锁占有72bits
 
* 具体锁了哪些记录
 
Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0 -- heap no 3 的记录被锁住了
0: len 4; hex 80000007; asc ;; --这是一个二级索引上的锁，7被锁住
1: len 4; hex 80000003; asc ;; --二级索引上面还会自带一个主键，所以主键值3也会被锁住
 
RECORD LOCKS space id 281 page no 3 n bits 72 index PRIMARY of table `lc_3`.`a` trx id 133588131 lock_mode X locks rec but not gap（这是一个记录锁，在主键上锁住的）
Record lock, heap no 3 PHYSICAL RECORD: n_fields 6; compact format; info bits 0
0: len 4; hex 80000003; asc ;; --第一个字段是主键3，占用4个字节，被锁住了
1: len 6; hex 000007f66444; asc dD;; --该字段为6个字节的事务id，这个id表示最近一次被更新的事务id
2: len 7; hex fc0000271d011d; asc ' ;; --该字段为7个字节的回滚指针，用于mvcc
3: len 4; hex 80000005; asc ;; --该字段表示的是此记录的第二个字段5
4: len 4; hex 80000007; asc ;; --该字段表示的是此记录的第三个字段7
5: len 4; hex 80000009; asc ;; --该字段表示的是此记录的第四个字段9
 
RECORD LOCKS space id 281 page no 5 n bits 72 index idx_c of table `lc_3`.`a` trx id 133588131 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 80000009; asc ;; --这是一个二级索引上的锁，9被锁住
1: len 4; hex 80000005; asc ;; --二级索引上面还会自带一个主键，所以主键值5被锁住

案例二(无索引的情况)

  • 前期准备
 
dba:lc_3> show create table t;
+-------+------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+------------------------------------------------------------------------------------+
| t | CREATE TABLE `t` (
`i` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
 
dba:lc_3> select * from t;
+------+
| i |
+------+
| 1 |
| 2 |
| 3 |
| 4 |
| 5 |
| 5 |
| 5 |
| 5 |
| 5 |
| 5 |
| 5 |
| 5 |
| 5 |
| 5 |
| 5 |
| 5 |
+------+
16 rows in set (0.00 sec)
  • 产生锁语句
 
dba:lc_3> set tx_isolation = 'repeatable-read';
Query OK, 0 rows affected (0.00 sec)
 
dba:lc_3> select * from t where i=1 for update;
+------+
| i |
+------+
| 1 |
+------+
1 row in set (0.00 sec)
  • show engine innodb status
------------
TRANSACTIONS
------------
Trx id counter 133588133
Purge done for trx's n:o < 133588131 undo n:o < 0 state: running but idle
History list length 836
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 421565826149088, not started
0 lock struct(s), heap size 1136, 0 row lock(s)
---TRANSACTION 133588132, ACTIVE 6 sec
2 lock struct(s), heap size 1136, 17 row lock(s)
MySQL thread id 118, OS thread handle 140001238955776, query id 904 localhost dba cleaning up
TABLE LOCK table `lc_3`.`t` trx id 133588132 lock mode IX
RECORD LOCKS space id 278 page no 3 n bits 88 index GEN_CLUST_INDEX of table `lc_3`.`t` trx id 133588132 lock_mode X
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
0: len 8; hex 73757072656d756d; asc supremum;;
 
Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000000dff05; asc ;;
1: len 6; hex 000007f66397; asc c ;;
2: len 7; hex fb0000271c0110; asc ' ;;
3: len 4; hex 80000001; asc ;;
 
Record lock, heap no 3 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000000dff06; asc ;;
1: len 6; hex 000007f663ea; asc c ;;
2: len 7; hex bb000027340110; asc '4 ;;
3: len 4; hex 80000002; asc ;;
 
Record lock, heap no 4 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000000dff07; asc ;;
1: len 6; hex 000007f66426; asc d&;;
2: len 7; hex e4000040210110; asc @! ;;
3: len 4; hex 80000003; asc ;;
 
Record lock, heap no 5 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000000dff08; asc ;;
1: len 6; hex 000007f66427; asc d';;
2: len 7; hex e5000040220110; asc @" ;;
3: len 4; hex 80000004; asc ;;
 
Record lock, heap no 6 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000000dff09; asc ;;
1: len 6; hex 000007f6642c; asc d,;;
2: len 7; hex e8000040230110; asc @# ;;
3: len 4; hex 80000005; asc ;;
 
Record lock, heap no 7 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000000dff0a; asc ;;
1: len 6; hex 000007f6642d; asc d-;;
2: len 7; hex e9000040240110; asc @$ ;;
3: len 4; hex 80000005; asc ;;
 
Record lock, heap no 8 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000000dff0b; asc ;;
1: len 6; hex 000007f66432; asc d2;;
2: len 7; hex ec0000273f0110; asc '? ;;
3: len 4; hex 80000005; asc ;;
 
Record lock, heap no 9 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000000dff0c; asc ;;
1: len 6; hex 000007f66433; asc d3;;
2: len 7; hex ed000040020110; asc @ ;;
3: len 4; hex 80000005; asc ;;
 
Record lock, heap no 10 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000000dff0d; asc ;;
1: len 6; hex 000007f66434; asc d4;;
2: len 7; hex ee000040030110; asc @ ;;
3: len 4; hex 80000005; asc ;;
 
Record lock, heap no 11 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000000dff0e; asc ;;
1: len 6; hex 000007f66435; asc d5;;
2: len 7; hex ef000040040110; asc @ ;;
3: len 4; hex 80000005; asc ;;
 
Record lock, heap no 12 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000000dff0f; asc ;;
1: len 6; hex 000007f66436; asc d6;;
2: len 7; hex f0000040050110; asc @ ;;
3: len 4; hex 80000005; asc ;;
 
Record lock, heap no 13 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000000dff10; asc ;;
1: len 6; hex 000007f66437; asc d7;;
2: len 7; hex f1000040060110; asc @ ;;
3: len 4; hex 80000005; asc ;;
 
Record lock, heap no 14 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000000dff11; asc ;;
1: len 6; hex 000007f66438; asc d8;;
2: len 7; hex f2000027130110; asc ' ;;
3: len 4; hex 80000005; asc ;;
 
Record lock, heap no 15 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000000dff12; asc ;;
1: len 6; hex 000007f66439; asc d9;;
2: len 7; hex f3000027140110; asc ' ;;
3: len 4; hex 80000005; asc ;;
 
Record lock, heap no 16 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000000dff13; asc ;;
1: len 6; hex 000007f6643a; asc d:;;
2: len 7; hex f4000027150110; asc ' ;;
3: len 4; hex 80000005; asc ;;
 
Record lock, heap no 17 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 6; hex 0000000dff14; asc ;;
1: len 6; hex 000007f6643b; asc d;;;
2: len 7; hex f5000027160110; asc ' ;;
3: len 4; hex 80000005; asc ;;
  • 锁解读
1. 这里只列出跟第一个案例不同的地方解读，其他的都一样
 
2. RECORD LOCKS space id 278 page no 3 n bits 88 index GEN_CLUST_INDEX of table `lc_3`.`t` trx id 133588132 lock_mode X
 
由于表定义没有显示的索引，而InnoDB又是索引组织表，会自动创建一个索引，这里面叫index GEN_CLUST_INDEX
 
3. 由于没有索引，那么会对每条记录都加上lock_mode X （next-key lock）
 
4. 这里有一个明显不一样的是：
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
0: len 8; hex 73757072656d756d; asc supremum;;
 
supremum 值得是页里面的最后一条记录(伪记录，通过select查不到的,并不是真实的记录)，heap no=1 , Infimum 表示的是页里面的第一个记录（伪记录）
 
可以简单的认为：
supremum 为upper bounds，正去穷大
Infimum 为Minimal bounds，负无穷大
 
那这里的加锁的意思就是：通过supremum 锁住index GEN_CLUST_INDEX的最大值到正无穷大的区间，这样就可以锁住全部记录，以及全部间隙，相当于表锁

锁开销

  • 锁10条记录和锁1条记录的开销是成正比的吗？
1. 由于锁的内存对象针对的是页而不是记录，所以开销并不是非常大
2. 锁10条记录和锁1条记录的内存开销都是一样的，都是heap size=1136个字节

最后

这里面select * from a where c=7 for update; 明明只锁一条记录，为什么却看到4把锁呢？
看到这里是不是有点晕，没关系，这个问题，后面会慢慢揭晓答案

posted @ 2024-03-04 22:23  翎野君  阅读(17)  评论(0)  编辑  收藏  举报