MySQL锁系列(二)之 锁解读-阿里云开发者社区

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MySQL锁系列(二)之 锁解读

简介:

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

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