内容简介：MySQL锁系列（二）之 锁解读
背景
- 锁系列第一期的时候介绍的锁，我们要如何去解读呢？
- 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 Purgedonefortrx'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, heapno 3PHYSICAL RECORD: n_fields4; 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, heapno 4PHYSICAL RECORD: n_fields4; 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, heapno 5PHYSICAL RECORD: n_fields4; 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, heapno 6PHYSICAL RECORD: n_fields4; 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, heapno 7PHYSICAL RECORD: n_fields4; 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, heapno 8PHYSICAL RECORD: n_fields4; 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, heapno 9PHYSICAL RECORD: n_fields4; 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, heapno 10PHYSICAL RECORD: n_fields4; 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, heapno 11PHYSICAL RECORD: n_fields4; 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, heapno 12PHYSICAL RECORD: n_fields4; 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, heapno 13PHYSICAL RECORD: n_fields4; 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, heapno 14PHYSICAL RECORD: n_fields4; 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, heapno 15PHYSICAL RECORD: n_fields4; 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, heapno 16PHYSICAL RECORD: n_fields4; 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, heapno 17PHYSICAL RECORD: n_fields4; 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
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持 码农网
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[美] Daniel P. Friedman、[美] Matthias Felleisen / 卢俊祥 / 电子工业出版社 / 2017-7 / 65.00
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