MySQL锁系列（七）之 锁算法详解

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摘要：能学到什么 隔离级别和锁的关系 重点讲解在RR隔离级别下的加锁算法逻辑 重点罗列了比较典型的几种加锁逻辑案例 对insert的加锁逻辑进行了深度剖析 实战中剖析加锁的全过程 InnoDB为什么要这样加锁 隔离级别和算法 repeatable-read 1.

能学到什么

隔离级别和锁的关系

重点讲解在RR隔离级别下的加锁算法逻辑

重点罗列了比较典型的几种加锁逻辑案例

对insert的加锁逻辑进行了深度剖析

实战中剖析加锁的全过程

InnoDB为什么要这样加锁

隔离级别和算法

repeatable-read

1.使用的是next-keylocking2.next-keylock  =  record lock + Gap lock

read-committed

1. 使用的是 recordlock2.当然特殊情况下(purge+uniquekey)，也会有Gaplock

我们接下来就以RR隔离级别来阐述，因为RC更加简单

锁的通用算法

RR隔离级别

1.锁是在索引上实现的2.假设有一个key，有5条记录，1，3，5，7，9.如果where id<5， 那么锁住的区间不是（-∞，5），而是(-∞,1],(1,3],(3,5] 多个区间组合而成3.RR隔离级别使用的是：next-keylock算法，即：锁住 记录本身+区间4.next-keylock 降级为 record lock的情况    如果是唯一索引，且查询条件得到的结果集是1条记录（等值，而不是范围），那么会降级为记录锁    典型的案例：where primary_key =1(会降级), 而不是 where primary_key <10（由于返回的结果集不仅仅一条，那么不会降级）5.上锁，不仅仅对主键索引加锁，还需要对辅助索引加锁，这一点非常重要

锁算法的案例剖析

RR隔离级别

表结构

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> 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)*设置RR隔离级别set tx_isolation = 'repeatable-read';

等值查询，非唯一索引的加锁逻辑

dba:lc_3> begin;Query OK,0rows affected (0.00sec)dba:lc_3>select* from a where c=9forupdate;+---+------+------+------+| a | b    | c    | d    |+---+------+------+------+|5|7|9|11|+---+------+------+------+1rowinset(0.00sec)TABLE LOCK table `lc_3`.`a` trx id133601815lock mode IXRECORD LOCKSspaceid281page no5n bits72index idx_c of table `lc_3`.`a` trx id133601815lock_mode XRecord lock, heap no4PHYSICAL RECORD: n_fields2; compact format; info bits00:len4;hex80000009;asc;;1:len4;hex80000005;asc;;RECORD LOCKSspaceid281page no3n bits72index PRIMARY of table `lc_3`.`a` trx id133601815lock_mode X locks rec butnotgapRecord lock, heap no4PHYSICAL RECORD: n_fields6; compact format; info bits00:len4;hex80000005;asc;;1:len6;hex000007f66444;ascdD;;2:len7;hexfc0000271d012a;asc'  *;;3:len4;hex80000007;asc;;4:len4;hex80000009;asc;;5:len4;hex8000000b;asc;;RECORD LOCKSspaceid281page no5n bits72index idx_c of table `lc_3`.`a` trx id133601815lock_mode X locks gap before recRecord lock, heap no5PHYSICAL RECORD: n_fields2; compact format; info bits00:len4;hex8000000b;asc;;1:len4;hex80000007;asc;;锁的结构如下：对二级索引idx_c：1.加next-key lock，((7,3),(9,5)] , ((9,5),(11,7)]，解读一下：((7,3),(9,5)] 表示：7是二级索引key，3是对应的主键2.这样写不太好懂，所以以后就暂时忽略掉主键这样写：next-key lock = (7,9],(9,11]对主键索引primary： 加record lock，[5]

等值查询，唯一键的加锁逻辑

dba:lc_3>select* from a where b=9forupdate;+---+------+------+------+| a | b    | c    | d    |+---+------+------+------+|7|9|11|13|+---+------+------+------+1rowinset(0.00sec)TABLE LOCK table `lc_3`.`a` trx id133601816lock mode IXRECORD LOCKSspaceid281page no4n bits72index idx_b of table `lc_3`.`a` trx id133601816lock_mode X locks rec butnotgapRecord lock, heap no5PHYSICAL RECORD: n_fields2; compact format; info bits00:len4;hex80000009;asc;;1:len4;hex80000007;asc;;RECORD LOCKSspaceid281page no3n bits72index PRIMARY of table `lc_3`.`a` trx id133601816lock_mode X locks rec butnotgapRecord lock, heap no5PHYSICAL RECORD: n_fields6; compact format; info bits00:len4;hex80000007;asc;;1:len6;hex000007f66444;ascdD;;2:len7;hexfc0000271d0137;asc'  7;;3:len4;hex80000009;asc;;4:len4;hex8000000b;asc;;5:len4;hex8000000d;asc;;锁的结构如下：对二级索引idx_b：1.加record lock，[9]对主键索引primary：1.加record lock，[7]

>= ，非唯一索引的加锁逻辑

dba:lc_3>select* from a where c>=9forupdate;+---+------+------+------+| a | b    | c    | d    |+---+------+------+------+|5|7|9|11||7|9|11|13|+---+------+------+------+2rowsinset(0.00sec)TABLE LOCK table `lc_3`.`a` trx id133601817lock mode IXRECORD LOCKSspaceid281page no5n bits72index idx_c of table `lc_3`.`a` trx id133601817lock_mode XRecord lock, heap no1PHYSICAL RECORD: n_fields1; compact format; info bits00:len8;hex73757072656d756d;ascsupremum;;Record lock, heap no4PHYSICAL RECORD: n_fields2; compact format; info bits00:len4;hex80000009;asc;;1:len4;hex80000005;asc;;Record lock, heap no5PHYSICAL RECORD: n_fields2; compact format; info bits00:len4;hex8000000b;asc;;1:len4;hex80000007;asc;;RECORD LOCKSspaceid281page no3n bits72index PRIMARY of table `lc_3`.`a` trx id133601817lock_mode X locks rec butnotgapRecord lock, heap no4PHYSICAL RECORD: n_fields6; compact format; info bits00:len4;hex80000005;asc;;1:len6;hex000007f66444;ascdD;;2:len7;hexfc0000271d012a;asc'  *;;3:len4;hex80000007;asc;;4:len4;hex80000009;asc;;5:len4;hex8000000b;asc;;Record lock, heap no5PHYSICAL RECORD: n_fields6; compact format; info bits00:len4;hex80000007;asc;;1:len6;hex000007f66444;ascdD;;2:len7;hexfc0000271d0137;asc'  7;;3:len4;hex80000009;asc;;4:len4;hex8000000b;asc;;5:len4;hex8000000d;asc;;锁的结构如下：对二级索引idx_c：1.加next-key lock， (7,9],(9,11],(11,∞]对主键索引primary：1.加record lock，[5],[7]

>= ，唯一索引的加锁逻辑

dba:lc_3>select* from a where b>=7forupdate;+---+------+------+------+| a | b    | c    | d    |+---+------+------+------+|5|7|9|11||7|9|11|13|+---+------+------+------+2rowsinset(0.00sec)TABLE LOCK table `lc_3`.`a` trx id133601820lock mode IXRECORD LOCKSspaceid281page no4n bits72index idx_b of table `lc_3`.`a` trx id133601820lock_mode XRecord lock, heap no1PHYSICAL RECORD: n_fields1; compact format; info bits00:len8;hex73757072656d756d;ascsupremum;;Record lock, heap no4PHYSICAL RECORD: n_fields2; compact format; info bits00:len4;hex80000007;asc;;1:len4;hex80000005;asc;;Record lock, heap no5PHYSICAL RECORD: n_fields2; compact format; info bits00:len4;hex80000009;asc;;1:len4;hex80000007;asc;;RECORD LOCKSspaceid281page no3n bits72index PRIMARY of table `lc_3`.`a` trx id133601820lock_mode X locks rec butnotgapRecord lock, heap no4PHYSICAL RECORD: n_fields6; compact format; info bits00:len4;hex80000005;asc;;1:len6;hex000007f66444;ascdD;;2:len7;hexfc0000271d012a;asc'  *;;3:len4;hex80000007;asc;;4:len4;hex80000009;asc;;5:len4;hex8000000b;asc;;Record lock, heap no5PHYSICAL RECORD: n_fields6; compact format; info bits00:len4;hex80000007;asc;;1:len6;hex000007f66444;ascdD;;2:len7;hexfc0000271d0137;asc'  7;;3:len4;hex80000009;asc;;4:len4;hex8000000b;asc;;5:len4;hex8000000d;asc;;锁的结构如下：对二级索引idx_b：1.加next-key lock， (5,7],(7,9],(9,∞]对主键索引primary：1.加record lock，[5],[7]

<= , 非唯一索引的加锁逻辑

dba:lc_3>select* from a where c<=7forupdate;+---+------+------+------+| a | b    | c    | d    |+---+------+------+------+|1|3|5|7||3|5|7|9|+---+------+------+------+2rowsinset(0.00sec)TABLE LOCK table `lc_3`.`a` trx id133601822lock mode IXRECORD LOCKSspaceid281page no5n bits72index idx_c of table `lc_3`.`a` trx id133601822lock_mode XRecord lock, heap no2PHYSICAL RECORD: n_fields2; compact format; info bits00:len4;hex80000005;asc;;1:len4;hex80000001;asc;;Record lock, heap no3PHYSICAL RECORD: n_fields2; compact format; info bits00:len4;hex80000007;asc;;1:len4;hex80000003;asc;;Record lock, heap no4PHYSICAL RECORD: n_fields2; compact format; info bits00:len4;hex80000009;asc;;1:len4;hex80000005;asc;;RECORD LOCKSspaceid281page no3n bits72index PRIMARY of table `lc_3`.`a` trx id133601822lock_mode X locks rec butnotgapRecord lock, heap no2PHYSICAL RECORD: n_fields6; compact format; info bits00:len4;hex80000001;asc;;1:len6;hex000007f66444;ascdD;;2:len7;hexfc0000271d0110;asc'  ;;3:len4;hex80000003;asc;;4:len4;hex80000005;asc;;5:len4;hex80000007;asc;;Record lock, heap no3PHYSICAL RECORD: n_fields6; compact format; info bits00:len4;hex80000003;asc;;1:len6;hex000007f66444;ascdD;;2:len7;hexfc0000271d011d;asc'  ;;3:len4;hex80000005;asc;;4:len4;hex80000007;asc;;5:len4;hex80000009;asc;;锁的结构如下：对二级索引idx_c：1.加next-key lock， (-∞,5],(5,7],(7,9]对主键索引primary：1.加record lock，[1],[3]

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