mysql innodb lock机制原理

发布时间：2020-08-06 16:39:59 作者：wwjfeng
来源：ITPUB博客阅读：214

一.lock 和 latch
latch 一般称为闩锁，目的是用来保证并发线程操作临界资源的正确性，无死锁检测机制。分为：mutex(互斥量)，rwlock(读写锁)
lock 的对象是事务，用来锁定的数据库中的对象。
mysql innodb lock机制原理

二.lock 种类
1.行级锁
共享锁(S lock)：允许事务读取一行数据
排他锁(X lock)：允许事务修改或删除一行数据
2.表级锁
意向共享锁(IS lock):在对行加S锁之前，先对其表追加IS锁
意向排他锁(IX lock):在对行加X锁之前，先对其表追加IX锁

表级意向锁和行级锁的兼容性：
mysql innodb lock机制原理

三.锁的应用场景
1.一致性非锁定读
mysql innodb lock机制原理

2.一致性锁定读
通过select * from table for update; 或 select * from table lock in share mode; 来锁定读取数据，在数据读取过程中其他事务不能修改该数据。

3.自增长与锁
含有自增长值的表，都有一个自增长计数器，当对该表进行插入操作时，执行如下语句来得到计数器的值。
select max(auto_inc_col) from table for update;
该锁不是事务完成之后才释放，而是insert命令执行完成后就释放该锁。

mysql5.1.22之前，该模式对于有自增值列的表的并发插入性能较差。
mysql5.1.22开始，innodb提供了一种轻量级的互斥量的自增长实现机制，这种机制大大提高了自增长值的插入性能。
相关参数，innodb_autoinc_lock_mode,默认值为1
mysql innodb lock机制原理

4.外键和锁
在innodb存储引擎下，外键列如果没有显式的建立index，mysql会为该列自动添加index，避免发生表锁。
对于外键值的插入和更新，会先select父表，但该select操作并不是一致性非锁定读，而是一致性锁定读（对父表追加S锁）。因此当父表被其他事务加上X锁时，子表的操作会被阻塞。

--主表

CREATE TABLE `wwj`.`t1` (

`deptno` INT NOT NULL,

`deptname` VARCHAR(45) NOT NULL,

`address` VARCHAR(45) NOT NULL,

PRIMARY KEY (`deptno`));

--子表

CREATE TABLE `wwj`.`t2` (

`empno` INT NOT NULL,

`empname` VARCHAR(45) NOT NULL,

`age` INT NOT NULL,

`deptno` INT NOT NULL,

PRIMARY KEY (`empno`),

INDEX `deptno_idx` (`deptno` ASC),

CONSTRAINT `deptno`

FOREIGN KEY (`deptno`)

REFERENCES `wwj`.`t1` (`deptno`)

ON DELETE NO ACTION

ON UPDATE NO ACTION);

insert into wwj.t1 values(1,'it','北京');

insert into wwj.t1 values(2,'product','天津');

insert into wwj.t1 values(3,'haha','上海');

mysql> select * from information_schema.innodb_locks\G;

*************************** 1. row ***************************

lock_id: 1303:26:3:2

lock_trx_id: 1303

lock_mode: S

lock_type: RECORD

lock_table: `wwj`.`t1`

lock_index: PRIMARY

lock_space: 26

lock_page: 3

lock_rec: 2

lock_data: 1

*************************** 2. row ***************************

lock_id: 1298:26:3:2

lock_trx_id: 1298

lock_mode: X

lock_type: RECORD

lock_table: `wwj`.`t1`

lock_index: PRIMARY

lock_space: 26

lock_page: 3

lock_rec: 2

lock_data: 1

2 rows in set, 1 warning (0.00 sec)

四.锁的算法
行锁的三种算法：
1.record lock
单个记录上的锁

2.gap lock
间隙锁，锁定一个范围，但不包含记录本身

3.next-key lock
record lock+gap lock 锁定一个范围，但不包含记录本身

--场景模拟

CREATE TABLE `wwj`.`t3` (

`idt3` INT NOT NULL,

`idt4` INT NOT NULL,

PRIMARY KEY (`idt3`),

INDEX `idx-1` (`idt4` ASC));

insert into wwj.t3 values(1,10);

insert into wwj.t3 values(3,30);

insert into wwj.t3 values(5,50);

mysql> select * from wwj.t3;

+------+------+

| idt3 | idt4 |

+------+------+

| 1 | 10 |

| 3 | 30 |

| 5 | 50 |

+------+------+

唯一索引的锁定范围

因为idt3上有唯一索引，因此锁定的只是idt3=3这个值，而不是(1,3)这个范围，即锁定由next-key lock降级为record lock

辅助索引的锁定范围
mysql innodb lock机制原理

五.一条sql的加锁范围
对于各种情况下加锁的分析

mysql> show full processlist;

+----+------+-----------+------+---------+------+----------+-----------------------+

+----+------+-----------+------+---------+------+----------+-----------------------+

+----+------+-----------+------+---------+------+----------+-----------------------+

杀掉线程:

KILL [CONNECTION | QUERY] thread_id

kill 13 --杀掉线程连接

kill query 13 --杀掉正在执行的语句，保留连接

mysql> show engine innodb status\G;

------------

TRANSACTIONS

------------

Trx id counter 1296

Purge done for trx's n:o < 1294 undo n:o < 0 state: running but idle

History list length 2

LIST OF TRANSACTIONS FOR EACH SESSION:

---TRANSACTION 421324408397424, not started

0 lock struct(s), heap size 1136, 0 row lock(s)

---TRANSACTION 1295, ACTIVE 396 sec inserting

mysql tables in use 1, locked 1

LOCK WAIT 2 lock struct(s), heap size 1136, 4 row lock(s), undo log entries 1

MySQL thread id 12, OS thread handle 139848225883904, query id 61 localhost root executing

insert into wwj.t3 select 4,20

------- TRX HAS BEEN WAITING 10 SEC FOR THIS LOCK TO BE GRANTED:

RECORD LOCKS space id 26 page no 4 n bits 72 index idx-1 of table `wwj`.`t3` trx id 1295 lock_mode X locks gap before rec insert intention waiting

Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0

0: len 4; hex 8000001e; asc ;;

1: len 4; hex 80000003; asc ;;

---------------------

TRANSACTION 1294, ACTIVE 449 sec

4 lock struct(s), heap size 1136, 3 row lock(s)

MySQL thread id 11, OS thread handle 139848226150144, query id 50 localhost root

Trx read view will not see trx with id >= 1294, sees < 1294

mysql> select * from information_schema.INNODB_LOCK_WAITS;

+-------------------+-------------------+-----------------+------------------+

+-------------------+-------------------+-----------------+------------------+

| 1302 | 1302:26:4:3 | 1301 | 1301:26:4:3 |

+-------------------+-------------------+-----------------+------------------+

1 row in set, 1 warning (0.00 sec)

mysql> select lock_id,lock_trx_id,lock_mode,lock_type,lock_table,lock_index from information_schema.INNODB_LOCKs;

+-------------+-------------+-----------+-----------+------------+------------+

+-------------+-------------+-----------+-----------+------------+------------+

| 1302:26:4:3 | 1302 | X,GAP | RECORD | `wwj`.`t3` | idx-1 |

| 1301:26:4:3 | 1301 | X | RECORD | `wwj`.`t3` | idx-1 |

+-------------+-------------+-----------+-----------+------------+------------+

2 rows in set, 1 warning (0.00 sec)

SELECT

        p2.`HOST` Blockedhost, #被阻塞方host
        p2.`USER` BlockedUser, #被阻塞方用户
        r.trx_id BlockedTrxId, #被阻塞方事务id
        r.trx_mysql_thread_id BlockedThreadId,      #被阻塞方线程号
        TIMESTAMPDIFF(
            SECOND,
            r.trx_wait_started,
            CURRENT_TIMESTAMP
        ) WaitTime,   #等待时间
        r.trx_query BlockedQuery,         #被阻塞的查询
        l.lock_table BlockedTable,        #阻塞方锁住的表
        m.`lock_mode` BlockedLockMode,    #被阻塞方的锁模式
        m.`lock_type` BlockedLockType,    #被阻塞方的锁类型(表锁还是行锁)
        m.`lock_index` BlockedLockIndex, #被阻塞方锁住的索引
        m.`lock_space` BlockedLockSpace, #被阻塞方锁对象的space_id
        m.lock_page BlockedLockPage,      #被阻塞方事务锁定页的数量
        m.lock_rec BlockedLockRec,        #被阻塞方事务锁定行的数量
        m.lock_data BlockedLockData,      #被阻塞方事务锁定记录的主键值
        p.`HOST` blocking_host,           #阻塞方主机
        p.`USER` blocking_user,           #阻塞方用户
        b.trx_id BlockingTrxid,           #阻塞方事务id
        b.trx_mysql_thread_id BlockingThreadId, #阻塞方线程号
        b.trx_query BlockingQuery,        #阻塞方查询
        l.`lock_mode` BlockingLockMode,   #阻塞方的锁模式
        l.`lock_type` BlockingLockType,   #阻塞方的锁类型(表锁还是行锁)
        l.`lock_index` BlockingLockIndex, #阻塞方锁住的索引
        l.`lock_space` BlockingLockSpace, #阻塞方锁对象的space_id
        l.lock_page BlockingLockPage,     #阻塞方事务锁定页的数量
        l.lock_rec BlockingLockRec,       #阻塞方事务锁定行的数量
        l.lock_data BlockingLockData,     #阻塞方事务锁定记录的主键值
       IF (p.COMMAND = 'Sleep', CONCAT(p.TIME,' seconds'), 0) idel_in_trx #阻塞方事务空闲的时间
    FROM
        information_schema.INNODB_LOCK_WAITS w
    INNER JOIN information_schema.INNODB_TRX b ON b.trx_id = w.blocking_trx_id
    INNER JOIN information_schema.INNODB_TRX r ON r.trx_id = w.requesting_trx_id
    INNER JOIN information_schema.INNODB_LOCKS l ON w.blocking_lock_id = l.lock_id AND l.`lock_trx_id`=b.`trx_id`
      INNER JOIN information_schema.INNODB_LOCKS m ON m.`lock_id`=w.`requested_lock_id` AND m.`lock_trx_id`=r.`trx_id`
    INNER JOIN information_schema. PROCESSLIST p ON p.ID = b.trx_mysql_thread_id
INNER JOIN information_schema. PROCESSLIST p2 ON p2.ID = r.trx_mysql_thread_id
    ORDER BY
        WaitTime DESC;

参考书籍:
MySQL技术内幕：InnoDB存储引擎

mysql innodb lock机制原理

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