How to check whether a table is locked in mysql

How to see if a deadlock occurs

在使用mysql的时候,How to check whether the table is locked？

查看表被锁状态和结束死锁步骤：

1.在mysql命令行执行sql语句

use dbName;  // Switch to a specific database
show engine innodb status;  // 查询db是否发生死锁

2.View the data table is locked status

show OPEN TABLES where In_use > 0; 

This statement can query the status of the current lock table

3.分析锁表的SQL
通过sql日志,分析相应SQL,给表加索引,常用字段加索引,Table associated fields plus indexes, etc. are pairedsql进行优化.

4.查看正在锁的事务

SELECT * FROM INFORMATION_SCHEMA.INNODB_LOCKS;

在5.5中,information_schema 库中增加了三个关于锁的表（innoDB引擎）：

innodb_trx ## 当前运行的所有事务
innodb_locks ## 当前出现的锁
innodb_lock_waits ## 锁等待的对应关系
先来看一下这三张表结构：

[email protected] : information_schema 13:28:38> desc innodb_locks;
+————-+———————+——+—–+———+——-+
| Field       | Type                | Null | Key | Default | Extra |
+————-+———————+——+—–+———+——-+
| lock_id     | varchar(81)         | NO   |     |         |       |#锁ID
| lock_trx_id | varchar(18)         | NO   |     |         |       |#拥有锁的事务ID
| lock_mode   | varchar(32)         | NO   |     |         |       |#锁模式
| lock_type   | varchar(32)         | NO   |     |         |       |#锁类型
| lock_table  | varchar(1024)       | NO   |     |         |       |#被锁的表
| lock_index  | varchar(1024)       | YES  |     | NULL    |       |#被锁的索引
| lock_space  | bigint(21) unsigned | YES  |     | NULL    |       |#被锁的表空间号
| lock_page   | bigint(21) unsigned | YES  |     | NULL    |       |#被锁的页号
| lock_rec    | bigint(21) unsigned | YES  |     | NULL    |       |#被锁的记录号
| lock_data   | varchar(8192)       | YES  |     | NULL    |       |#被锁的数据
+————-+———————+——+—–+———+——-+
10 rows in set (0.00 sec)
   
[email protected] : information_schema 13:28:56> desc innodb_lock_waits;
+——————-+————-+——+—–+———+——-+
| Field             | Type        | Null | Key | Default | Extra |
+——————-+————-+——+—–+———+——-+
| requesting_trx_id | varchar(18) | NO   |     |         |       |#请求锁的事务ID
| requested_lock_id | varchar(81) | NO   |     |         |       |#请求锁的锁ID
| blocking_trx_id   | varchar(18) | NO   |     |         |       |#当前拥有锁的事务ID
| blocking_lock_id  | varchar(81) | NO   |     |         |       |#当前拥有锁的锁ID
+——————-+————-+——+—–+———+——-+
4 rows in set (0.00 sec)
   
[email protected] : information_schema 13:29:05> desc innodb_trx ;
+—————————-+———————+——+—–+———————+——-+
| Field                      | Type                | Null | Key | Default             | Extra |
+—————————-+———————+——+—–+———————+——-+
| trx_id                     | varchar(18)         | NO   |     |                     |       |#事务ID
| trx_state                  | varchar(13)         | NO   |     |                     |       |#事务状态：
| trx_started                | datetime            | NO   |     | 0000-00-00 00:00:00 |       |#事务开始时间;
| trx_requested_lock_id      | varchar(81)         | YES  |     | NULL                |       |#innodb_locks.lock_id
| trx_wait_started           | datetime            | YES  |     | NULL                |       |#事务开始等待的时间
| trx_weight                 | bigint(21) unsigned | NO   |     | 0                   |       |#
| trx_mysql_thread_id        | bigint(21) unsigned | NO   |     | 0                   |       |#事务线程ID
| trx_query                  | varchar(1024)       | YES  |     | NULL                |       |#具体SQL语句
| trx_operation_state        | varchar(64)         | YES  |     | NULL                |       |#事务当前操作状态
| trx_tables_in_use          | bigint(21) unsigned | NO   |     | 0                   |       |#事务中有多少个表被使用
| trx_tables_locked          | bigint(21) unsigned | NO   |     | 0                   |       |#事务拥有多少个锁
| trx_lock_structs           | bigint(21) unsigned | NO   |     | 0                   |       |#
| trx_lock_memory_bytes      | bigint(21) unsigned | NO   |     | 0                   |       |#事务锁住的内存大小（B）
| trx_rows_locked            | bigint(21) unsigned | NO   |     | 0                   |       |#事务锁住的行数
| trx_rows_modified          | bigint(21) unsigned | NO   |     | 0                   |       |#事务更改的行数
| trx_concurrency_tickets    | bigint(21) unsigned | NO   |     | 0                   |       |#事务并发票数
| trx_isolation_level        | varchar(16)         | NO   |     |                     |       |#事务隔离级别
| trx_unique_checks          | int(1)              | NO   |     | 0                   |       |#是否唯一性检查
| trx_foreign_key_checks     | int(1)              | NO   |     | 0                   |       |#是否外键检查
| trx_last_foreign_key_error | varchar(256)        | YES  |     | NULL                |       |#最后的外键错误
| trx_adaptive_hash_latched  | int(1)              | NO   |     | 0                   |       |#
| trx_adaptive_hash_timeout  | bigint(21) unsigned | NO   |     | 0                   |       |#
+—————————-+———————+——+—–+———————+——-+
22 rows in set (0.01 sec)

5.查看等待锁的事务

SELECT * FROM INFORMATION_SCHEMA.INNODB_LOCK_WAITS.

6.View the type and status of the lock：

show status like '%lock%';

7.查询进程

show processlist

8.Kill the deadlocked process
如果发生死锁,Through this statement, you can query the process for which the table is locked,然后通过kill命令杀掉该进程.

Others look at table locks：

# 查看表锁的情况：
mysql> show status like 'table%';
+----------------------------+---------+
| Variable_name | Value |
+----------------------------+---------+
| Table_locks_immediate | 100 |
| Table_locks_waited | 11 |
+----------------------------+---------+

# 查看InnoDB_row_lock状态变量来分析系统上的行锁的争夺情况：
mysql> show status like 'InnoDB_row_lock%';
+-------------------------------+--------+
| Variable_name                 | Value  |
+-------------------------------+--------+
| Innodb_row_lock_current_waits | 0      |
| Innodb_row_lock_time          | 159372 |
| Innodb_row_lock_time_avg      | 39843  |
| Innodb_row_lock_time_max      | 51154  |
| Innodb_row_lock_waits         | 4      |
+-------------------------------+--------+
5 rows in set (0.01 sec)

mysql>

在分析innodb中锁阻塞时,几种方法的对比情况：

（1）使用show processlist查看不靠谱;
（2）直接使用show engine innodb status查看,无法判断到问题的根因;
（3）使用mysqladmin debug查看,能看到所有产生锁的线程,但无法判断哪个才是根因;
（4）开启innodb_lock_monitor后,再使用show engine innodb status查看,能够找到锁阻塞的根因.

Deadlock occurrence and cause

产生原因

所谓死锁：是指两个或两个以上的进程在执行过程中,因争夺资源而造成的一种互相等待的现象.若无外力作用,它们都将无法推进下去.此时称系统处于死锁状态或系统产生了死锁,这些永远在互相等待的进程称为死锁进程.表级锁不会产生死锁.所以解决死锁主要还是针对于最常用的InnoDB.

死锁的关键在于：两个(或以上)的Session加锁的顺序不一致.
那么对应的解决死锁问题的关键就是：让不同的session加锁有次序

Several situations in which deadlock occurs

There are several cases of deadlocks
We have two tables with the same structure,分别为t1和t2：

id：integer	
token：varchar	
message：varchar

其中id是主键（自增）,token是非聚集索引,message没有索引.

1、A table with two rows of records crosses to apply for a mutual exclusion lock

A执行到第二步时,等待BRelease the lock on the first step,而BThe lock needs to be released after the second step is completed to end the transaction;
B执行到第二步时,等待ARelease the speed of the first step,这样A和B都无法进行下去,A deadlock phenomenon occurs.

2、Two tables and two rows of records intersect to apply for a mutual exclusion lock

这种情况与1中的类似.

3、A clustered index conflicts with a nonclustered index

This does not necessarily result in a deadlock,It doesn't even appear on the surface.

假设AThe locking sequence of records that meet the conditions in is （5,4,3,2,1）,BThe order of locking in is （1,2,3,4,5）,The ordering here is correspondencerecord的主键;
（InnoDBThe lock is acquired incrementally,Instead of acquiring all the locks needed at once.）
有可能A加锁了5和4,B加锁了1、2、3,If it goes further, there will be a situation where they wait for each other and become deadlocked,就是死锁.

4、Clustered index conflict

这种情况与3中的类似.

5、Gap lock conflict

这种情况是因为AThe first step uses a gap lock,在A释放锁之前BThe second step could not be completed,也会形成死锁.

innodb提供了wait-for graph算法来主动进行死锁检测,每当加锁请求无法立即满足需要并进入等待时,wait-for graph算法都会被触发,Check if a waiting loop occurs.当检测到死锁时,InnoDBA transaction with a lower cost will be selected for rollback.

参考

mysql死锁
MySQLHow to analyze deadlock problems&After locking the table, see deadlocks and quick solutions to remove deadlocks
mysqlHow to check if a table is locked
mysql 查看死锁和去除死锁
MySQL 死锁产生原因及解决方法
mysqlUnder what circumstances can a deadlock occur

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