MySQL SQL更新锁定

矫情吗；* 2022-04-13 04:54 218阅读 0赞

MySQL数据库锁是实现数据一致性，解决并发问题的重要手段。数据库是一个多用户共享的资源，当出现并发的时候，就会导致出现各种各样奇怪的问题，就像程序代码一样，出现多线程并发的时候，如果不做特殊控制的话，就会出现意外的事情，比如“脏“数据、修改丢失等问题。所以数据库并发需要使用事务来控制，事务并发问题需要数据库锁来控制，所以数据库锁是跟并发控制和事务联系在一起的。本文主要描述基于更新SQL语句来理解MySQL锁定。

### 一、构造环境 ###

(root@localhost) [user]> show variables like 'version';
    +---------------+------------+
    | Variable_name | Value |
    +---------------+------------+
    | version | 5.7.23-log |
    +---------------+------------+
    
    (root@localhost) [user]> desc t1;
    +-------------+--------------+------+-----+---------+----------------+
    | Field | Type | Null | Key | Default | Extra |
    +-------------+--------------+------+-----+---------+----------------+
    | id | int(11) | NO | PRI | NULL | auto_increment |
    | n | int(11) | YES | | NULL | |
    | table_name | varchar(64) | YES | | NULL | |
    | column_name | varchar(64) | YES | | NULL | |
    | pad | varchar(100) | YES | | NULL | |
    +-------------+--------------+------+-----+---------+----------------+
    
    (root@localhost) [user]> select count(*) from t1;
    +----------+
    | count(*) |
    +----------+
    | 3406 |
    +----------+
    
    (root@localhost) [user]> create unique index idx_t1_pad on t1(pad);
    Query OK, 0 rows affected (0.35 sec)
    Records: 0 Duplicates: 0 Warnings: 0
    
    (root@localhost) [user]> create index idx_t1_n on t1(n);
    Query OK, 0 rows affected (0.03 sec)
    Records: 0 Duplicates: 0 Warnings: 0
    (root@localhost) [user]> show index from t1;
    +-------+------------+------------+--------------+-------------+-----------+-------------+------+------------+
    | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Null | Index_type |
    +-------+------------+------------+--------------+-------------+-----------+-------------+------+------------+
    | t1 | 0 | PRIMARY | 1 | id | A | 3462 | | BTREE |
    | t1 | 0 | idx_t1_pad | 1 | pad | A | 3406 | YES | BTREE |
    | t1 | 1 | idx_t1_n | 1 | n | A | 12 | YES | BTREE |
    +-------+------------+------------+--------------+-------------+-----------+-------------+------+------------+
    select 'Leshami' author,'http://blog.csdn.net/leshami' Blog;
    +---------+------------------------------+
    | author | Blog |
    +---------+------------------------------+
    | Leshami | http://blog.csdn.net/leshami |
    +---------+------------------------------+

### 二、基于主键更新 ###

(root@localhost) [user]> start transaction;
    Query OK, 0 rows affected (0.00 sec)
    
    (root@localhost) [user]> update t1 set table_name='t1' where id=1299;
    Query OK, 1 row affected (0.00 sec)
    Rows matched: 1 Changed: 1 Warnings: 0
    
    SELECT trx_id,
           trx_state,
           trx_started,
           trx_mysql_thread_id,
           trx_tables_locked,
           trx_rows_locked,
           trx_rows_modified,
           trx_isolation_level
    FROM INFORMATION_SCHEMA.INNODB_TRX \G
    
    -- 从下面的结果可知，trx_rows_locked，一行被锁定    
    *************************** 1. row ***************************
                 trx_id: 6349647
              trx_state: RUNNING
            trx_started: 2018-11-06 16:54:12
    trx_mysql_thread_id: 2
      trx_tables_locked: 1
        trx_rows_locked: 1
      trx_rows_modified: 1
    trx_isolation_level: REPEATABLE READ    
    
    (root@localhost) [user]> rollback;
    Query OK, 0 rows affected (0.01 sec)

### 三、基于二级唯一索引 ###

(root@localhost) [user]> start transaction;
    Query OK, 0 rows affected (0.00 sec)
    
    (root@localhost) [user]> update t1 set table_name='t2' where pad='4f39e2a03df3ab94b9f6a48c4aecdc0b';
    Query OK, 1 row affected (0.00 sec)
    Rows matched: 1 Changed: 1 Warnings: 0
    
    SELECT trx_id,
           trx_state,
           trx_started,
           trx_mysql_thread_id,
           trx_tables_locked,
           trx_rows_locked,
           trx_rows_modified,
           trx_isolation_level
    FROM INFORMATION_SCHEMA.INNODB_TRX \G
    
    -- 从下面的查询结果可知，trx_rows_locked，2行被锁定
    *************************** 1. row ***************************
                 trx_id: 6349649
              trx_state: RUNNING
            trx_started: 2018-11-06 16:55:22
    trx_mysql_thread_id: 2
      trx_tables_locked: 1
        trx_rows_locked: 2
      trx_rows_modified: 1
    trx_isolation_level: REPEATABLE READ  
    
    (root@localhost) [user]> rollback;
    Query OK, 0 rows affected (0.00 sec)

### 三、基于二级非唯一索引 ###

(root@localhost) [user]> start transaction;
    Query OK, 0 rows affected (0.00 sec)
    
    (root@localhost) [user]> update t1 set table_name='t3' where n=8;
    Query OK, 350 rows affected (0.01 sec)
    Rows matched: 351 Changed: 351 Warnings: 0
    
    SELECT trx_id,
           trx_state,
           trx_started,
           trx_mysql_thread_id,
           trx_tables_locked,
           trx_rows_locked,
           trx_rows_modified,
           trx_isolation_level
    FROM INFORMATION_SCHEMA.INNODB_TRX \G
        
    --从下面的查询结果可知，703行被锁定
    *************************** 1. row ***************************
                 trx_id: 6349672
              trx_state: RUNNING
            trx_started: 2018-11-06 17:06:53
    trx_mysql_thread_id: 2
      trx_tables_locked: 1
        trx_rows_locked: 703
      trx_rows_modified: 351
    trx_isolation_level: REPEATABLE READ 
    
    (root@localhost) [user]> rollback;
    Query OK, 0 rows affected (0.00 sec)

### 四、无索引更新 ###

(root@localhost) [user]> start transaction;
    Query OK, 0 rows affected (0.00 sec)
    
    (root@localhost) [user]> update t1 set table_name='t4' where column_name='id';
    Query OK, 26 rows affected (0.00 sec)
    Rows matched: 26 Changed: 26 Warnings: 0
    
    SELECT trx_id,
           trx_state,
           trx_started,
           trx_mysql_thread_id,
           trx_tables_locked,
           trx_rows_locked,
           trx_rows_modified,
           trx_isolation_level
    FROM INFORMATION_SCHEMA.INNODB_TRX \G
    
    -- 从下面的查询结果可知，trx_rows_locked，3429行被锁定，而被更新的仅仅为26行
    -- 而且这个结果超出了表上的总行数3406
    *************************** 1. row ***************************
                 trx_id: 6349674
              trx_state: RUNNING
            trx_started: 2018-11-06 17:09:41
    trx_mysql_thread_id: 2
      trx_tables_locked: 1
        trx_rows_locked: 3429
      trx_rows_modified: 26
    trx_isolation_level: REPEATABLE READ 
    
    (root@localhost) [user]> rollback;
    Query OK, 0 rows affected (0.00 sec)
    
    -- 也可以通过show engine innodb status进行观察
    
    show engine innodb status\G
    
    ------------
    TRANSACTIONS
    ------------
    Trx id counter 6349584
    Purge done for trx's n:o < 0 undo n:o < 0 state: running but idle
    History list length 0
    LIST OF TRANSACTIONS FOR EACH SESSION:
    ---TRANSACTION 421943222819552, not started
    0 lock struct(s), heap size 1136, 0 row lock(s)
    ---TRANSACTION 6349583, ACTIVE 2 sec
    2 lock struct(s), heap size 1136, 1 row lock(s), undo log entries 1
    
    
    ------------
    TRANSACTIONS
    ------------
    Trx id counter 6349586
    Purge done for trx's n:o < 6349585 undo n:o < 0 state: running but idle
    History list length 1
    LIST OF TRANSACTIONS FOR EACH SESSION:
    ---TRANSACTION 421943222819552, not started
    0 lock struct(s), heap size 1136, 0 row lock(s)
    ---TRANSACTION 6349585, ACTIVE 8 sec
    3 lock struct(s), heap size 1136, 2 row lock(s), undo log entries 1
    MySQL thread id 2, OS thread handle 140467640694528, query id 29 localhost root

### 五、锁相关查询SQL ###

1：查看当前的事务
    SELECT * FROM INFORMATION_SCHEMA.INNODB_TRX;
    
    2：查看当前锁定的事务
    
    SELECT * FROM INFORMATION_SCHEMA.INNODB_LOCKS;
    
    3：查看当前等锁的事务
    SELECT * FROM INFORMATION_SCHEMA.INNODB_LOCK_WAITS;
    
    SELECT trx_id,
           trx_state,
           trx_started,
           trx_mysql_thread_id thr_id,
           trx_tables_locked tb_lck,
           trx_rows_locked rows_lck,
           trx_rows_modified row_mfy,
           trx_isolation_level is_lvl
    FROM INFORMATION_SCHEMA.INNODB_TRX;
    
    SELECT r.`trx_id` waiting_trx_id,
           r.`trx_mysql_thread_id` waiting_thread,
           r.`trx_query` waiting_query,
           b.`trx_id` bolcking_trx_id,
           b.`trx_mysql_thread_id` blocking_thread,
           b.`trx_query` block_query
    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`;

### 六、小结 ###

1、MySQL表更新时，对记录的锁定根据更新时where谓词条件来确定锁定范围  
2、对于聚簇索引过滤，由于索引即数据，因为仅仅锁定更新行，这是由聚簇索引的性质决定的  
3、对于非聚簇唯一索引过滤，由于需要回表，因此锁定为唯一索引过滤行数加上回表行数  
4、对于非聚簇非唯一索引过滤，涉及到了间隙锁，因此锁定的记录数更多  
5、如果过滤条件无任何索引或无法使用到索引，则锁定整张表上所有数据行