基于更新SQL语句理解MySQL锁定详解

前言

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。下面话不多说了，来一起看看详细的介绍吧

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

总结

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