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

以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作具有一定的参考学习价值，如果有疑问大家可以留言交流，谢谢大家对的支持。