MySQL-8.0 事务隔离级别

查看数据库事务隔离级别

mysql> show variables like 'transaction%';
mysql> select @@transaction_isolation;
mysql> SELECT @@GLOBAL.transaction_isolation, @@GLOBAL.transaction_read_only;
mysql> SELECT @@SESSION.transaction_isolation, @@SESSION.transaction_read_only;

修改事务隔离级别的方式

• 由低到高分别为
  • read uncommitted (读未提交)
  • read committed (读提交)
  • repeatable read (重复)
  • Serializable (序列化)

1.只在本次会话有效

mysql> set session transaction isolation level read committed;

2.设置全局的事务隔离级别,该设置不会影响当前已经连接的会话,新会话,将使用新设置的事务隔离级别

mysql> set global transaction isolation level read committed;

3.修改配置文件,重启后生效

[mysqld]
transaction-isolation = REPEATABLE-READ
transaction-read-only = OFF

select id,name,count from ent_stock;
select id,name,count from ent_stock lock in share mode;
update ent_stock s set s.count = s.count-1 where s.name = '库存-1';
insert into ent_stock(id,name,count) values(uuid(),'库存-3',1000);

级别	脏读	不可重复读	幻读
ru	是	是	是
rc	否	是	是
rr	否	否	是
ru	否	否	否

• Read-Uncommitted

Transication A	Transication B
begin;	begin;
	-
-
	-
-	commit;
	-

• Read-Committed(不可重复读)
  1.解决脏读

Transication A	Transication B
begin;	begin;
	-
-
	-
-	commit;
	-

• Repeatable Read(可重复读)

Transication A	Transication B
begin;	begin;
	-
-
	-
-	commit
	-

• 事务A: update ent_stock s set s.count = count-1 where s.name = '库存-1';
• 事务B: update ent_stock s set s.count = count-1 where s.name = '库存-1';
• 事务A给库存-1加了X锁,事务B更新时只能阻塞等待事务A提交后才可提交此次更新,反之同理
• 普通select是查询快照读,select ... lock in share mode 使用当前读,当其他事务有update操作,本事务会阻塞,直到其他事务释放锁