前言
oracle可以查询v$transaction实时了解数据库事务的执行情况，v$transaction有个列flag，引起我的注意，见下：
延伸阅读 oracle数据库事务transaction锁lock模式思考之一
oracle提供2种事务隔离级别,read committed及serializable。前者是默认值。如果采用不同的事务隔离级别发起数据库事务， v$transaction之flag有何区别呢。
不同事务隔离级别的表现
read committed事务隔离级别
事务隔离级别 read committed，各种dml产生的事务 flag全是3587 SQL> set transaction isolation level read committed name 'tran76'; Transaction set. SQL> insert into t_test select 3,3 from dual; 1 row created. SQL> / NAME ADDR FLAG ---------------------------------------- ---------------- ---------- tran76 00000000843521C0 3587 SQL> delete from t_test where a=1; 1 row deleted. SQL> / NAME ADDR FLAG ---------------------------------------- ---------------- ---------- tran76 00000000843521C0 3587 SQL> update t_test set a=11; 2 rows updated. SQL> / NAME ADDR FLAG ---------------------------------------- ---------------- ---------- tran76 00000000843521C0 3587
serializable事务隔离级别
事务隔离级别serializable各种dml产生的事务的flag全是268439043 SQL> set transaction isolation level serializable name 'tran76'; Transaction set. SQL> insert into t_test select 3,3 from dual; 1 row created. SQL> select addr,flag from v$transaction; ADDR FLAG ---------------- ---------- 00000000840B04D8 268439043 SQL> update t_test set a=11 where a=1; 1 row updated. SQL> / NAME ADDR FLAG ---------------------------------------- ---------------- ---------- tran76 00000000843521C0 268439043 SQL> delete from t_test; 2 rows deleted. SQL> / NAME ADDR FLAG ---------------------------------------- ---------------- ---------- tran76 00000000843521C0 268439043 SQL> select addr,flag from v$transaction; ADDR FLAG ---------------- ---------- 00000000840B04D8 268439043
lock mode语句产生的事务隔离级别
lock table不会产生事务，当然也不会产生事务的flag SQL> lock table t_test in row share mode; Table(s) Locked. SQL> lock table t_test in row exclusive mode; Table(s) Locked. SQL> lock table t_test in share row exclusive mode; Table(s) Locked. SQL> lock table t_test in exclusive mode; Table(s) Locked. SQL> lock table t_test in share mode; Table(s) Locked.
思考
---可以对flag作文章，就可以知道事务是哪种事务隔离级别 select name,addr,flag,decode(flag,'268439043','serializable','3587','read committed') from v$transaction where addr=(select taddr from v$session where sid=76) NAME ADDR FLAG DECODE(FLAG,'2 ---------------------------------------- ---------------- ---------- -------------- tran76 00000000843521C0 268439043 serializable
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