设置事务
set transaction isolation level read committed;set transaction isolation level serializable;rollback;set transaction isolation level serializable;rollback;set transaction read only;rollback;begin set transaction read only; rollback;end;/--exec dbms_transaction.read_only;alter session set isolation_level=serializable;rollback;alter session set isolation_level=read committed;begin execute immediate 'alter session set isolation_level=serializable';end;/rollback;ALTER SESSION SET isolation_level 与 SET TRANSACTION ISOLATION LEVEL 区别1: set transaction readonly只能设置SERIALIZABLE | READ COMMITTED 而ALTER SESSION SET isolation_level 可以设置任何事务2:SET TRANSACTION ISOLATION LEVEL 设置的事务在rollback 或commit 后，就恢复到默认read COMMIT而ALTER SESSION SET是在会话级别 有效设置事务只能在事务语句的第一句
