time 1:

session a:

scott@kenny> set transaction read only;

transaction set.

time2:

session b:

scott@kenny> update emp set sal=4000 where ename='smith';

1 row updated.

scott@kenny> commit;

commit complete.

time 3:

session a:

scott@kenny> select * from emp where ename='smith';

empno ename job mgr hiredate sal comm deptno
---------- ---------- --------- ---------- --------- 
7369 smith clerk 7902 17-dec-80 2000 20
我们可以看到，在session a下所查到的值仍然是更改前的sal：2000。

=================================================

sql>set transaction isolation level serializable;