实验步骤如下：
create undo tablespace undotbs2 datafile '/u01/app/oracle/oradata/ocp/undotbs2.dbf' size 10M;
alter system set undo_tablespace=undotbs2;
alter system set undo_retention=2 scope=both;
第1步、session1: 目标是让b表报快照过旧的报错
conn gyj/gyj
create table a (id int,cc varchar2(10));
insert into a values(1,'hello');
commit;
create table b(id int,cc varchar2(10));
insert into b values(10,'AAAAAA');
commit;
select * from a;
select * from b;
var x refcursor;
exec open :x for select * from b;
第2步、session2:修改b表,字段cc前镜像"OK"保存在UDNO段中
update b set cc='BBBBBB' where id= 10;
commit;
第3步、session 3:该条语句就是刷新缓存
conn / as sysdba
SQL> alter session set events = 'immediate trace name flush_cache'; --9i提供强制刷缓存
(alter system flush buffer_cache;--10g提供的一种刷缓存方法)
第4步、 session2: 在A表上行大的事务,多运行几次以确保，回滚段被覆盖
begin
for i in 1..20000 loop
update a set cc='HELLOWWWW';
commit;
end loop;
end;
/
第5步、session 1:在B表上执行查询(第一步的查询)
SQL> print :x
ERROR:
ORA-01555: snapshot too old: rollback segment number 21 with name "_SYSSMU21$" too small
