重现ORA-01555错误

第1步修改undo表空间大小

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2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3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4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5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6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

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