Why Am I able to view flashback query longer than undo_retention?

Question

Hi
this is what I did
SQL> select sysdate from dual;
SYSDATE
2008-11-23 16:55:39
--to find out the lower transaction end
SQL> exec :v_lower_timestamp:='2008-11-23 15:55:39';
--adjust the timestamp to a lower value than will overshot the undo_retention
SQL> update emp set sal=sal*2 where empno=:v_empno;
1 row updated.
SQL> commit;
Commit complete.
SQL> select sysdate from dual;
SYSDATE
2008-11-23 16:59:14
SQL> exec :v_upper_timestamp:='2008-11-23 16:59:14';
SQL> SELECT * FROM emp AS OF TIMESTAMP TO_TIMESTAMP(:v_lower_timestamp, :v_date_ format) WHERE empno=:v_empno;
EMPNO ENAME JOB MGR HIREDATE SAL
COMM DEPTNO
7934 MILLER CLERK 7782 1982-01-23 00:00:00 650
10
SQL> SELECT * FROM emp AS OF TIMESTAMP TO_TIMESTAMP(:v_upper_timestamp, :v_date_ format) WHERE empno=:v_empno;
EMPNO ENAME JOB MGR HIREDATE SAL
COMM DEPTNO
7934 MILLER CLERK 7782 1982-01-23 00:00:00 1300
10
but when I try to do that for flashback transaction query
SQL> SELECT versions_startscn, versions_endscn, versions_xid, versions_operation ,
ename, sal
FROM emp
VERSIONS BETWEEN TIMESTAMP
TO_TIMESTAMP(:v_lower_timestamp, :v_date_format)
AND TO_TIMESTAMP(:v_upper_timestamp, :v_date_format)
WHERE empno=:v_empno;
2 3 4 5 6 7 FROM emp
ERROR at line 3:
ORA-30052: invalid lower limit snapshot expression
--but the moment I raise the lower timestamp limit
SQL> exec :v_lower_timestamp:='2008-11-23 16:55:39';
VERSIONS_STARTSCN VERSIONS_ENDSCN VERSIONS_XID V ENAME SAL
619162 0300250051010000 U MILLER 1300
619162 MILLER 650
flashback transaction works, this is understandable.
but my question is why flashback query does allow the snapshot to be capture even longer than the undo_retention specified by but not flashback version query?
thanks a lot!

Answer 1

Select 10 files and open the files, then select and open the next 10 files.