SCN_TO_TIMESTAMP and TIMESTAMP_TO_SCN Behaviour

Question

Dear all,
I'm trying to understand how both SCN_TO_TIMESTAMP and TIMESTAMP_TO_SCN work by executing the following:
SQL> select scn_to_timestamp(ora_rowscn) test, last_name from employees where la
st_name='King';
TEST                 LAST_NAME
16-DEC-10 01.34.23.0 King
00000000 PM
16-DEC-10 01.34.23.0 King
00000000 PM
SQL> select ora_rowscn, last_name from employees where last_name='King';
ORA_ROWSCN LAST_NAME
    559374 King
    559374 King
SQL> select timestamp_to_scn('16-DEC-10 01.34.23.000000000 PM') from dual;
TIMESTAMP_TO_SCN('16-DEC-1001.34.23.000000000PM')
                                           559370why doesn't the last query resulting 559374 but instead 559370?
best regards,
valerie

Answer 1

Hi,
From the documentation :
SCN_TO_TIMESTAMP takes as an argument a number that evaluates to a system
change number (SCN), and returns the approximate timestamp associated with that
SCN.The important word being approximate.
You cannot rely on it to be as precise as a systimestamp.
timestamp_to_scn(scn_to_timestamp(SCN)) == SCN is FALSE
(even if it might happen to be true sometime...)