Q: How does changing the system clock effect Oracle?? specifically, point
in time recovery?? Is it different with V6 and Oracle7??
A: Changing the system clock should really have no effect on the Oracle kernel
until it comes time to do time-based media recovery. Both in V6 and V7 there
may be problems if you use RECOVER DATABASE UNTIL TIME if time has changed
between the time you recover and the time the backup is taken.
Consider:
3pm 4pm 4.30 5pm–>4pm 4.16pm 4.30 5pm
|———|——-|——-|—–|——-|——-|——–|
cold/hot T1 T2 T3 clock T4 T5 T6
backup change
If you wish to recover to the T5, you will not be able to since Oracle will
recover to the first occurance of 4:30 which is T2. In Oracle7 you can recover
to a specific SCN (RECOVER … UNTIL CHANGE) which is systemclock independent,
but when using RECOVER … UNTIL TIME will not give the desired results.
So the lesson to be learned, is when the clock is set back, you should take
full database backup immediately after the system clock change.
NOTE: this only applies in the fall when we set the clocks back.