Let’s discuss about some of the recovery
situations in database
· Online Block Recovery àOracle automatically do this using pmon when a process dies while
changing a buffer. Oracle will reconstruct the buffer using the online redo
logs and writes it to disk. No action required from your side J
· Thread Recovery à Oracle automatically do this when an instance crashes while
having the database open. Oracle applies all the redo changes in the
thread that occurred since the last time the thread was checkpointed. No action
required from your side J
· Media Recovery à You need to do this when a data file is restored from backup and the
checkpoint count in the data files here are not equal to the check point count
in the control file.
At this moment it is critical to understand Redo vs Undo.
Redo information is recorded so that all commands that took place can be repeated
during recovery (it is complete information about redoing operations).
Undo information is recorded so that database can
undo changes made by the current transaction but were not committed. The Redo
Logs are used to Roll Forward the changes made, both committed and non-
committed changes. Then from the Undo segments, the undo information is
rollback the uncommitted changes.