Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 9 ■ REDO AND UNDO
critical information and commit the operation, you can restore a backup of the affected
data and recover it to the point in time immediately prior to the “accident” using online and
archived redo log files.
Archived redo log files are simply copies of old, full online redo log files. As the system
fills up log files, the ARCH process will make a copy of the online redo log file in another location,
and optionally make several other copies into local and remote locations as well. These
archived redo log files are used to perform media recovery when a failure is caused by a disk
drive going bad or some other physical fault. Oracle can take these archived redo log files and
apply them to backups of the data files to catch them up to the rest of the database. They are
the transaction history of the database.
■Note With the advent of the Oracle 10g, we now have flashback technology. This allows us to perform
flashback queries (i.e., query the data as of some point in time in the past), un-drop a database table, put a
table back the way it was some time ago, and so on. As a result, the number of occasions where we need to
perform a conventional recovery using backups and archived redo logs has decreased. However, performing
a recovery is the DBA’s most important job. Database recovery is the one thing a DBA is not allowed to get
wrong.
Every Oracle database has at least two online redo log groups with at least a single member
(redo log file) in each group. These online redo log groups are used in a circular fashion.
Oracle will write to the log files in group 1, and when it gets to the end of the files in group 1, it
will switch to log file group 2 and begin writing to that one. When it has filled log file group 2,
it will switch back to log file group 1 (assuming you have only two redo log file groups; if you
have three, Oracle would, of course, proceed to the third group).
Redo logs, or transaction logs, are one of the major features that make a database a database.
They are perhaps its most important recovery structure, although without the other
pieces such as undo segments, distributed transaction recovery, and so on, nothing works.
They are a major component of what sets a database apart from a conventional file system.
The online redo logs allow us to effectively recover from a power outage—one that might happen
while Oracle is in the middle of a write. The archived redo logs allow us to recover from
media failures when, for instance, the hard disk goes bad or human error causes data loss.
Without redo logs, the database would not offer any more protection than a file system.
What Is Undo?
Undo is conceptually the opposite of redo. Undo information is generated by the database as
you make modifications to data to put it back the way it was before the modifications, in the
event the transaction or statement you are executing fails for any reason or if you request it
with a ROLLBACK statement. Whereas redo is used to replay a transaction in the event of failure—
to recover the transaction—undo is used to reverse the effects of a statement or set of
statements. Undo, unlike redo, is stored internally in the database in a special set of segments
known as undo segments.