Flashback vs. rollback mechanism
Mladen Gogala
undoing the effect of transactions: rollback and flashback. Rollback
method is transactional, it simply undoes the changes made to the blocks
during the failed transaction while the flashback method is more akin to
recovery. The differences between those two methods are numerous but the
outcome will, in many cases be the same.
Should we expect to see some blending between rollback and flashback? I
believe that in the future releases the fine line between the rollback and
flashback will be blurred, possibly by introducing "fast rollback" which
will use recovery-like methods characteristic of the flashback but will
be used within transaction as a "get me my old values back" mechanism.
This, of course, is just a mental experiment. Any thoughts?
--
Mladen Gogala
http://mgogala.freehostia.com
Michael Austin
> In the newer versions of Oracle there are 2 distinct mechanisms for
> undoing the effect of transactions: rollback and flashback. Rollback
> method is transactional, it simply undoes the changes made to the blocks
> during the failed transaction while the flashback method is more akin to
> recovery. The differences between those two methods are numerous but the
> outcome will, in many cases be the same.
Except not. The rollback only "restores" the data involved in the
current transaction. FLASHBACK table/database - restores back to an
SCN-x (point in time) losing all other data post-SCN-x.
From:
Oracle® Database Administrator's Guide 10g Release 2
>>http://download.oracle.com/docs/cd/B19306_01/server.102/b14231/tables.htm#sthref2371
* Restores all data in a specified table to a previous point
in time described by a timestamp or SCN.
* Performs the restore operation online.
* Automatically maintains all of the table attributes, such as
indexes, triggers, and constraints that are necessary for an application
to function with the flashed-back table.
.....
* Maintains data integrity as specified by constraints. Tables
are flashed back provided none of the table constraints are violated.
This includes any referential integrity constraints specified between a
table included in the FLASHBACK TABLE statement and another table that
is not included in the FLASHBACK TABLE statement.
....
> Should we expect to see some blending between rollback and flashback? I
Don't think so. Using a FLASHBACK-like mechanism to do a ROLLBACK is
like using a 20lb sledge hammer to nail a miniature picture frame
together (ie. does a lot more damage than good... :) ) If you were only
executing one very large transaction that inserted 100's or thousands of
records AND was the only thing running AND the transaction committed -
then a flashback-like mechanism is definitely an option.
> believe that in the future releases the fine line between the rollback and
> flashback will be blurred, possibly by introducing "fast rollback" which
> will use recovery-like methods characteristic of the flashback but will
> be used within transaction as a "get me my old values back" mechanism.
>
> This, of course, is just a mental experiment. Any thoughts?
>
I would say your experiment has a long way to go for success... :)
Mark D Powell
wrote:
I attempted to reply to this twice on Friday but both attempts
returned an error message from Google. So here is another attempt.
There are several flashback mechanisms in Oracle including flashback
database, flashback query, and flashback table. Flashback database is
similar in nature to running a reverse recovery. That is instead of
starting at a point in the past and rolling forward using the archived
redo logs you start in the present and roll back using flashback
logs. Being that flashback database is database wide I do no thing
you can compare it to rollback.
However, all other flashback methods actually depend on undo segment
data so I do not think you can consider rollback to be transactional
and consider flashback table or query to be non-transitional. Since
flashback table and query are based on undo the features might be said
to be multi-transactional in that each of capable of rolling back
through more than one transaction. That is where a rollback will undo
one failed or cancelled transaction the flashback table and query
features will rollback as many transactions as necessary to get to the
requested point in time providing the undo tablespace still retains
the necessary data.
IMHO – Mark D Powell
Matthias Hoys
while the other flashback options don't need this requirement ?
Matthias