redo and undo

[kvdeodhar]

gfgffdf when we execute any SQL or PL-sql script/statement it creates an entry in REDO log buffer and UNDO Sengment. Undo is used for storing changed data ie. data changed during transaction and used for rollback pupose... thn wh is the use of REDO entry ????

[yxez]

ambot sa imo...gadangas lang na imong agtang just a wild guess,
I think they just serve for the same purpose hunnie...redo is used
to store the "after" image of the updated data and undo
is used to store the "before" image of the data.

[davey23uk]

well they are definitely not for the same purpose

redo contains all the changes which can be used in case of a database crash, undo is used to privde a consistent view of the data and for rollback

[pando]

redo is absolutely everything, if you have archive log then it lasts forever as long as you dont delete the archive logs of curse

undo mostly DML changes, it only lasts during a transaction (mostly again)

[Bore]

Redo is ReDo - contains a description of all the changes in the database. Looks like "In file A block B field C change value from 10 to 20" That allows you to REDO that change in case of media failure. Then you return files from backup AND REDO all the changes made after the moment of backup so you do not loose data. The REDO is saved in redo log files and archived redo log files OUT OF THE DATAFILES
UNDO is UnDo - contains the BEFORE immages of your data, and is generally kept until the end of the transaction. It is used to UNDO your changes in case of transaction failure or user rollback. It is also used for read consistent view of the data i.e. when you have changed the data, but still not commited, the other users sees the OLD STATE of the data, not the changed. That's taken from the UNDO.
The UNDO is saved in the segments (like tables) and these segments by themselves are covered by the REDO generation.
You'll understand clearly the difference when become more experienced :-)

[yxez]

wow i'm impressed..very well explained...at least my wild guess is similar

[Bore]

I often teach Oracle courses :-)
Ipersonally realized clearly the difference in my 2th year as DBA :-)

[abhaysk]

Ipersonally realized clearly the difference in my 2th year as DBA :-)

You must be a brilliant student then...