databse object/structures

[lnr]

Hi
I have question which was asked to me recently .

database statistics about "undo" and with "redo",what datbase objects/structures do they refer to ?what sort of data do they contain .

I didnt understand the question at all ,can someone explain me clearly what does it mean and so on .

Thanks
lnreddy

[rohitsn]

Hi,
In Oracle "undo" and "redo" are 2 different concepts."Undo" refers to rollback segment and "redo" refers to redo log files.Whenever Oracle instance crashes,Oracle performs Instance recovery using Undo and Redo methodology.
Undo contains all the transaction entries which are not yet commited in the database.Any change made to the data,its previous value is stored in the rollback segement as well as redo log file.So in case a user rollbacks the transcation,the previous value is stored back in the table.
Redo contains both pre-image and post-image.Redo log files contains all the changes made to the database buffer cache.It is also used in recovery purpose to bring the database to the point of failure.
I hope so that I have answered the question of urs?If u have any doubt please be free to ask to me at rohitsn@altavista.com

regards,
Rohit Nirkhe,Oracle DBA,OCP
rohitsn@altavista.com

[QUOTE][i]Originally posted by lnr [/i]
[B]Hi
I have question which was asked to me recently .

database statistics about "undo" and with "redo",what datbase objects/structures do they refer to ?what sort of data do they contain .

I didnt understand the question at all ,can someone explain me clearly what does it mean and so on .

Thanks
lnreddy [/B][/QUOTE]