Difference between Redolog and Undosegment

[Landan]

I am so confused about, redo log and undo segment both components are being used to store the previous objects after updating anything. Why ? oracle database always using it,One of them can also be used to perform the operation of any previous changes. It is asked in a seminar and now I want to know the reason. please help me to provide such information.

[Reegan]

The redolog in database :

The redo log is being used to store the committed and uncommitted changes that is made on datafile and written by LGWR process from redo buffer.It stores committed and uncommitted changes both.

The redo log can use two different part: The first one is "online redo log" and second one is "archived redo log". The redo can use both components to recover the lost data after any failure of database.It contains every transaction on a redo log file and the archive process is being used to save these contents on a different locations.

[opaper]

Undo segment of database :

The undo segment is an object of the database that is being used to store the previous records. Undo records can be stored in either rollback segments or undo tablespaces and then the rollback segment performs a rollback operation to remove the uncommitted changes in the data file.

There are two ways to manage the UNDO space of database :Automatic and Manual. The best thing to use the manual management of UNDO spaces.

[MindSpace]

Effect of rollback in database :

The rollback statement perform the undoing changes to data that have been updated during the transaction using SQL. If we are working on any database and performing some updation using SQL statement and we have done the updating operation. Now we need to have the oldest data.

In this case, we used to issue the rollback statement.

[Modifier]

Effect of using COMMIT statement :

The commit statement signal to the checkpoint process.Then related processes comes to active and play its own role.

After a commit, the Log writer(LGWR)process writes the redo information from redo buffer to the online redo log files. Every process activated in parallel way.
If a user get disconnects from oracle database then a commit happens.