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23. Oracle UNDO management
The undo data stores the old values of modified data . it lets a database rollback active changes during failure or abort.
b. Undo changes= if an user cancels a transaction, the system reads the undo data to put old values back.
c. Recovery after loss= after power loss, the database uses the undo logs to put back the uncommited changes during startup.
c. Reconstract data = when a active changes a modified row, the database uses undo logs to build the old , clean the version of this row.
SAFETY > Never drop a active undo tablespace. Confirm the active transactions , rollback segment status , database role , recovery requirments before removing the undo.
##.
> A query needs an older vertion of a block. if the required undo record has already been overwritten ,then Oracle cannot reconstract the consistent image and raises the ORA-01555.
The undo data stores the old values of modified data . it lets a database rollback active changes during failure or abort.
b. Undo changes= if an user cancels a transaction, the system reads the undo data to put old values back.
c. Recovery after loss= after power loss, the database uses the undo logs to put back the uncommited changes during startup.
c. Reconstract data = when a active changes a modified row, the database uses undo logs to build the old , clean the version of this row.
