When we update some data to one table,oracle will do something as the next describing:
1.Create a redo change vector describing how to insert an undo record into an undo block.
2.Create a redo change vector for the data block change.
3.Combine the redo change vectors into a redo record and write it to the log buffer.
4.Insert the undo record into the undo block.
5.Change the data block.
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