title: Oracle：Why Oracle create a redo change vector describing the change to the undo block when user issue an instruction to change an item of data
body: <p>Generally，when a user  issue an instruction to change an item of data,  Oracle works through four critical steps to make the change happen as follow:</p>
  
  <p><strong>1.  Create a redo change vector describing the change to the data block.</strong> </p>
  
  <p><strong>2.  Create an undo record for insertion into an undo block in the undo tablespace.</strong> </p>
  
  <p><strong>3.  Create a redo change vector describing the change to the undo block.</strong> </p>
  
  <p><strong>4.  Change the data block.</strong></p>
  
  <p>why we need step 3 to finish the change?</p>