Chapter 19, Problem 7PE

Explanation of Solution

Actions taken during the undo phase of recovery resulting in an incorrect database state:

Consider the given modifications to the recovery algorithm mentioned in section 19.4.

• For example, consider the below log records that were written with the given modifications to the recovery algorithm as follows:
• 1. 1. 1 start>
  2. 2. < T₁, B, 200, 900>
  3. 3. 2 start>
  4. 4. < T₂, B, 200, 500>
  5. 5. 2 commit>
• Here, in the above given algorithm, assume that a rollback occurred between step 2 and 3, but as specified, no log records were generated for the same.
• Consider what happens when the modified recovery algorithm processes these log records.
  • At the end of the redo phase, transaction T₁ will get added to the undo-list and the value of data item B is 5000...