Chapter 19, Problem 1PE

Explanation of Solution

Processing log records for transactions on the undo-list in reverse order:

Each transaction must create a log record before modifying the database. These log records are used for the following purposes:

• Undo these changes in the database in case of transaction abortion.
• Redo these changes in the database if the transaction is committed but the system failed to store these changes in the disk or database.

Transaction log records on the undo-list must be processed in reverse order as given below:

• Initially, consider the case of a single transaction.
  • Consider the situation when a data item is updated more than one time. Let the data item Q have an initial value as one, it is changed from 1 to 4, and then 4 to 9.
  • Then, try to process the undo log records in forward order.
    • The final undo value of the data item Q will be set to 4, which is an incorrect value.
  • Whereas, when the undo log records are processed in reverse order, the final undo value of the data item Q will be set to 1, which is the correct value.
• Similar logic for data items that can be updated by more than transaction on undo list is used. Thus, they must also be processed in reverse order.

Transaction log records on the redo-list must be processed in forward order as given below:

• Initially, consider the case of a single transaction.
  • Consider the situation when a data item is updated more than one time. Let the data item Q have an initial value as one, it is changed from 1 to 4, and then 4 to 9. Finally, let the transaction commit.
  • Then, try to process the redo log records in reverse order.
    • The final redo value of the data item Q will be set to 4, which is an incorrect value.
  • Whereas, when the redo log records are processed in forward order, the final redo value of the data item Q will be set to 9, which is the correct value.
• Similar logic for data items that can be updated by more than transaction on redo list is used. Thus, they must also be processed in forward order.