Question-34

Consider the following log sequence of two transactions on a bank account, with initial balance 12000, that transfer 2000 to a mortgage payment and then apply a 5% interest.

1. T1: start
   
2. T1: B old =12000, new =10000
   
3. T1: M old =0, new =2000
   
4. T1: commit
   
5. T2: start
   
6. T2: B old =10000, new =10500
   
7. T2: commit

Suppose the database system crashes just before log record 7 is written. When the system is restarted, which one statement is true of the recovery procedure?

• We must redo log record 6 to set B to 10500
  
• We must undo log record 6 to set B to 10000 and then redo log records 2 and 3
  
• We need not redo log records 2 and 3 because transaction T1 has committed
  
• We can apply redo and undo operations in arbitrary order because they are idempotent

Answer

• We must redo log record 6 to set B to 10500
  
• We must undo log record 6 to set B to 10000 and then redo log records 2 and 3
  
• We need not redo log records 2 and 3 because transaction T1 has committed
  
• We can apply redo and undo operations in arbitrary order because they are idempotent