GATE CSE 2006 | Question: 20, ISRO2015-17

Question

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? 

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

Comments

https://drive.google.com/file/d/1bPUx7LD3pW0rBiQa4bs_el2CS6SYjg7C/view

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Steps after database crashes:-

Step 1) UNDO the uncommitted transactions. Even UNDO the uncommitted records which are before the checkpoint. UNDO happens in reverse order (from bottom to top).

Step 2) REDO the committed transactions. No need to REDO the records which are already committed before the last checkpoint. REDO happens in original order (from top to bottom).

With these simple steps any questions on deferred database (recovery) can be solved very easily.

Similar qs:-

Databases: GATE CSE 2015 Set 2 | Question: 46 (gateoverflow.in)

Databases: GATE CSE 2021 Set 1 | Question: 13 (gateoverflow.in)

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@Abhrajyoti00 Great Man :) 

Answer 1

C.

Once a transaction is committed, no need to redo or undo operations

Comments

ans should be B here we arent using checkpoints so redo  log records 2 and 3 and undo log record 6