| 71. | Consider the following transactions with data items P and Q initialized to zero: T1: read (P) ; read (Q) ; if P = 0 then Q : = Q + 1 ; write (Q) ; T2: read (Q) ; read (P) ; if Q = 0 then P : = P + 1 ; write (P) ; Any non-serial interleaving of T1 and T2 for concurrent execution leads to |
| a. | A serializable schedule |
| b. | A schedule that is not conflict serializable |
| c. | A conflict serializable schedule |
| d. | A schedule for which a precedence graph cannot be drawn |
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Answer: (b).A schedule that is not conflict serializable
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| 72. | Which of the following concurrency control protocols ensure both conflict serialzability and freedom from deadlock? I. 2-phase locking II. Time-stamp ordering |
| a. | I only |
| b. | II only |
| c. | Both I and II |
| d. | Neither I nor II |
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Answer: (b).II only
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| 73. | Consider the transactions T1, T2, and T3 and the schedules S1 and S2 given below. T1: r1(X); r1(Z); w1(X); w1(Z) T2: r2(Y); r2(Z); w2(Z) T3: r3(Y); r3(X); w3(Y) S1: r1(X); r3(Y); r3(X); r2(Y); r2(Z); w3(Y); w2(Z); r1(Z); w1(X); w1(Z) S2: r1(X); r3(Y); r2(Y); r3(X); r1(Z); r2(Z); w3(Y); w1(X); w2(Z); w1(Z) Which one of the following statements about the schedules is TRUE? |
| a. | Only S1 is conflict-serializable |
| b. | Only S2 is conflict-serializable |
| c. | Both S1 and S2 are conflict-serializable |
| d. | Neither S1 nor S2 is conflict-serializable |
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Answer: (a).Only S1 is conflict-serializable
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| 74. | 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 |
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Answer: (b).We must undo log record 6 to set B to 10000 and then redo log records 2 and 3
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| 75. | Which of the following scenarios may lead to an irrecoverable error in a database system ? |
| a. | A transaction writes a data item after it is read by an uncommitted transaction |
| b. | A transaction reads a data item after it is read by an uncommitted transaction |
| c. | A transaction reads a data item after it is written by a committed transaction |
| d. | A transaction reads a data item after it is written by an uncommitted transaction |
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Answer: (d).A transaction reads a data item after it is written by an uncommitted transaction
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| 76. | Consider the following transaction involving two bank accounts x and y. read(x); x := x – 50; write(x); read(y); y := y + 50; write(y) The constraint that the sum of the accounts x and y should remain constant is that of |
| a. | Atomicity |
| b. | Consistency |
| c. | Isolation |
| d. | Durability |
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Answer: (b).Consistency
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| 77. | Consider a simple checkpointing protocol and the following set of operations in the log. (start, T4); (write, T4, y, 2, 3); (start, T1); (commit, T4); (write, T1, z, 5, 7); (checkpoint); (start, T2); (write, T2, x, 1, 9); (commit, T2); (start, T3); (write, T3, z, 7, 2); If a crash happens now and the system tries to recover using both undo and redo operations, what are the contents of the undo list and the redo list |
| a. | Undo: T3, T1; Redo: T2 |
| b. | Undo: T3, T1; Redo: T2, T4 |
| c. | Undo: none; Redo: T2, T4, T3; T1 |
| d. | Undo: T3, T1, T4; Redo: T2 |
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Answer: (a).Undo: T3, T1; Redo: T2
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| 78. | Which level of locking provides the highest degree of concurrency in a relational data base? |
| a. | Page |
| b. | Table |
| c. | Row |
| d. | Page, table and row level locking allow the same degree of concurrency |
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Answer: (c).Row
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| 79. | Which one of the following is NOT a part of the ACID properties of database transactions? |
| a. | Atomicity |
| b. | Consistency |
| c. | Isolation |
| d. | Deadlock-freedom |
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Answer: (d).Deadlock-freedom
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| 80. | Consider the following two phase locking protocol. Suppose a transaction T accesses (for read or write operations), a certain set of objects {O1,...,Ok}. This is done in the following manner: Step 1. T acquires exclusive locks to O1, . . . , Ok in increasing order of their addresses. Step 2. The required operations are performed. Step 3. All locks are released. This protocol will |
| a. | guarantee serializability and deadlock-freedom |
| b. | guarantee neither serializability nor deadlock-freedom |
| c. | guarantee serializability but not deadlock-freedom |
| d. | guarantee deadlock-freedom but not serializability |
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Answer: (a).guarantee serializability and deadlock-freedom
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