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

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

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

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

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

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

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

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

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

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