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Transaction & Concurrency Control GATE CS PYQ Quiz
Question 1
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 serializable schedule
A schedule that is not conflict serializable
A conflict serializable schedule
A schedule for which a precedence graph cannot be drawn
Question 2
Consider the following schedule for transactions T1, T2 and T3:
Which one of the schedules below is the correct serialization of the above?
T1->>T3->>T2
T2->>T1->>T3
T2->>T3->>T1
T3->>T1->>T2
Question 3
Consider two transactions T1 and T2, and four schedules S1, S2, S3, S4 of T1 and T2 as given below:
T1 = R1[X] W1[X] W1[Y]
T2 = R2[X] R2[Y] W2[Y]
S1 = R1[X] R2[X] R2[Y] W1[X] W1[Y] W2[Y]
S2 = R1[X] R2[X] R2[Y] W1[X] W2[Y] W1[Y]
S3 = R1[X] W1[X] R2[X] W1[Y] R2[Y] W2[Y]
S4 = R2[X] R2[Y] R1[X] W1[X] W1[Y] W2[Y]
Which of the above schedules are conflict-serializable?
S1 and S2
S2 and S3
S3 only
S4 only
Question 4
Which of the following scenarios may lead to an irrecoverable error in a database system ?
A transaction writes a data item after it is read by an uncommitted transaction
A transaction reads a data item after it is read by an uncommitted transaction
A transaction reads a data item after it is written by a committed transaction
A transaction reads a data item after it is written by an uncommitted transaction
Question 5
Consider the following schedule S of transactions T1, T2, T3, T4:
Which one of the following statements is CORRECT?
S is conflict-serializable but not recoverable
S is not conflict-serializable but is recoverable
S is both conflict-serializable and recoverable
S is neither conflict-serializable nor is it recoverable
Question 6
Consider the following partial Schedule S involving two transactions T1 and T2. Only the read and the write operations have been shown. The read operation on data item P is denoted by read(P) and the write operation on data item P is denoted by write(P).
Suppose that the transaction T1 fails immediately after time instance 9. Which one of the following statements is correct?
T2 must be aborted and then both T1 and T2 must be re-started to ensure transaction atomicity
Schedule S is non-recoverable and cannot ensure transaction atomicity
Only T2 must be aborted and then re-started to ensure transaction atomicity
Schedule S is recoverable and can ensure atomicity and nothing else needs to be done
Question 7
Which of the following concurrency control protocols ensure both conflict serializability and freedom from deadlock? I. 2-phase locking II. Time-stamp ordering
I only
II only
Both I and II
Neither I nor II
Question 8
Consider the following four schedules due to three transactions (indicated by the subscript) using read and write on a data item x, denoted by r(x) and w(x) respectively. Which one of them is conflict serializable.
A
B
C
D
Question 9
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?
Only S1 is conflict-serializable.
Only S2 is conflict-serializable.
Both S1 and S2 are conflict-serializable.
Neither S1 nor S2 is conflict-serializable.
Question 10
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
There are 31 questions to complete.