Schedule S is R1(A), W1(A), R2(A), W2(A), R1(B), W1(B), R2(B), W2(B). Which serial schedule is S conflict-equivalent to?
Correct answer D. T1 then T2
Subject / 421 questions
Programming MCQs
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T2 reads a value of X written by T1. For the schedule to be recoverable, what must hold?
Correct answer B. T1 must commit before T2 commits
Under the two-phase locking (2PL) protocol, what is a transaction forbidden to do once it has released its first lock?
Correct answer D. Acquire any new lock
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What does strict two-phase locking add to basic 2PL, and what problem does it remove?
Correct answer B. Exclusive locks are held until commit or abort, which removes cascading aborts
T1 holds a shared (S) lock on item X. T2 now requests an exclusive (X) lock on the same item. What happens under standard lock compatibility?
Correct answer B. T2 must wait until T1 releases its shared lock
Under the wait-die deadlock prevention scheme, an older transaction T1 requests a lock held by a younger transaction T2. What happens?
Correct answer B. T1 waits for T2
Under basic timestamp ordering, transaction T with timestamp 10 tries to read item X whose write timestamp W-TS(X) is 15. What does the protocol do?
Correct answer D. Aborts T and restarts it with a new timestamp
What does the write-ahead logging (WAL) rule require?
Correct answer B. Log records describing a change must reach stable storage before the changed data page does
What is the main purpose of taking a checkpoint in a log-based recovery system?
Correct answer B. To limit how much of the log must be scanned and replayed after a crash
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At a crash, T1 had committed but some of its updated pages were still only in memory, and T2 was still active with some of its updates already on disk. What does log-based recovery do?
Correct answer C. Redo T1 and undo T2
A buffer manager uses the STEAL and NO-FORCE policies. Which recovery actions must the log support?
Correct answer D. Both undo and redo
Table Student(roll_no, cnic, email, name) has roll_no, cnic and email each unique and never NULL. If roll_no is chosen as the primary key, what are cnic and email called?
Correct answer C. Alternate keys
In table OrderItem(order_id, product_id, quantity), neither order_id nor product_id is unique alone, so the primary key is declared as (order_id, product_id). What is such a key called?
Correct answer B. Composite key
Relation R(A, B, C) has A as its only candidate key. How many superkeys does R have?
Correct answer B. 4
Which integrity rule states that no attribute of a primary key may contain NULL?
Correct answer C. Entity integrity
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Emp.dept_id is a foreign key referencing Dept(id), declared with no ON DELETE action. What happens if you try to delete a Dept row that some Emp rows still reference?
Correct answer D. The delete is rejected with a foreign key violation
Emp.dept_id is a foreign key to Dept(id) and is not declared NOT NULL. What happens when you insert an Emp row with dept_id set to NULL?
Correct answer B. The insert succeeds, because a NULL foreign key is not checked
Relation R(A, B) contains tuples (1,x), (1,y) and (2,x). How many tuples are in the relational algebra result of projection on A, written pi_A(R)?
Correct answer B. 2
For the relational algebra expression R union S to be valid, what must be true of R and S?
Correct answer A. They must have the same number of attributes with compatible domains
Supply(s, p) contains (s1,p1), (s1,p2), (s2,p1), (s3,p1), (s3,p2), (s3,p3), and Parts(p) contains p1 and p2. What is Supply divided by Parts?
Correct answer A. {s1, s3}