24 real Concurrency Control questions from the SQL bank, as asked in Indian campus drives and tech interviews. Every question has a verified answer and an AI-tutor explanation on placd — free to start.
1. What is MVCC?
Mid
A.a write lock that only one transaction may hold and that blocks all other readers and writers on the resource
B.the engine's promotion of many fine-grained row locks into a single coarser table lock to reduce locking overhead
C.a read lock that several transactions may hold on the same resource at once but which blocks any writer
D.a technique that keeps multiple timestamped versions of a row so readers see a consistent snapshot without blocking writers
A.Optimistic locking — a strategy that assumes conflicts are rare and checks a version or timestamp only at commit time, retrying if the row changed
B.Optimistic locking — a protocol with a growing phase that only acquires locks and a shrinking phase that only releases them, guaranteeing serializability
C.Optimistic locking — the engine's promotion of many fine-grained row locks into a single coarser table lock to reduce locking overhead
D.Optimistic locking — a named marker within a transaction that lets you roll back part of the work without aborting the whole transaction
A.Pessimistic locking — a protocol with a growing phase that only acquires locks and a shrinking phase that only releases them, guaranteeing serializability
B.Pessimistic locking — a strategy that acquires locks up front before reading or writing, blocking other transactions until they are released
C.Pessimistic locking — a write lock that only one transaction may hold and that blocks all other readers and writers on the resource
D.Pessimistic locking — a named marker within a transaction that lets you roll back part of the work without aborting the whole transaction
A.Exclusive lock — a write lock that only one transaction may hold and that blocks all other readers and writers on the resource
B.Exclusive lock — the engine's promotion of many fine-grained row locks into a single coarser table lock to reduce locking overhead
C.Exclusive lock — a read lock that several transactions may hold on the same resource at once but which blocks any writer
D.Exclusive lock — a protocol with a growing phase that only acquires locks and a shrinking phase that only releases them, guaranteeing serializability
23. Which term means: "a protocol with a growing phase that only acquires locks and a shrinking phase that only releases them, guaranteeing serializability"?
A.Two-phase locking — a write lock that only one transaction may hold and that blocks all other readers and writers on the resource
B.Two-phase locking — a strategy that assumes conflicts are rare and checks a version or timestamp only at commit time, retrying if the row changed
C.Two-phase locking — a named marker within a transaction that lets you roll back part of the work without aborting the whole transaction
D.Two-phase locking — a protocol with a growing phase that only acquires locks and a shrinking phase that only releases them, guaranteeing serializability
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