Transactional concurrency control: If two transactions run at the same time and the combined result is the same as if they had executed one after the other in some order, are those transactions considered serializable?

Introduction / Context:
Serializable isolation is the gold standard for correctness in concurrent transaction processing. It ensures that interleaved execution of transactions yields a final database state equivalent to some serial (one-after-another) execution. This question checks whether that equivalence property is precisely what “serializable” means.

Given Data / Assumptions:

• Two or more transactions may overlap in time (concurrent execution).
• We compare the outcome of the concurrent schedule to the outcome of a serial schedule.
• No requirement to match a particular serial order—only that some serial order exists.

Concept / Approach:
A schedule is serializable if it is conflict-serializable or view-serializable, meaning there exists a serial ordering of the same transactions that produces the same effect on the database. Concurrency control mechanisms (e.g., two-phase locking or optimistic schemes) strive to allow parallelism while preserving this serializability property.

Step-by-Step Solution:

Verification / Alternative check:
Use precedence (conflict) graphs: if the graph is acyclic, the schedule is conflict-serializable, guaranteeing equivalence to a serial order.

Why Other Options Are Wrong:

• “Incorrect” contradicts the formal definition.
• “Only under read uncommitted isolation” is wrong; serializability is stronger than read uncommitted.
• “Only when no locks are used” is wrong; locking (2PL) is a common way to enforce serializability.

Common Pitfalls:
Confusing serial execution (no overlap) with serializable execution (may overlap but is equivalent to a serial order). Also, assuming there must be a specific serial order match—any serial order that yields the same result suffices.

Final Answer:
Correct