Which isolation level offers the highest degree of isolation, preventing dirty, non-repeatable, and phantom reads?

• A. Read Uncommitted
• B. Read Committed
• C. Repeatable Read
• D. Serializable

Answer: (D)

Maximal isolation means transactions appear to execute in some serial order, even when others are running concurrently. This is achieved by the Serializable level, which makes every transaction see a consistent, uninterrupted view of the data as if no other transactions were in progress.

Why this is the best fit: it prevents all three types of anomalies that can occur in concurrent execution. Dirty reads are avoided because you never see uncommitted changes from others. Non-repeatable reads are avoided because the data you read stays fixed for the duration of your transaction. Phantom reads are avoided because the set of rows matching your query won’t change due to other transactions inserting new rows that would match your criteria. This level often uses stricter locking (such as range locks) or equivalent mechanisms to ensure a serial schedule, so the results are deterministic and consistent.

In contrast, lighter levels relax some guarantees: reading uncommitted data allows dirty reads; read committed stops dirty reads but can still show different data on repeated reads or allow new rows to appear for the same query; repeatable read prevents non-repeatable reads but can still allow phantom rows to appear. Those weaker options trade safety for performance, which is why they don’t completely eliminate the three anomalies Serializable eliminates.