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You're correct that a table-level lock is a useful tool to ensure serializability, but it's also a drastic one since it blocks other transactions requesting a higher access level. The concurrency control system is trying to provide a generic way to both guarantee consistency and maintain a high throughout on a variety of workloads, and locking tables always would diminish throughout for many common workloads. For example, one where inserts and reads tend to occur simultaneously at high volume.