Suppose that transactions T1 and T2 can be decomposed into the subtransactions T1 : T1,1, T1,2 and T2 : T2,1, T2,2 such that each subtransaction individually maintains the consistency constraints of the database. Instead of guaranteeing that all schedules involving T1 and T2 are serializable, suppose that a concurrency control guarantees that all subtransactions are always executed serializably.

a. Will T1 always be serializable with T2? Explain.
b. Will integrity constraints be maintained by all possible schedules?
c. What possible problems might arise if the concurrency control schedules transactions in this way?

a. No. See previous exercise.
b. Yes, because each subtransaction starts with a consistent database and hence
produces a consistent database.
c. The database states produced might not correspond to any serial schedule and hence might not be a correct model of the state of the world. However, this situation might be acceptable to some applications. A similar problem might arise with respect to the information returned to the transaction.