In database systems, a transaction schedule is a sequence of instructions that comes from concurrently executing transactions. Each instruction belongs to a particular transaction, and the order of instructions in the schedule is the order in which the instructions are executed.
If two transactions are running concurrently, the database system will interleave the actions (read, write, commit, abort) of the transactions. The result is a schedule of the transactions.
Let's take a simple example with two transactions, T1 and T2.
Transaction T1: Read(A), Write(A), Read(B), Write(B)
Transaction T2: Read(C), Write(C), Read(D), Write(D)
A possible schedule for these transactions might be:
Schedule S: Read(A)-T1, Write(A)-T1, Read(C)-T2, Write(C)-T2, Read(B)-T1, Write(B)-T1, Read(D)-T2, Write(D)-T2
In the above schedule, operations of T1 and T2 are interleaved.
Schedules are vital in understanding the concurrent execution of transactions. There are two types of schedules:
Serial schedule: In this schedule, transactions are ordered one after the other. If there are 'n' transactions, there will be 'n!' possible serial schedules. Though this type of schedule is simple and ensures database consistency, it may not provide sufficient concurrency.
Concurrent schedule: In this schedule, operations from a set of transactions are interleaved. While the concurrent schedule improves performance by allowing transactions to run in parallel, it increases the complexity of ensuring the ACID properties.
It's also important to note the concept of conflict and conflict-serializability in the context of schedules. Two operations are said to be in conflict if they belong to different transactions, operate on the same data item, and at least one of them is a write operation. A schedule is conflict-serializable if it can be transformed into a serial schedule by swapping non-conflicting operations.
In practice, the goal of a database system is to allow as much concurrency as possible, while ensuring that the execution of concurrent transactions yields consistent and correct results. The transaction scheduler in a DBMS is responsible for creating a schedule that maintains the appropriate balance between performance and correctness.