Databases Test 6

Total number of schedule possible = \(\frac{{\left( {T_{1} + T_{2} + T_{3}} \right)!}}{{T_{1}!\times T_{2}!\times T_{3}!}} \)

Total number of schedule possible = \(\frac{{\left( {6 + 3 + 4} \right)!}}{{6!\times 3! \times 4!}} = 60060\)

Total number of serial schedule possible = \(n!\)

where n is total number of transaction

Total number of serial schedules = 3! = 6

Total number of non-serial schedules = 60060 - 6 = 60054

Concept:

Ti requests a data item that is held by Tj then

If TS(Ti)<TS(Tj) abort Tj

Otherwise, Ti will wait

Explanation:

Data item held by T23 and T22 request a data item

Since T22 < T23, therefore, T23 will rollback

Data item held by T23 and T24 request a data item

Since T23 < T24, therefore, T24 will die

Note:

TS → Time Stamp

ACID Properties:

A transaction in a database system must maintain Atomicity, Consistency, Isolation, and Durability − commonly known as ACID properties − in order to ensure accuracy, completeness, and data integrity.

Atomicity − This property states that a transaction must be treated as an atomic unit, that is, either all its operations are executed or none.

Consistency − The database must remain in a consistent state after any transaction. No transaction should have any adverse effect on the data residing in the database.

Isolation − In a database system where more than one transaction is being executed simultaneously and in parallel, the property of isolation states that all the transactions will be carried out and executed as if it is the only transaction in the system.

Durability − The database should be durable enough to hold all its latest updates even if the system fails or restarts.

Therefore, Isolation is the correct answer.

Concept:

Two schedules are said to be conflict equivalent if the order of any two conflicting operations is the same in both the schedules. A schedule S is said to be conflict serializable if it is conflict equivalent to some serial schedule S’.

Explanation:

Schedule S:

T1(read(P)) →  T2(write (P)); T2(read (Q)) → T1(write(Q))

For this, precedence graph contains cycle. So, the concurrent execution of T1 and T2 leads to a non-conflict serializable schedule.

A transaction read a data item after it has been written by an uncommitted transaction leads to an irrecoverable error in a database system.

∴ statement I false

Time stamp ordering protocol ensures freedom from deadlock

∴ statement II false

In the ACID property, the durability property makes changes successfully in the database in case of:

Power failure, dish crash or operating system crash

∴ statement III false

Cascadeless schedule is one where, for each pair of transactions T_i and T_j such that T_j reads a data item previously written by T_i, the commit operation of T_i appears before the read operation of T_j.

Therefore, only Schedule 2 is the cascadeless schedule.