Consider a schedule of transactions T₁ and T₂:

Here, RX stands for “Read(X)” and WX stands for “Write(X)”. Which one of the following schedules is conflict equivalent to the above schedule?

Consider the following transactions with data items P and Q initialized to zero:

T1: read (P) ;
    read (Q) ;
    if P = 0 then Q : = Q + 1 ;
    write (Q) ;
T2: read (Q) ;
    read (P) ;
    if Q = 0 then P : = P + 1 ;
    write (P) ;

Any non-serial interleaving of T1 and T2 for concurrent execution leads to

Which of the following scenarios may lead to an irrecoverable error in a database system?

Consider three data items D1, D2 and D3 and the following execution schedule of transactions T1, T2 and T3. In the diagram, R(D) and W(D) denote the actions reading and writing the data item D respectively.

Which of the following statements is correct?

Consider the following two transactions: T₁ and T₂.

T1: read(A);
               read(B);
               if A=0 then B ← B+1;
               write(B);
T2: read(B);
               read(A);
               if B≠0 then A ← A-1;
               write(A); 
 
Which of the following schemes, using shared and exclusive locks, satisfy the requirements for strict two phase locking for the above transactions? 
      

Consider the following three schedules of transactions T1, T2 and T3. [Notation: In the following NYO represents the action Y (R for read, W for write) performed by transac­tion N on object O.]

(S1)	2RA	2WA	3RC	2WB	3WA	3WC	1RA	1RB	1WA	1WB
(S2)	3RC	2RA	2WA	2WB	3WA	1RA	1RB	1WA	1WB	3WC
(S3)	2RA	3RC	3WA	2WA	2WB	3WC	1RA	1RB	1WA	1WB

Consider the following transaction involving two bank account x and y.

The constraint that the sum of the accounts x and y should remain constant is that of

Consider a simple checkpointing protocol and the following set of operations in the log.

(start, T₄); (write, T₄, y, 2, 3); (start, T₁); (commit, T₄); (write, T₁, z, 5, 7);
(checkpoint);
(start, T₂); (write, T₂, x, 1, 9); (commit, T₂); (start, T₃); (write, T₃, z, 7, 2);

If a crash happens now and the system tries to recover using both undo and redo operations, what are the contents of the undo list and the redo list

Suppose a database schedule S involves transactions T₁, ..., T_n. Construct the precedence graph of S with vertices representing the transactions and edges representing the conflicts. If S is serializable, which one of the following orderings of the vertices of the precedence graph is guaranteed to yield a serial schedule?

Consider the following partial Schedule S involving two transactions T1 and T2. Only the read and the write operations have been shown. The read operation on data item P is denoted by read(P) and the write operation on data item P is denoted by write(P).

Consider the following four schedules due to three transactions (indicated by the subscript) using read and write on a data item x, denoted by r(x) and w(x) respectively. Which one of them is conflict serializable?

Consider the following log sequence of two transactions on a bank account, with initial balance 12000, that transfer 2000 to a mortgage payment and then apply a 5% interest.

         
            1. T1 start
            2. T1 B old=12000 new=10000
            3. T1 M old=0 new=2000
            4. T1 commit
            5. T2 start
            6. T2 B old=10000 new=10500
            7. T2 commit

Suppose the database system crashes just before log record 7 is written. When the system is restarted, which one statement is true of the recovery procedure?

Consider two transactions T1 and T2, and four schedules S1, S2, S3, S4 of T1 and T2 as given below:

T1 = R1[X] W1[X] W1[Y]
T2 = R2[X] R2[Y] W2[Y]
S1 = R1[X] R2[X] R2[Y] W1[X] W1[Y] W2[Y]
S2 = R1[X] R2[X] R2[Y] W1[X] W2[Y] W1[Y]
S3 = R1[X] W1[X] R2[X] W1[Y] R2[Y] W2[Y]
S1 = R1[X] R2[Y]R2[X]W1[X] W1[Y] W2[Y]

Which of the above schedules are conflict-serializable?

Which of the following concurrency control protocols ensure both conflict serializability and freedom from deadlock?

I. 2-phase locking
II. TIme-stamp ordering