Consider the following database schedule with two transactions, T₁ and T₂

 S = r2(X); r1(X); r2(Y); w1(X); r1(Y); w2(X); a1; a2

where r_i(Z) denotes a read operation by transaction T_i on a variable Z, w_i(Z) denotes a write operation by T_i on a variable Z and a_i denotes an abort by transaction T_i .

Which one of the following statements about the above schedule is TRUE?

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 $$(P).$$

Suppose that the transaction $$T1$$ fails immediately after time instance $$9.$$ Which one of the following statements is correct?

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 recovver using both undo and redo operations, what are the contents of the undo list and the redo list?

Consider the following schedule S of transactions T1, T2, T3, T4: