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 two phase locking protocol. Suppose a transaction $$T$$ accesses (for read or write operations), a certain set of objects $$\left\{ {{O_1},...,{O_k}} \right\}.$$ This is done in the following manner:

Step 1. T acquires exclusive locks to $${{O_1},...,{O_k}}$$ in increasing order of their
addresses.
Step 2. The required operations are performed.
Step 3. All locks are released.

This protocol will

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?