Consider the following schedules involving two transactions. Which one of the following statements is TRUE? $S_1 :r_1(X); r_1(Y); r_2(X); r_2(Y); w_2(Y); w_1(X)$ $S_2 :r_1(X); r_2(X); r_2(Y); w_2(Y); r_1(Y); w_1(X)$

🚀 Solve in Practice Mode

📖 Explanation

To determine conflict serializability, we construct a precedence graph for each schedule and check for cycles. An edge $T_i \to T_j$ exists if $T_i$ accesses an item before $T_j$ , and at least one of these accesses is a write operation (i.e., they conflict).

For $S_1: r_1(X); r_1(Y); r_2(X); r_2(Y); w_2(Y); w_1(X)$ :

$r_1(Y)$ and $w_2(Y)$ conflict (read-write on $Y$ ), so $T_1 \to T_2$ .
$r_2(X)$ and $w_1(X)$ conflict (read-write on $X$ ), so $T_2 \to T_1$ .
The precedence graph contains a cycle: $T_1 \to T_2 \to T_1$ . Thus, $S_1$ is not conflict serializable.

For $S_2: r_1(X); r_2(X); r_2(Y); w_2(Y); r_1(Y); w_1(X)$ :

$r_2(X)$ and $w_1(X)$ conflict (read-write on $X$ ), so $T_2 \to T_1$ .
$w_2(Y)$ and $r_1(Y)$ conflict (write-read on $Y$ ), so $T_2 \to T_1$ .
The precedence graph contains only edges $T_2 \to T_1$ . There is no cycle. Thus, $S_2$ is conflict serializable.

Therefore, $S_1$ is not conflict serializable, and $S_2$ is conflict serializable.

The final answer is $\boxed{C}$

Q45GATE 2005MCQ

A company maintains records of sales made by its salespersons and pays them commission based on each individual's total sales made in a year. This data is maintained in a table with following schema: salesinfo = (salespersonid, totalsales, commission) In a certain year, due to better business results, the company decides to further reward its salespersons by enhancing the commission paid to them as per the following formula: If commission $\leq 50000$ , enhance it by 2% If 50000 < commission $\leq 100000$ , enhance it by 4% If commission > 100000, enhance it by 6% The IT staff has written three different SQL scripts to calculate enhancement for each slab, each of these scripts is to run as a separate transaction as follows: T1 Update salesinfo Set commission = commission * 1.02 Where commission < = 50000; T2 Update salesinfo Set commission = commission * 1.04 Where commission > 50000 and commission is < = 100000; T3 Update salesinfo Set commission = commission * 1.06 Where commission > 100000; Which of the following options of running these transactions will update the commission of all salespersons correctly

🚀 Solve in Practice Mode

📖 Explanation

To ensure each record is updated correctly exactly once, we must prevent a commission value from being modified by one transaction and then falling into the criteria of another. Executing $T_1$ first could push a value $> 50000$ , causing it to be updated again by $T_2$ . Similarly, $T_2$ could push a value $> 100000$ , triggering $T_3$ . By executing $T_3$ first, all values $> 100000$ are updated to values still $> 100000$ and are not affected by subsequent scripts. Next, $T_2$ updates values in $(50000, 100000]$ , resulting in values that will not satisfy the condition for $T_1$ ( $\leq 50000$ ). Finally, $T_1$ updates the remaining values $\leq 50000$ . This descending order $T_3 \rightarrow T_2 \rightarrow T_1$ ensures no record is processed twice.