Data Mgmt Lab_Part_57 - Concurrency Control 169 (c)...

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Concurrency Control 169 (c) Otherwise, T writes O and WTS(O) is set to TS(T). The justifcation is as Follows: had TS ( T ) <RTS ( O ), T would have been aborted and we would not have bothered to check the WTS(O). So to decide whether to abort T based on WTS(O), we can assume that TS ( T ) > = RTS ( O ). IF TS ( T ) > = RTS ( O )and TS ( T ) <WTS ( O ), then RTS ( O ) <WTS ( O ), which means the previous write occurred immediately beFore this planned-new-write oF O and was never read by anyone, thereFore the previous write can be saFely ignored. 5. IF two schedules over the same set oF actions oF the transactions are conflict equiv- alent, they must order every pair oF conflicting actions oF two committed trans- actions in the same way. Let’s assume that two schedules are conflict equivalent, but they are not view equivalent, then one oF the three conditions held under view equivalency must be violated. But as we can see iF every pair oF conflicting actions is ordered in the same way, this cannot happen. Thus we can conclude that iF two schedules are conflict equivalent, they are also view equivalent. 6. The Following example is a serializable schedule, but it’s not strict. T1:R(X), T2:R(X), T2:W(X), T1:W(X), T2:Commit, T1:Commit 7. The Following example is a strict schedule, but it’s not serializable. T1:R(X), T2:R(X), T1:W(X), T1:Commit, T2:W(X), T2:Commit
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This note was uploaded on 01/17/2012 for the course EGN 4302 taught by Professor Dr.vishak during the Fall '12 term at University of Central Florida.

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Data Mgmt Lab_Part_57 - Concurrency Control 169 (c)...

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