database

Question

s1:R1(X)R2(Z)R1(Z)R3(X)R3(Y)W1(X)C1W3(Y)C3R2(Y)W2(Z)W2(Y)C2 S2:R1(X)R2(Z)R1(Z)R3(X)R3(Y)W1(X)W3(Y)R2(Y)W2(Z)W2(Y)C1C2C3

S3:R1(X)R2(Z)R3(Z)R1(Z)R2(Y)W1(X)C1W2(Z)W3(Y)W2(Y)C2C3

WHICH of this is true ? please verify

a)s1 cascadeless, recoverable but not strict

b) s1 and s2 irrecoverable

c) s1 s3 cascadeless and recoverable and s2 recoverable

d)all irrecoverable

Comments

https://gateoverflow.in/39590/gate2016-2-51
https://gateoverflow.in/34329/testbook-test-series-database

Answer 1

Recoverability

A recoverable schedule is one where, for each pair of Transaction T_i and T_j such that T_j  reads data item previously written by T_i   the commit operation of T_i  appears before the commit operation T_j .

 

Cascadeless Schedule

Formally, a cascadeless schedule is one where for each pair of transaction T_i  and T_j  such that T_j  reads data item, previously written by T_i   the commit operation of T_i  appears before the read operation of T_j .

 

Strict

A schedule is strict - has the strictness property - if for any two transactions T1, T2, if a write operation of T1 precedes a conflicting operation of T2 (either read or write), then the commit event of T1 also precedes that conflicting operation of T2.

 

here w(y) of T3 happens before r(y) of T2

and commit operation c3 is before the read operation of T2.

So, it is cascadeless schedule

And we know Every Cascadeless schedule is also recoverable schedule.

http://coddicted.com/recoverable-and-cascadeless-schedules/

it is also irrecoverable and no chance for cascadeless

it is cascadeless and recoverable. Because it is not violating any rule of cascadeless or recoverable

S1- cascadeless recoverable and  strict

S2- cascading irrecoverable

S3- cascadeless recoverable

Comments

i guess non of the option is correct ..

how did you come s2 and s3  recoverable ... they are not recoverable ...

if you see in s2 , t2 transaction read value of y , written by t3 transaction but t2 commit before t3 while t3 must be commited before t2 ..other wise it will not be recoverable rt ..

same in s3 also ...

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plz rechk @sonam

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@srestha how u concluded S1 is not strict?? There is no WW conflict so it should be strict recoverable...

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@Arpit ya it is strict , there is WW, and RW conflict

And commit of T3 is before commit of T2

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@minal

@srestha mam

how to check for strict???

i am confused for both S1 and S3

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https://gateoverflow.in/274982/strict-schedule-or-not

Strict schedule is if $T_{j}$ writes/reads the value written by $T_{i}$, then $T_{i}$ must commit before $T_{j}$ reads/writes