MadeEasy Subject Test: Databases - Transactions

Question

S: R1(A), R3(D), W1(B), R2(B), R4(B), W2(C), R5(C), W4(E), R5(E), W5(B)

Number of serial schedules which are view equal to above S are __.

Polygraph that I drew:

Answer 1

Rules :

1.  Initial Read must be same
2. There is W-R dependency
3.  Final write on some item should be  same

 There are 4 Write-Read conflict .

• W1-R4 on B ,
• W1-R2 on B
• W2-R5 on C
• W4-R5 on E

Transaction  T5 is performing final write on items B  , and it is fixed .

Initial Read is done by T1 on A.

so all 3 rules are satisfying here.

--------T1------- T2-------T4--------T5--------(T3 can be placed anywhere).....

--------T1------- T4-------T2--------T5--------(T3 can be placed anywhere)....

Therefore, total orderings = 5 +5=10 .

Comments

okay. thanks a lot :)

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Sir if you T3 placed before T1, will not it voilate initial read condition because now iniial read will be done by T3

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See the schedule in the question .

T₃ is performed by a Read operation, R3(D) . Not by a write operation so if T₃ place before T₁ there is no W-R dependency , only R-R dependency .

Same logic also apply if T₃ place after T₅ . 

Answer 2

Correct me if I'm wrong!

Comments

T2 and T4 have no W-R dependency thats why they can only interchange their position.

T2 and T4 have W-W dependency. [ write-write ]

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bikram sir , W - W is not considered as dependency rt.??

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R-R only not consider,

W-R and W-W both consider.

Answer 3

Now, the topo sort of the above schedule will give the orderings.

Comments

yes, 10 is the correct answer.

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@Lokesh. Have changed the graph. Just check.

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question is duplicate of this : https://gateoverflow.in/113129/madeeasy