Two schedules S1 and S2 are said to be view-equivalent if below conditions are satisfied :
1) Initial Read
If a transaction T1 reading data item A from initial database in S1 then in S2 also T1 should read A from initial database.
T1 T2 T3 ------------------- R(A) W(A) R(A) R(B)
Transaction T2 is reading A from initial database.
If Ti is reading A which is updated by Tj in S1 then in S2 also Ti should read A which is updated by Tj.
T1 T2 T3 T1 T2 T3 ------------------- ---------------- W(A) W(A) W(A) R(A) R(A) W(A)
Above two schedule are not view-equivalent as in S1 :T3 is reading A updated by T2, in S2 T3 is reading A updated by T1.
3)Final Write operation
If a transaction T1 updated A at last in S1, then in S2 also T1 should perform final write operations.
T1 T2 T1 T2 ------------ --------------- R(A) R(A) W(A) W(A) W(A) W(A)
Above two schedules are not view-equivalent as Final write operation in S1 is done by T1 while in S2 done by T2.
View Serializability: A Schedule is called view serializable if it is view equal to a serial schedule (no overlapping transactions).
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