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Condition of schedules to View-equivalent

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  • Difficulty Level : Easy
  • Last Updated : 28 Jun, 2021
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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 database in S1 then in S2 also T1 should read A from database. 


  T1     T2     T3

Transaction T2 is reading A from database. 

2)Updated Read 
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). 

Below are the previous Year Gate Question asked on this topic 

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