Explain the rules for creating a labelled precedence graph, Database Management System

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Explain the rules for creating a labelled precedence graph for testing view serializability.          

 Ans: A schedule S is view serializable if it is view equivalent to a serial schedule 

  • Each conflict serializable schedule is also view serializable
  • A schedule that is view serializable but not conflict serializable:

Testing for Serializability 

  • Refer some schedule of a set of transactions T1, T2... Tn
  • Precedence graph is a direct graph in which the vertices denote the transactions
  • We illustrate an arc from Ti to Tj if the two transaction conflict and Ti accessed the data item earlier on which the conflict arose
  • We may label the arc through the item on which the conflict arose. A schedule is conflict serializable if and only if its precedence graph is acyclic
  • If precedence graph is acyclic, the serializability order can be acquired by a topological sorting of the graph.
  • The trouble of checking if a schedule is view serializable falls in the class of NP-complete problems
  • Though practical algorithms that just check few sufficient conditions for view serializability can still be employed.

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