What is a schedule Database Management Systems 10CS54 VTU notes JUNJULY 2013Jan 2016

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What is a schedule? . Database Management Systems - 10CS54 VTU notes (JUN/JULY 2013/Jan 2016)

Explain with example serial, non serial and conflict serializable schedules?


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A schedule (or history) S of n transactions T1, T2, ..., Tn is an ordering of the operations of the transactions subject to the constraint that, for each transaction Ti that participates in S, the operations of Ti in S must appear in the same order in which they occur in Ti. Note, however, that operations from other transactions Tj can be interleaved with the operations of Ti in S. For now, consider the order of operations in S to be a total ordering, although it is possible theoretically to deal with schedules whose operations form partial orders.  

Two operations in a schedule are said to conflict if they satisfy all three of the following conditions: 
1. they belong to different transactions;
 2. they access the same item X; and
 3. at least one of the operations is a write_item(X). For example, in schedule , the operations 
operations w1(X) and w2(X). However, the operations r1(X) and r2(X) do not conflict, since they are both read operations; the operations w2(X) and w1(Y) do not conflict, because they operate on distinct data items X and Y; and the operations r1(X) and w1(X) do not conflict, because they belong to the same transaction. • A schedule S is serial if, for every transaction T all the operations of T are executed consecutively in the schedule. • A schedule S of n transactions is serializable if it is equivalent to some serial VTUlive.com 67 schedule of the same n transactions


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