Serializability of concurrent scheduling

Different scheduling of concurrent transactions by database management system may produce different results
Serial scheduling is correct
It is also true that the execution result is equivalent to serial scheduling , It is called serializable scheduling

Serializable scheduling

Serializable (Serializable) Dispatch

Concurrent execution of multiple transactions is correct , If and only if the result is the same as when these transactions are executed serially in a certain order

Serializability (Serializability)

It is the criterion for the correct scheduling of concurrent transactions
A given concurrent schedule , If and only if it is serializable , I think it's the right scheduling

[ example 11.2] Now there are two transactions , The following operations are included respectively ：

Business T1： read B;A=B+1; Write back to A
Business T2： read A;B=A+1; Write back to B
Different scheduling strategies for these two transactions are given
Serial scheduling , Correct scheduling

hypothesis A、B The initial values of are 2.
Press T1→T2 The sequence execution result is A=3,B=4
Serial scheduling policy , Correct scheduling

hypothesis A、B The initial values of are 2.
T2→T1 The sequence execution result is B=3,A=4
Serial scheduling policy , Correct scheduling
Non serializable scheduling , Wrong scheduling

Execution results and (a)、(b) The results are all different
It's the wrong scheduling
Serializable scheduling , Correct scheduling

Execution results and serial scheduling (a) The execution results are the same
Is the right scheduling

Conflicts can be scheduled serially

Conflicts can be serialized

A more stringent condition than serializability
The scheduler in the commercial system adopts
Conflicting operations ： It refers to the reading and writing operations of different transactions on the same data ：

  Ri(x) And Wj(x)	       /* Business Ti read x,Tj Write x, among i≠j*/
  Wi(x) And Wj(x)         /* Business Ti Write x,Tj Write x, among i≠j*/

Other operations are non conflicting operations

It can't be exchanged （Swap） The action of :

Two operations of the same transaction
Conflicting operations of different transactions
A dispatch Sc In the case of keeping the order of conflicting operations unchanged , Get another schedule by exchanging the order of two non conflicting transactions Sc’, If Sc’ It's serial , Call scheduling Sc Conflict serializable scheduling
If a scheduling conflict is serializable , Then it must be serializable scheduling
This method can be used to determine whether a schedule is conflict serializable

[ example 11.3] There is a dispatch today

Sc2 Equivalent to a serial scheduling T1,T2. therefore Sc1 Conflict serializable scheduling
Conflict serializable scheduling is a sufficient condition for serializable scheduling , It's not necessary . There is also serializable scheduling that does not meet the conflict serializability condition .
[ example 11.4] Yes 3 One transaction T1=W1(Y)W1(X),T2=W2(Y)W2(X),T3=W3(X)
Dispatch L1=W1(Y)W1(X)W2(Y)W2(X) W3(X) Is a serial scheduler .
Dispatch L2=W1(Y)W2(Y)W2(X)W1(X)W3(X) Does not satisfy conflict serializability . But scheduling L2 It's serializable , because L2 Execution results and scheduling L1 identical ,Y All the values of are equal to T2 Value ,X All the values of are equal to T3 Value
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