Consider a relation R(product, value). There are initially two tuples in R: (A,1

Question

Consider a relation R(product, value). There are initially two tuples in R: (A,10) and (B,20). The following two transactions are executed concurrently: T1 BEGIN TRANSACTION S1: Update R set value = value +5 S2: Insert into R values (C, 30) COMMIT;

T2 BEGIN TRANSACTION S3: Select sum(value) as s from R S4: Select max(value) as m from R COMMIT;

T1 always executes with isolation level Serializable. Assume both transactions successfully commit, and the individual statements S1, S2, S3, and S4 within the transactions each execute atomically.
a) If transaction T2 also executes with isolation level Serializable, which possible pair of values for the sum (s) and max (m) can be returned by T2? Explain

b) If transaction T2 executes with isolation level Read Committed, which possible pair of values for the sum (s) and max (m) can be returned by T2? Explain

c) If transaction T2 executes with isolation level Read Uncommitted, which possible pair of values for the sum (s) and max (m) can be returned by T2? Explain

Problem 5 Consider a relation R(product, value). There are initially two tuples in R: (A,10) and (B,20). The following two transactions are executed concurrently: T1 BEGIN TRANSACTION S1: Update R set value value +5 S2: Insert into R values (C, 30) COMMIT T2 BEGIN TRANSACTION S3: Select sum(value) as s from R S4: Select max(value) as m from R COMMIT T1 always executes_ with_isolation level Serializable. Assume both transactions successfully commit, and the individual statements S1, S2, S3, and S4 within the transactions each execute atomically. a) If transaction T2 also executes with isolation level Serializable, which possible pair of values for the sum (s) and max (m) can be returned by T2? Explain b) If transaction T2 executes with isolation level Read Committed, which possible pair of values for the sum (s) and max (m) can be returned by T2? Explain If transaction T2 executes with isolation level Read Uncommitted, which possible pair of values for the sum (s) and max (m) can be returned by T2? Explain c)

Explanation / Answer

Solution:

a)

In this case, S3 will return 15+25+30= 70

and

S4 will return 30

Explanation:
In Isolation level serializable the transaction reads data which is committed by the transaction

b)

In this case

S3 will return 10+20= 30

and

S4 will return 20

Explanation:

READ COMMITTED. Specifies that statements cannot read data that has been modified but not committed by other transactions.  

c)

S3 will return 15+25= 40

S4 will return 30= 70

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