Problem 1 Consider a database consisting of a single relation M: 10 0 2 a) The f

Question

Problem 1 Consider a database consisting of a single relation M: 10 0 2 a) The following two transactions T1 and T2 run concurrently on this database: T1 BEGIN TRANSACTION S1: update M set B -B-10 where A-1 S2: update M set B = B+10 where A-2 COMMIT T2 BEGIN TRANSACTION S3: select sum(B) from M COMMIT Indicate at least one isolation level (if exist) that may make possible to see zero as output for T2. The following two transactions run concurrently on this database: T1 BEGIN TRANSACTION b) S1: insert into M values (3,100) COMMIT T2 BEGIN TRANSACTION S2: select sum(B) from M S3: select sum(B) from M; COMMIT For what isolation levels, T2 may see a different value as the output of the select sum(B) from M statements?

Explanation / Answer

Solution:

a)

When in T1, S1 gets executed then B= B-10 = 0

and just after that T2 reads then the sum(B) will be 0

the T1 doesn't complete the work and T2 finishes

b)

Different values will be seen when

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