Following Questions have been asked in GATE 2012 exam.
1) Consider the following transactions with data items P and Q initialized to zero:
T1: read (P) ; read (Q) ; if P = 0 then Q : = Q + 1 ; write (Q) ; T2: read (Q) ; read (P) ; if Q = 0 then P : = P + 1 ; write (P) ;
Any non-serial interleaving of T1 and T2 for concurrent execution leads to
(A) A serializable schedule
(B) A schedule that is not conflict serializable
(C) A conflict serializable schedule
(D) A schedule for which a precedence graph cannot be drawn
Two or more actions are said to be in conflict if:
1) The actions belong to different transactions.
2) At least one of the actions is a write operation.
3) The actions access the same object (read or write).
The schedules S1 and S2 are said to be conflict-equivalent if the following conditions are satisfied:
1) Both schedules S1 and S2 involve the same set of transactions (including ordering of actions within each transaction).
2) The order of each pair of conflicting actions in S1 and S2 are the same.
A schedule is said to be conflict-serializable when the schedule is conflict-equivalent to one or more serial schedules.
Source: Wiki Page for Schedule
In the given scenario, there are two possible serial schedules:
1) T1 followed by T2
2) T2 followed by T1.
In both of the serial schedules, one of the transactions reads the value written by other transaction as a first step. Therefore, any non-serial interleaving of T1 and T2 will not be conflict serializable.
2) Consider the following relations A, B, C. How many tuples does the result of the following relational algebra expression contain? Assume that the schema of A U B is the same as that of A.
Table A Id Name Age ---------------- 12 Arun 60 15 Shreya 24 99 Rohit 11 Table B Id Name Age ---------------- 15 Shreya 24 25 Hari 40 98 Rohit 20 99 Rohit 11 Table C Id Phone Area ----------------- 10 2200 02 99 2100 01
Result of AUB will be following table Id Name Age ---------------- 12 Arun 60 15 Shreya 24 99 Rohit 11 25 Hari 40 98 Rohit 20 The result of given relational algebra expression will be Id Name Age Id Phone Area --------------------------------- 12 Arun 60 10 2200 02 15 Shreya 24 10 2200 02 99 Rohit 11 10 2200 02 25 Hari 40 10 2200 02 98 Rohit 20 10 2200 02 99 Rohit 11 99 2100 01 98 Rohit 20 99 2100 01
3) Consider the above tables A, B and C. How many tuples does the result of the following SQL query contains?
SELECT A.id FROM A WHERE A.age > ALL (SELECT B.age FROM B WHERE B. name = "arun")
The meaning of “ALL” is the A.Age should be greater than all the values returned by the subquery. There is no entry with name “arun” in table B. So the subquery will return NULL. If a subquery returns NULL, then the condition becomes true for all rows of A (See this for details). So all rows of table A are selected.
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