Give an example of an instance of the Transcript relation and a hash function on the attribute sequence StudId, Grade that sends two identical tuples in ? StudId,Semester(Transcript) into di?erent hash buckets. (This shows that such a hash-based access path cannot be used to compute the projection.)
What will be an ideal response?
Consider the following two tuples: 111111111, EE101, F1997, A and 111111111,MAT123,F1997,B. Since these tuples have identical projections on the attributes StudId,Semester, they become one in the projected relation ?StudId,Semester(Transcript).
Consider nowthe following hash function on StudId,Grade:
f (t )=(t .StudId +(t .Grade ? 'A')) mod 1000
The ?rst tuple will then be sent to the bucket 111111111 mod 1000 = 111 and the second to 111111112 mod 1000 = 112.
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