Relation: x=(d,b,c,f,g,h) Functional dependencies: d→bc,g→hd Decomposed into: a=(d,b,c)Consider relation R(A,B,C,D,E,F,G)

Consider relation R(A,B,C,D,E,F,G) with functional dependencies F = {AD→BF, CD→EGC, BD→F, E→D, F→C, D→F}. After finding the minimal cover, R is decomposed into R1(A,B,C,D,E) and R2(A,D,F,G). Determine whether this decomposition is lossless and dependency preserving.

  1. A.

    Decomposition is lossless and dependency preserving

  2. B.

    Decomposition is lossy and dependency preserving

  3. C.

    Decomposition is lossy and not dependency preserving

  4. D.

    Decomposition is lossless and not dependency preserving

Attempted by 24 students.

Show answer & explanation

Correct answer: D

First reduce the dependencies to a minimal-cover form. Splitting RHS attributes and removing redundant parts gives the useful dependencies:
AD→B, CD→E, CD→G, E→D, F→C, D→F.

Lossless join check:
The decomposed relations are R1(A,B,C,D,E) and R2(A,D,F,G).
Their common attributes are {A,D}.

Compute {A,D}+ using the dependencies:
AD→B gives B.
D→F gives F.
F→C gives C.
Now C and D are available, so CD→E gives E and CD→G gives G.
Thus {A,D}+ = {A,B,C,D,E,F,G}.

Since the common attributes {A,D} determine all attributes, the decomposition is lossless.

Dependency preservation check:
Some dependencies are preserved in the projections, but CD→G is not. C and D are in R1, while G is in R2, and the projected dependencies cannot derive G from CD without joining the relations. Therefore the decomposition is not dependency preserving.

Hence, the decomposition is lossless and not dependency preserving. Option D is correct.

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