ics321-20091119-fdnf3 - ICS 321 Fall 2009 Schema Refinement...

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ICS 321 Fall 2009 Schema Refinement & Normal Forms (iii) Asst. Prof. Lipyeow Lim Information & Computer Science Department University of Hawaii at Manoa 11/19/2009 1 Lipyeow Lim -- University of Hawaii at Manoa
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Decompositions Reduces redundancies and anomalies, but could have the following potential problems: Some queries become more expensive. Given instances of the decomposed relations, we may not be able to reconstruct the corresponding instance of the original relation! Checking some dependencies may require joining the instances of the decomposed relations. Two desirable properties: Lossless-join decomposition Dependency-preserving decomposition 11/19/2009 Lipyeow Lim -- University of Hawaii at Manoa 2
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Lossless-join Decomposition Decomposition of R into X and Y is lossless-join w.r.t. a set of FDs F if, for every instance r that satisfies F: π X (r) join π Y (r) = r In general one direction π X (r) join π Y (r) r is always true, but the other may not hold. Definition extended to decomposition into 3 or more relations in a straightforward way. It is essential that all decompositions used to deal with redundancy be lossless! (Avoids Problem (2).) 11/19/2009 Lipyeow Lim -- University of Hawaii at Manoa 3
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Conditions for Lossless Join The decomposition of R into X and Y is lossless- join wrt F if and only if the closure of F contains: X Y X, or X Y Y In particular, the decomposition of R into UV and R - V is lossless- join if U V holds over R. 11/19/2009 Lipyeow Lim -- University of Hawaii at Manoa 4 A B C 1 2 3 4 5 6 7 2 8 A B 1 2 4 5 7 2 B C 2 3 5 6 2 8 A B C 1 2 3 4 5 6 7 2 8 1 2 8 7 2 3
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Dependency-preserving Decomposition
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