Chapter 6: Query Decomposition and Data Localization
Chapter 6: Query Decomposition and Data Localization
Chapter 6: Query Decomposition and Data Localization
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<strong>Data</strong> <strong>Localization</strong>s Issues – Reduction for VF<br />
• Reduction for Vertical Fragmentation<br />
– Recall, VF distributes a relation based on projection, <strong>and</strong> the reconstruction operator<br />
is the join.<br />
– Similar to HF, it is possible to identify useless intermediate relations, i.e., fragments<br />
that do not contribute to the result.<br />
– Assume a relation R(A) with A = {A 1 , . . . , A n }, which is vertically fragmented as<br />
R i = π A<br />
′<br />
i<br />
(R), where A ′ i ⊆ A.<br />
– Rule 3: π D,K (R i ) is useless if the set of projection attributes D is not in A ′ i <strong>and</strong> K<br />
is the key attribute.<br />
– Note that the result is not empty, but it is useless, as it contains only the key attribute.<br />
DDB 2008/09 J. Gamper Page 24
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