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Unformatted text preview: In that case, the join with ASG would not return an empty set. Therefore, that subtree remains. Because we have only one subtree left, the union operation is also eliminated. The reduced subtree is: Π budget σ PNO=”P4” r PNO PROJ2 ASG The corresponding query is: SELECT BUDGET FROM PROJ2, ASG WHERE PROJ2.PNO = ASG.PNO AND ASG.PNO = ''P4'' Based on the Figure 2.4, the Budget attribute is in the project relation, and the query doesn’t select any attributes from the ASG relation. According to Ozsu and Valduriez (1999) definition, we have a useless join that needs to be eliminated. The resulting query tree is: Π budget σ PNO=”P4” PROJ2 The corresponding query is: SELECT BUDGET FROM PROJ2 WHERE PNO=”P4” References Ozsu, M.T. & Valduriez, P. (1999). Principles of distributed database systems . New Jersey: Prentice Hall....
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- Spring '09
- fragmentation, fragmentation rule