Query Processing and Optimization

Some Transformation Rules
• Cascade of σ: σ c1 ∧ c 2 ∧ ... ∧ c n
(R) = σ c1 (σ c2 (...(σ cn (R))...))
• Commutativity of σ: σ c1 (σ c2 (R)) = σ c2 (σ c1 (R))
• Commuting σ with π: π L (σ c (R)) = σ c (π L (R))
• Only if c involves solely attributes in L.
• Commuting σ with n : σ c (R
n S) = σ c (R)
n S
• Only if c involves solely attributes in R.
• Commuting σ with set operations: σ c (R θ S) = σ c (R) θ σ c (S)
• Where θ is one of ∪, ∩, or -.
• Commutativity of ∪, ∩, and n: R θ S = S θ R
• Where θ is one of ∪, ∩ and n.
• Associativity of n, ∪, ∩: (R θ S) θ T = R θ (S θ T)
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