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Estimating the result set size of γ_L(R)

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• Range of the result set of γ_L(R)
  • Range:

    1 ≤ T( γL(R) ) ≤ T(R) ^ ^ | | all tuples all tuples have same have diff attr value attr values for attributes L

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• Recommended estimate for γ_L(R)
  • Recommended:

    If the database maintains statistics on the attribute values: T( γL(R) ) = V(R,L) If no statistics available: T( γL(R) ) = (1/2)n(L) × T(R) n(L) = number of attributes in the attribute list L