Size Estimation and Reduction Factors
•Consider a
query block:
•Maximum # tuples in
result is the product of the cardinalities of relations in the FROM clause.
•Reduction factor (RF)
associated with each term reflects the impact of the term in reducing result size. Result cardinality = Max # tuples *
product of all RF’s.
–Implicit assumption that terms are independent!
–Term col=value has
RF 1/NKeys(I), given index I on col
–Term col1=col2 has
RF 1/MAX(NKeys(I1), NKeys(I2))
–Term col>value has
RF (High(I)-value)/(High(I)-Low(I))
SELECT
attribute list
FROM
relation list
WHERE term1 AND ... AND termk