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Estimating Selection Result Size (cont)

Estimating size of result for e.g. 

select * from Enrolment where year > 2005;

Could estimate by using:

  • uniform distribution assumption,   r,   min/max years
Assume: min(year)=2000, max(year)=2009, |Enrolment|=105
  • 105 from 2000-2009 means approx 10000 enrolments/year
  • this suggests 40000 enrolments since 2006
Heuristic used by some systems:   | σA>c(R) | ≅ r/3