Example
•If we have an Index on rating:
–(1/NKeys(I))
* NTuples(R) = (1/10) * 40000 tuples retrieved.
–Clustered index: (1/NKeys(I)) * (NPages(I)+NPages(R)) = (1/10) * (50+500) pages are retrieved (= 55).
–Unclustered index: (1/NKeys(I)) * (NPages(I)+NTuples(R)) = (1/10) * (50+40000) pages are
retrieved.
•If we have an index on sid:
–Would have to
retrieve all tuples/pages. With a clustered index, the cost is 50+500.
•Doing a file scan: we retrieve all file pages (500).
SELECT S.sid
FROM Sailors S
WHERE
S.rating=8