Simple Selections
•Of the form
•With no index, unsorted: Must
essentially scan the whole relation; cost is M (#pages
in R).
•With an index on
selection attribute: Use index to find qualifying data entries, then retrieve corresponding data
records. (Hash index useful only for equality selections.)
•Result size estimation:
(Size of R) * reduction
factor.
More on this
later.
SELECT *
FROM Person R
WHERE
R.phone < ‘543%’
