© Copyright, 2002 Howard F.
Arner, Jr. All Rights Reserved. Unauthorized duplication prohibited.
What selectivity means
•Files have records. Indexes have Keys. The selectivity of a key
indicates how many records will be returned, (on average),
for one instance of a key
•Selectivity can be skewed, for
example a status field might have only four entries,
but a I status only occurs in one out of 10,000 records
•Understanding the
selectivity of an index helps in planning queries