Hi all,
I recently read the following in a book(oracle 9i for dummies by
Carol McCullough Dieter)
The fastest way to retrieve rows from a table is to access the row
with exact row id. An
index is the second fastest way, but it decreases in performance as
the pro****tion of the rows retrieved increases. if you are retrieving
approximately 20 % of the rows in a table, using a index is just as
fast. But beyond that magic 20 %, not using 20% is faster.
keep this rule in mind when you create indexes intended to help speed
up a query.
Queries vary in the rows that they select from a table. if you have a
query that you use often,
determine the number of rows that it selects from the table. if this
number is more than 20%
of the total no: of rows in the table , an index on the table may not
improve the performance
of the query. you may just want to try both methods. if the number of
rows is less than 20%, an index will almost certainly help
performance.
my question is to what extent this rule is true ?
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