On Jul 24, 9:01 am, aark...@[EMAIL PROTECTED]
wrote:
> 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 ?
As with all performance questions the answer is "it depends".
That is why there is EXPLAIN PLAN and other performance measures.
Here's the simple counter example for the 20% "rule"
SELECT * FROM sometable WHERE ColumnWith20Percent IS NULL;
since indices do not track NULLs.
You best bet is to remember that this is not a hard rule, but a
general guideline. The paragraph you quoted (or at least paraphrased)
hints at this since it says: "you may just want to try both methods".
to quote the raven: It's a guideline, nothing more.
HTH,
ed
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