Re: The 20% rule

From: Gints Plivna <>
Date: Thu, 24 Jul 2008 07:08:57 -0700 (PDT)
Message-ID: <>

On 24 Jūl., 17:00, 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 proportion 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 ?

See this artcile fom Hotsos to understand why this rule simply cannot be measured using %. Simply cannot.

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Gints Plivna Received on Thu Jul 24 2008 - 09:08:57 CDT

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