Re: index usage

"Holger Baer" <holger.baer_at_science-computing.de> wrote in message news:csm2hc$ssu$1_at_news.BelWue.DE...
> hastenthunder wrote:
>> Hello,
>>
>> I've read many documentations online stating to only create an index if
>> queries against this table frequently retrieve less than 15% of the rows.
>> However, if the query returns, say, 40% of the rows, wouldn't indexing
>> the
>> column still help by cutting the work by roughly half?
>>
>>
>> hastenthunder
>>
>>
>
> The number actually varies (down to 2% in some cases) and, as they say,
> depends
> on many factors.

Hi Holger,

And much lower than 2% in other cases ...

>
> But just think about it:
> If you're going to read about half a book - would you really go through
> the index
> or would you rather just leafe through the book and skip those pages that
> don't
> interest you?

The point that some people find hard "to picture" is that to read half a book, you might need to actually access *every* page. Why, because you might only be interested on average with the information on 1/2 of every page. 1/2 a book doesn't necessarily mean 1/2 the pages but 1/2 of every page.

With Oracle "pages", this is more likely the higher the selectivity and the worse the CF of the table in relation to the required data set.

Cheers

Richard Received on Thu Jan 20 2005 - 05:58:10 CST