Re: index usage

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.

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 same logic applies to table access via an index. So in fact using an index might increase the work instead of cutting down. And the threshold where using the index is increases the work depends on many factors, that's why those numbers when an index makes sense vary so much.

HTH
Holger Received on Wed Jan 19 2005 - 10:40:10 CST