Re: nonunique index on unique values

From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Date: Tue, 28 Aug 2007 09:14:33 +0100
Message-ID: <q5OdnYNXT4fJRE7bnZ2dnUVZ8v6dnZ2d@bt.com>

"ciapecki" <ciapecki_at_gmail.com> wrote in message news:1188285737.311072.16590_at_l22g2000prc.googlegroups.com...
> hi,
>
> is there a big lost in performance if there is a nonunique index on
> the column that holds only unique values (like primary key)?
>
> thanks
> chris
>

The index content will be one byte per row larger, and there are a few cases where the arithmetic the optimizer uses changes if you switch a unique index to non-unique.

Neither is necessarily a problem - but you can always find a few unlucky cases where performance changes dramatically because an execution plan has changed.

-- 
Regards

Jonathan Lewis
http://jonathanlewis.wordpress.com

Author: Cost Based Oracle: Fundamentals
http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html

The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/faq/ind_faq.html

Received on Tue Aug 28 2007 - 03:14:33 CDT