Optimizing table (shall I create a primary field?)

[Charles Lambach]

Hi.

My hosting provider recommended me to optimize my 200,000 record table in
order to save resources.

I do _always_ this query:
SELECT * FROM books WHERE isbn='foo' LIMIT 1

The primary key of this table was 'id', and 'isbn' was and INDEX field.

I've modified this:
ALTER TABLE books DROP PRIMARY KEY, ADD INDEX ('isbn')
ALTER TABLE books ADD PRIMARY KEY ('isbn')

Is this a good change? Am I going to waste less resources with 'isbn' field
as primary key?

Thank you very much.