Indexes are built on key fields, so yes. When a field is identified as a 
key it is indexed.

To answer your second question, it is the index which maintains the 
uniqueness of a field.

A book is a really good analogy. If you wanted to look up the references to 
"string", you can do it very quickly in the index. Even if you don't come 
close to the "s" section, you immediately know whether to look next to the 
right or to the left. If you push the issue, and act extremely dumb (like a 
computer), you'll find the page containing "string" within 7 hits; once 
you're on that page it's a short read to find the term. That's what makes 
searching on indexed fields so fast.

To find the term in the book requires you to start reading on page 1 and to 
read text until you find the term. Which why searches which do not take 
advantage of key fields are so slow.

Indexes are your friend, they don't slow down the addition of records that 
much, and make retrieval of records extremely fast.

Regards - Miles Thompson

At 02:13 PM 5/2/2002 +0200, Christoph Starkmann wrote:
>Hi there!
>I guess I got a very easy question for the pros here...
>I've been searching the documentaion of mySQL, but didn't
>find the answer (even though I'm sure it's out there
>somewhere :))...
>Is a primary key in mySQL automatically indexed? And is a
>unique field indexed automatically ?(I don't think and
>don't hope so, but one never knows)
>It's not a bug, it's a feature.
>christoph starkmann
>ICQ: 100601600
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