title: MySQL 5.0 indexes - Unique vs Non Unique
body: <p>What is the difference between MySQL unique and non-unique index in terms of performance?   </p>
  
  <p>Let us say I want to make an index on a combo of 2 columns, and the combination is unique, but I create a non-unique index. Will that have any significant effect on the performance or the memory MySQL uses?</p>
  
  <p>Same question, is there is difference between <em>primary</em> key and <em>unique</em> index?</p>