title: How to make MySQL field unique without creating B+Tree INDEX
body: <p>As I know, MySQL creates indexes automatically, if you trying to add unique_key to some field.</p>
  
  <p>But what should I do, if I need to add, for example, unique_key for VARCHAR(256) field type.
  Doing this in usually way, MySQL create an index by default. </p>
  
  <p>Should take into account that MySQL works too bad with looooong VARCHAR indexes.
  What should I do to create unique_key without index?</p>