Permalink change each post update in wordpress

I want to have a custom permalink for each new post in WordPress like: http://mysite.com/x5Kvy6.

function wp_unique_post_slug($col,$table='wp_posts'){
     global $wpdb;

     $alphabet = array_merge( range(0, 9), range('a','z') );

     $already_exists = true;
     do {

         $guidchr = array();
         for ($i=0; $i<32; $i++)
         $guidchr[] = $alphabet[array_rand( $alphabet )];


         $guid = sprintf( "%s", implode("", array_slice($guidchr, 0, 12, true)) );

       // check that GUID is unique
       $already_exists = (boolean) $wpdb->get_var("
       SELECT COUNT($col) as the_amount FROM $table WHERE $col = '$guid'
       ");

      } while (true == $already_exists);

     return $guid;
}

This script works well when i replace the fontion in post.php (wordpress core) but unfortunately the permalink change at each post uptade.
How to avoid this? And how to edit a custom optional keyword (http://mysite.com/keyword).

Any idea is welcome!