Find all user created wordpress custom field keys

Sometime we need to find all user created custom field key names (or meta key) in wordpress. This may be needed to find all posts and pages having a certain custom field key and value for audit/review purpose. Here is code which can be used for this:

  global $wpdb;
  $querystr = "
    SELECT distinct $wpdb->postmeta.meta_key 
    FROM $wpdb->postmeta
    WHERE $wpdb->postmeta.meta_key NOT BETWEEN '_' AND '_z'
    ORDER BY $wpdb->postmeta.meta_key
  ";
  $res_list = $wpdb->get_results($querystr);
  if ($res_list) {
    echo "custom keys:\n";
    foreach ($res_list as $obj) {
      $key = $obj->meta_key;
      echo "$key\n";
    }
  }