how can I filter the list for address > "country" field in "new individual"
Welcome to CiviCRM SE, Could you please tell why you want to add 'is_active' field to country table? Can't you just disable the countries from appearing in drop down(or include only countries that you need in your system) from CiviCRM &gt;&gt; Administer &gt;&gt; Localization &gt;&gt; Languages, Currency, Locations?