Простой Оптимизация запросов - MYSQL

Question

Я использую sakila db.

Как улучшить полное сканирование индекса по таблице стран ??

select country.country, count(customer_id) as Clients from customer
 INNER JOIN address on customer.address_id = address.address_id
INNER JOIN city on address.city_id = city.city_id
INNER JOIN country on city.country_id = country.country_id
GROUP BY country.country
order by Clients DESC
limit 5;