SELECT count(*) 
FROM contacts_lists 
     JOIN plain_contacts 
          ON contacts_lists.contact_id = plain_contacts.contact_id 
     JOIN contacts 
          ON contacts.id = plain_contacts.contact_id 
WHERE plain_contacts.has_email 
      AND NOT contacts.email_bad 
      AND NOT contacts.email_unsub 
      AND contacts_lists.list_id =67339

¿Cómo puedo optimizar esta consulta? ¿Podría explicarme ...