checkin - around 13m records
users - around 250k records
friends - around 1.5m records

SELECT checkin_id, created_at
FROM checkin
WHERE (user_id IN (SELECT friend_id from friends where user_id = 1 and is_approved = 1) OR user_id = 1)
ORDER by created_at DESC
LIMIT 0, 15

id     select_type     table   type    possible_keys   key     key_len     ref     rows    Extra
 1  PRIMARY     checkin     index   user_id,user_id_2   created_at  8   NULL    15  Using where
 2  DEPENDENT SUBQUERY friends  eq_ref    user_id,friend_id,is_approved,friend_looku...     PRIMARY     8   const,func  1   Using where

SELECT c.checkin_id, c.created_at
FROM checkin c
INNER JOIN friends f ON c.user_id = f.friend_id
WHERE f.user_id =1
AND f.is_approved =1
ORDER BY c.created_at DESC
LIMIT 0 , 15

id     select_type     table   type    possible_keys   key     key_len     ref     rows    Extra
 1  SIMPLE  f   ref     PRIMARY,user_id,friend_id,is_approved,friend_looku...   friend_lookup   5   const,const     938     Using temporary; Using filesort
 1  SIMPLE  c   ref     user_id,user_id_2   user_id     4   untappd_prod.f.friend_id    71  Using where

id     select_type     table   type    possible_keys   key     key_len     ref     rows    Extra
 1  SIMPLE  f   index_merge     PRIMARY,user_id,friend_id,is_approved,friend_looku...    user_id,friend_lookup  4,5     NULL    11  Using intersect(user_id,friend_lookup); Using wher...
 1  SIMPLE  c   ref     user_id,user_id_2   user_id     4   untappd_prod.f.friend_id    71  Using where

SELECT c.checkin_id, c.created_at
FROM checkin c
INNER JOIN friends f
   ON c.user_id = f.friend_id
WHERE f.user_id = 1
   AND f.is_approved = 1
ORDER by c.created_at DESC
LIMIT 0, 15