OPTIMIZATION: Add index to `post_timings` and adjust the query

app/models/user_stat.rb

@@ -30,10 +30,9 @@ class UserStat < ActiveRecord::Base
               FROM
               (SELECT pt.user_id,
                       COUNT(*) AS c
-               FROM post_timings AS pt
-               WHERE pt.user_id IN (
-                  SELECT u1.id FROM users u1 where u1.last_seen_at > :seen_at
-               )
+               FROM users AS u
+               INNER JOIN post_timings AS pt ON pt.user_id = u.id
+               WHERE u.last_seen_at > :seen_at
                GROUP BY pt.user_id) AS X
                WHERE X.user_id = user_stats.user_id AND
                      X.c <> posts_read_count

db/migrate/20140801170444_create_post_timings_user_index.rb

@@ -0,0 +1,5 @@
+class CreatePostTimingsUserIndex < ActiveRecord::Migration
+  def change
+    add_index :post_timings, :user_id
+  end
+end