PERF: Keep track of first unread PM and first unread group PM for user.

This optimization helps to filter away topics so that the joins on
related tables when querying for unread messages is not expensive.

spec/models/user_stat_spec.rb

@@ -88,6 +88,46 @@ describe UserStat do
       post.user.user_stat.reload
       expect(post.user.user_stat.first_unread_at).to eq_time(Time.zone.now)
     end
+
+    it 'updates first unread pm timestamp correctly' do
+      pm_topic = Fabricate(:private_message_topic)
+      user = pm_topic.user
+      user.update!(last_seen_at: Time.zone.now)
+      create_post(user: user, topic_id: pm_topic.id)
+
+      TopicUser.change(user.id, pm_topic.id,
+        notification_level: TopicUser.notification_levels[:tracking]
+      )
+
+      # user that is not tracking PM topic
+      user_2 = Fabricate(:user, last_seen_at: Time.zone.now)
+      pm_topic.allowed_users << user_2
+
+      TopicUser.change(user_2.id, pm_topic.id,
+        notification_level: TopicUser.notification_levels[:regular]
+      )
+
+      # User that has not been seen
+      user_3 = Fabricate(:user)
+      pm_topic.allowed_users << user_3
+
+      TopicUser.change(user_3.id, pm_topic.id,
+        notification_level: TopicUser.notification_levels[:tracking]
+      )
+
+      freeze_time 10.minutes.from_now
+
+      post = create_post(
+        user: Fabricate(:admin),
+        topic_id: pm_topic.id
+      )
+
+      UserStat.ensure_consistency!
+
+      expect(user.user_stat.reload.first_unread_pm_at).to eq_time(post.topic.updated_at)
+      expect(user_2.user_stat.reload.first_unread_pm_at).to_not eq_time(post.topic.updated_at)
+      expect(user_3.user_stat.reload.first_unread_pm_at).to_not eq_time(post.topic.updated_at)
+    end
   end
 
   describe 'update_time_read!' do