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.

app/models/user_stat.rb

@@ -12,10 +12,59 @@ class UserStat < ActiveRecord::Base
     update_distinct_badge_count
     update_view_counts(last_seen)
     update_first_unread(last_seen)
+    update_first_unread_pm(last_seen)
   end
 
-  def self.update_first_unread(last_seen, limit: 10_000)
-    DB.exec(<<~SQL, min_date: last_seen, limit: limit, now: 10.minutes.ago)
+  UPDATE_UNREAD_MINUTES_AGO = 10
+  UPDATE_UNREAD_USERS_LIMIT = 10_000
+
+  def self.update_first_unread_pm(last_seen, limit: UPDATE_UNREAD_USERS_LIMIT)
+    DB.exec(<<~SQL, archetype: Archetype.private_message, now: UPDATE_UNREAD_MINUTES_AGO.minutes.ago, last_seen: last_seen, limit: limit)
+    UPDATE user_stats us
+    SET first_unread_pm_at = COALESCE(Z.min_date, :now)
+    FROM (
+      SELECT
+        Y.user_id,
+        Y.min_date
+      FROM (
+        SELECT
+          u1.id user_id,
+          X.min_date
+        FROM users u1
+        LEFT JOIN (
+          SELECT
+            tau.user_id,
+            MIN(t.updated_at) min_date
+          FROM topic_allowed_users tau
+          INNER JOIN topics t ON t.id = tau.topic_id
+          INNER JOIN users u ON u.id = tau.user_id
+          LEFT JOIN topic_users tu ON t.id = tu.topic_id AND tu.user_id = tau.user_id
+          WHERE t.deleted_at IS NULL
+          AND t.archetype = :archetype
+          AND tu.last_read_post_number < CASE
+                                         WHEN u.admin OR u.moderator
+                                         THEN t.highest_staff_post_number
+                                         ELSE t.highest_post_number
+                                         END
+          AND (COALESCE(tu.notification_level, 1) >= 2)
+          GROUP BY tau.user_id
+        ) AS X ON X.user_id = u1.id
+      ) AS Y
+      WHERE Y.user_id IN (
+        SELECT id
+        FROM users
+        WHERE last_seen_at IS NOT NULL
+        AND last_seen_at > :last_seen
+        ORDER BY last_seen_at DESC
+        LIMIT :limit
+      )
+    ) AS Z
+    WHERE us.user_id = Z.user_id
+    SQL
+  end
+
+  def self.update_first_unread(last_seen, limit: UPDATE_UNREAD_USERS_LIMIT)
+    DB.exec(<<~SQL, min_date: last_seen, limit: limit, now: UPDATE_UNREAD_MINUTES_AGO.minutes.ago)
       UPDATE user_stats us
       SET first_unread_at = COALESCE(Y.min_date, :now)
       FROM (