PERF: Cache user badge count in user_stats table (#8610)

This means that we no longer need to run a `count()` on the user_badges table when serializing a user

app/models/badge.rb

@@ -114,6 +114,7 @@ class Badge < ActiveRecord::Base
 
   after_commit do
     SvgSprite.expire_cache
+    UserStat.update_distinct_badge_count if saved_change_to_enabled?
   end
 
   # fields that can not be edited on system badges

app/models/user.rb

@@ -945,7 +945,7 @@ class User < ActiveRecord::Base
   end
 
   def badge_count
-    user_badges.select('distinct badge_id').count
+    user_stat&.distinct_badge_count
   end
 
   def featured_user_badges(limit = 3)

app/models/user_badge.rb

@@ -18,11 +18,13 @@ class UserBadge < ActiveRecord::Base
 
   after_create do
     Badge.increment_counter 'grant_count', self.badge_id
+    UserStat.update_distinct_badge_count self.user_id
     DiscourseEvent.trigger(:user_badge_granted, self.badge_id, self.user_id)
   end
 
   after_destroy do
     Badge.decrement_counter 'grant_count', self.badge_id
+    UserStat.update_distinct_badge_count self.user_id
     DiscourseEvent.trigger(:user_badge_removed, self.badge_id, self.user_id)
   end
 

app/models/user_stat.rb

@@ -6,6 +6,7 @@ class UserStat < ActiveRecord::Base
 
   def self.ensure_consistency!(last_seen = 1.hour.ago)
     reset_bounce_scores
+    update_distinct_badge_count
     update_view_counts(last_seen)
     update_first_unread(last_seen)
   end
@@ -126,6 +127,29 @@ class UserStat < ActiveRecord::Base
     SQL
   end
 
+  def self.update_distinct_badge_count(user_id = nil)
+    sql = <<~SQL
+      UPDATE user_stats
+      SET distinct_badge_count = x.distinct_badge_count
+      FROM (
+        SELECT users.id user_id, COUNT(distinct user_badges.badge_id) distinct_badge_count
+        FROM users
+        LEFT JOIN user_badges ON user_badges.user_id = users.id
+                              AND (user_badges.badge_id IN (SELECT id FROM badges WHERE enabled))
+        GROUP BY users.id
+      ) x
+      WHERE user_stats.user_id = x.user_id AND user_stats.distinct_badge_count <> x.distinct_badge_count
+    SQL
+
+    sql = sql + " AND user_stats.user_id = #{user_id.to_i}" if user_id
+
+    DB.exec sql
+  end
+
+  def update_distinct_badge_count
+    self.class.update_distinct_badge_count(self.user_id)
+  end
+
   # topic_reply_count is a count of posts in other users' topics
   def update_topic_reply_count
     self.topic_reply_count = Topic