FIX: more performance improvement for PostAlert job (#22487)

Simplified query based on SiteSettings to join only relevant user_options rows.
In addition, index was added to 'watched_precedence_over_muted` column in `user_options` table to speed up query

app/models/category_user.rb

@@ -289,7 +289,8 @@ end
 #
 # Indexes
 #
-#  idx_category_users_category_id_user_id            (category_id,user_id) UNIQUE
-#  idx_category_users_user_id_category_id            (user_id,category_id) UNIQUE
-#  index_category_users_on_user_id_and_last_seen_at  (user_id,last_seen_at)
+#  idx_category_users_category_id_user_id                      (category_id,user_id) UNIQUE
+#  idx_category_users_user_id_category_id                      (user_id,category_id) UNIQUE
+#  index_category_users_on_category_id_and_notification_level  (category_id,notification_level)
+#  index_category_users_on_user_id_and_last_seen_at            (user_id,last_seen_at)
 #

app/models/topic_user.rb

@@ -582,6 +582,7 @@ end
 #
 # Indexes
 #
-#  index_topic_users_on_topic_id_and_user_id  (topic_id,user_id) UNIQUE
-#  index_topic_users_on_user_id_and_topic_id  (user_id,topic_id) UNIQUE
+#  index_topic_users_on_topic_id_and_notification_level  (topic_id,notification_level)
+#  index_topic_users_on_topic_id_and_user_id             (topic_id,user_id) UNIQUE
+#  index_topic_users_on_user_id_and_topic_id             (user_id,topic_id) UNIQUE
 #

app/models/user_option.rb

@@ -295,6 +295,7 @@ end
 #
 # Indexes
 #
-#  index_user_options_on_user_id                       (user_id) UNIQUE
-#  index_user_options_on_user_id_and_default_calendar  (user_id,default_calendar)
+#  index_user_options_on_user_id                        (user_id) UNIQUE
+#  index_user_options_on_user_id_and_default_calendar   (user_id,default_calendar)
+#  index_user_options_on_watched_precedence_over_muted  (watched_precedence_over_muted)
 #

app/services/post_alerter.rb

@@ -268,19 +268,26 @@ class PostAlerter
   end
 
   def category_or_tag_muters(topic)
+    user_option_condition_sql_fragment =
+      if SiteSetting.watched_precedence_over_muted
+        "uo.watched_precedence_over_muted IS false"
+      else
+        "(uo.watched_precedence_over_muted IS NULL OR uo.watched_precedence_over_muted IS false)"
+      end
+
     user_ids_sql = <<~SQL
-      SELECT user_id FROM category_users WHERE category_id = #{topic.category_id.to_i} AND notification_level = #{CategoryUser.notification_levels[:muted]}
-      UNION
-      SELECT user_id FROM tag_users tu JOIN topic_tags tt ON tt.tag_id = tu.tag_id AND tt.topic_id = #{topic.id} AND tu.notification_level = #{TagUser.notification_levels[:muted]}
-      EXCEPT
-      SELECT user_id FROM topic_users tus WHERE tus.topic_id = #{topic.id} AND tus.notification_level = #{TopicUser.notification_levels[:watching]}
-    SQL
-    User
-      .where("id IN (#{user_ids_sql})")
-      .joins("LEFT JOIN user_options ON user_options.user_id = users.id")
-      .where(
-        "user_options.watched_precedence_over_muted IS false OR (user_options.watched_precedence_over_muted IS NULL AND #{!SiteSetting.watched_precedence_over_muted})",
-      )
+        SELECT uo.user_id FROM user_options uo
+        LEFT JOIN topic_users tus ON tus.user_id = uo.user_id AND tus.topic_id = #{topic.id}
+        LEFT JOIN category_users cu ON cu.user_id = uo.user_id AND cu.category_id = #{topic.category_id.to_i}
+        LEFT JOIN tag_users tu ON tu.user_id = uo.user_id
+        JOIN topic_tags tt ON tt.tag_id = tu.tag_id AND tt.topic_id = #{topic.id}
+        WHERE
+          (tus.id IS NULL OR tus.notification_level != #{TopicUser.notification_levels[:watching]})
+          AND (cu.notification_level = #{CategoryUser.notification_levels[:muted]} OR tu.notification_level = #{TagUser.notification_levels[:muted]})
+          AND #{user_option_condition_sql_fragment}
+        SQL
+
+    User.where("id IN (#{user_ids_sql})")
   end
 
   def notify_first_post_watchers(post, user_ids, notified = nil)

db/migrate/20230707031122_add_watched_precedence_over_muted_index_to_user_options.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddWatchedPrecedenceOverMutedIndexToUserOptions < ActiveRecord::Migration[7.0]
+  def change
+    add_index :user_options, :watched_precedence_over_muted
+  end
+end

db/migrate/20230712011946_add_topic_id_notification_level_index_to_topic_users.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+#
+class AddTopicIdNotificationLevelIndexToTopicUsers < ActiveRecord::Migration[7.0]
+  def change
+    add_index :topic_users, %i[topic_id notification_level]
+  end
+end

db/migrate/20230712013248_add_category_id_notification_level_index_to_category_users.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddCategoryIdNotificationLevelIndexToCategoryUsers < ActiveRecord::Migration[7.0]
+  def change
+    add_index :category_users, %i[category_id notification_level]
+  end
+end