FIX: improve performance of post alerter job (discourse#22378)

Recently, SQL query returning users who have muted category or tag were introduced, and it is causing performance issues.

It is much more effective to first get IDs of users who have CategoryUser/TagUsers related to specific topic and then in second query get relevant users.

app/services/post_alerter.rb

@@ -270,15 +270,12 @@ def category_watchers(topic)
   end
 
   def category_or_tag_muters(topic)
-    User
-      .joins(
-        "LEFT JOIN category_users ON users.id = category_users.user_id AND category_users.category_id = #{topic.category_id.to_i} AND category_users.notification_level = #{CategoryUser.notification_levels[:muted].to_i}",
-      )
-      .joins("LEFT JOIN topic_tags ON topic_tags.topic_id = #{topic.id.to_i}")
-      .joins(
-        "LEFT JOIN tag_users ON users.id = tag_users.user_id AND tag_users.tag_id = topic_tags.tag_id AND tag_users.notification_level = #{TagUser.notification_levels[:muted].to_i}",
-      )
-      .where("category_users.id IS NOT NULL OR tag_users.id IS NOT NULL")
+    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]}
+    SQL
+    User.where("id IN (#{user_ids_sql})")
   end
 
   def notify_first_post_watchers(post, user_ids, notified = nil)