More performance optimization

* Added 2 additional indexes
* Added the viewing window as an additional and condition so the query
  optimizer is able to push it into the subqueries, greatly speeding up
  new topic notifications on search pages.

Author: dutow

app/controllers/topics_controller.rb

@@ -221,17 +221,18 @@ def assign_branch_segments!
   def apply_cursor_pagination(base_query)
     @viewing_since = viewing_since_param
 
-    query_with_window = base_query.joins(:messages)
-                                   .group('topics.id')
-                                   .having('MAX(messages.created_at) <= ?', @viewing_since)
-                                   .select('topics.*, MAX(messages.created_at) as last_activity')
+    windowed_query = base_query.joins(:messages)
+                               .where(messages: { created_at: ..@viewing_since })
+                               .group('topics.id')
+                               .having('MAX(messages.created_at) <= ?', @viewing_since)
+                               .select('topics.*, MAX(messages.created_at) as last_activity')
 
     if params[:cursor].present?
       cursor_time, cursor_id = params[:cursor].split('_')
-      @topics = query_with_window.having('(MAX(messages.created_at), topics.id) < (?, ?)',
+      @topics = windowed_query.having('(MAX(messages.created_at), topics.id) < (?, ?)',
                                           Time.zone.parse(cursor_time), cursor_id.to_i)
     else
-      @topics = query_with_window
+      @topics = windowed_query
     end
 
     @topics = @topics.order('MAX(messages.created_at) DESC, topics.id DESC')
@@ -573,6 +574,7 @@ def viewing_since_param
 
   def count_new_topics(base_query, viewing_since)
     base_query.joins(:messages)
+              .where(messages: { created_at: viewing_since.. })
               .group('topics.id')
               .having('MAX(messages.created_at) > ?', viewing_since)
               .count

db/migrate/20251215151000_add_message_topic_created_at_index.rb

@@ -0,0 +1,7 @@
+class AddMessageTopicCreatedAtIndex < ActiveRecord::Migration[8.0]
+  def change
+    add_index :messages, [:topic_id, :created_at, :id],
+              order: { created_at: :desc, id: :desc },
+              name: "index_messages_on_topic_id_and_created_at_desc_id_desc"
+  end
+end

db/migrate/20251215151100_add_message_read_ranges_recency_index.rb

@@ -0,0 +1,7 @@
+class AddMessageReadRangesRecencyIndex < ActiveRecord::Migration[8.0]
+  def change
+    add_index :message_read_ranges, [:user_id, :topic_id, :range_end_message_id],
+              order: { range_end_message_id: :desc },
+              name: "index_message_read_ranges_on_user_topic_range_end_desc"
+  end
+end