Find the messages a refresh replaces through an index

Every room visit and every reconnection asks for the messages updated
since the page was rendered. That query filtered the room by updated_at
and sorted it by created_at, so SQLite walked every message in the room:
about 220-360 ms in a room of a million messages. An index on
(room_id, updated_at) finds the few updated messages directly.

Sorting on +created_at keeps SQLite on that index once messages are also
indexed by (room_id, created_at): with both, the planner otherwise walks
the room in creation order looking for updated rows.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JVFo3Lt9T8M5NR7KxVvsZ2

app/models/message/pagination.rb

@@ -36,7 +36,10 @@ module Message::Pagination
     scope :page_after, ->(message) { after(message).first_page }
 
     scope :page_created_since, ->(time) { where("created_at > ?", time).first_page }
-    scope :page_updated_since, ->(time) { where("updated_at > ?", time).last_page }
+    # Sorting on +created_at, which no index covers, has SQLite find the few messages updated since
+    # through (room_id, updated_at) and sort just those, instead of walking the whole room by
+    # (room_id, created_at) looking for them.
+    scope :page_updated_since, ->(time) { Page.load(where("updated_at > ?", time).reorder(Arel.sql("+messages.created_at")), :last, PAGE_SIZE) }
   end
 
   class_methods do

db/migrate/20261005022000_add_room_id_and_updated_at_index_to_messages.rb

@@ -0,0 +1,5 @@
+class AddRoomIdAndUpdatedAtIndexToMessages < ActiveRecord::Migration[8.2]
+  def change
+    add_index :messages, %i[ room_id updated_at ]
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.2].define(version: 2025_12_12_154340) do
+ActiveRecord::Schema[8.2].define(version: 2026_10_05_022000) do
   create_table "accounts", force: :cascade do |t|
     t.datetime "created_at", null: false
     t.text "custom_styles"
@@ -101,6 +101,7 @@ ActiveRecord::Schema[8.2].define(version: 2025_12_12_154340) do
     t.integer "room_id", null: false
     t.datetime "updated_at", null: false
     t.index ["creator_id"], name: "index_messages_on_creator_id"
+    t.index ["room_id", "updated_at"], name: "index_messages_on_room_id_and_updated_at"
     t.index ["room_id"], name: "index_messages_on_room_id"
   end
 

test/models/message/pagination_test.rb

@@ -0,0 +1,31 @@
+require "test_helper"
+
+class Message::PaginationTest < ActiveSupport::TestCase
+  test "messages updated since a time are found through the room and update time index" do
+    assert_match(/USING INDEX index_messages_on_room_id_and_updated_at \(room_id=\? AND updated_at>\?\)/, plan_of_page_updated_since)
+  end
+
+  test "messages updated since a time are the newest page of them, in creation order" do
+    room = rooms(:watercooler)
+    first, second, third = room.messages.ordered.first(3)
+
+    travel 1.minute do
+      [ third, first, second ].each(&:touch)
+
+      stub_const(Message::Pagination, :PAGE_SIZE, 2) do
+        assert_equal [ second, third ], room.messages.page_updated_since(30.seconds.ago).to_a
+      end
+    end
+  end
+
+  private
+    def plan_of_page_updated_since
+      statements = []
+      callback = ->(*, payload) { statements << payload[:sql] if payload[:sql].start_with?(%(SELECT "messages")) }
+      ActiveSupport::Notifications.subscribed(callback, "sql.active_record") do
+        rooms(:watercooler).messages.page_updated_since(1.minute.ago)
+      end
+
+      Message.connection.select_rows("EXPLAIN QUERY PLAN #{statements.sole}").map(&:last).join(" | ")
+    end
+end