Index messages by room and creation time

Every page of a room's messages (opening the room, scrolling back, the
refresh after reconnecting, the bot API) selects WHERE room_id = ? ORDER BY
created_at LIMIT 40. With only the room_id index, SQLite reads every
message in the room and sorts them in a temporary B-tree to return 40, so
these requests grow with the room's history.

A composite index on (room_id, created_at) lets SQLite read the 40 rows in
order straight from the index. It covers every lookup the room_id index
served, so that one goes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NuTtwJKhb77Dv7EQqv2C3C

db/migrate/20261005010000_add_room_id_and_created_at_index_to_messages.rb

@@ -0,0 +1,6 @@
+class AddRoomIdAndCreatedAtIndexToMessages < ActiveRecord::Migration[8.2]
+  def change
+    add_index :messages, %i[ room_id created_at ]
+    remove_index :messages, :room_id
+  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_010000) do
   create_table "accounts", force: :cascade do |t|
     t.datetime "created_at", null: false
     t.text "custom_styles"
@@ -101,7 +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"], name: "index_messages_on_room_id"
+    t.index ["room_id", "created_at"], name: "index_messages_on_room_id_and_created_at"
   end
 
   create_table "push_subscriptions", force: :cascade do |t|

test/models/message_test.rb

@@ -65,7 +65,30 @@ class MessageTest < ActiveSupport::TestCase
     assert_equal room.messages.ordered.first(Message::Pagination::PAGE_SIZE).map(&:id), room.messages.first_page.map(&:id)
   end
 
+  test "pages read a room's messages from an index instead of sorting the whole room" do
+    room, message = rooms(:watercooler), messages(:sixth)
+    statements = capture_message_selects do
+      room.messages.last_page
+      room.messages.page_before(message)
+      room.messages.page_after(message)
+      room.messages.page_created_since(1.day.ago)
+    end
+
+    assert_equal 4, statements.size
+    statements.each do |sql|
+      plan = Message.connection.select_rows("EXPLAIN QUERY PLAN #{sql}").map(&:last).join(" | ")
+      assert_no_match(/TEMP B-TREE/, plan, sql)
+    end
+  end
+
   private
+    def capture_message_selects(&block)
+      statements = []
+      callback = ->(*, payload) { statements << payload[:sql] if payload[:sql].start_with?(%(SELECT "messages")) }
+      ActiveSupport::Notifications.subscribed(callback, "sql.active_record", &block)
+      statements
+    end
+
     def create_new_message_in(room)
       room.messages.create!(creator: users(:jason), body: "Hello", client_message_id: "123")
     end