Migrations: Make user_id index on notifications better for query.

priv/repo/migrations/20200527104138_change_notification_user_index.exs

@@ -0,0 +1,8 @@
+defmodule Pleroma.Repo.Migrations.ChangeNotificationUserIndex do
+  use Ecto.Migration
+
+  def change do
+    drop_if_exists(index(:notifications, [:user_id]))
+    create_if_not_exists(index(:notifications, [:user_id, "id desc nulls last"]))
+  end
+end