On a SaaS ticket board support constantly runs «open tickets sorted by priority descending, first 50». A simple index on status covers the filter but not the sort — Postgres still sorts the result separately. A composite index over two columns at once — status plus priority descending — covers both the filter and the sort in one pass. Build a tickets table with id (primary key), status (string up to 20 chars), priority (integer), and created_at (timestamp with timezone), then create an index named tickets_status_priority_idx where status is the first column and priority is the second in descending order.