The N+1 query problem

ORMs load related objects lazily by default, meaning only when you first touch them. That causes the N+1 problem. You fetch 50 orders, then read order.customer.name in a loop or a template, and 50 more queries run, one per order.

In Django, select_related follows foreign keys using a join. prefetch_related fetches many-to-many or reverse relations in one extra query. In SQLAlchemy, use joinedload or selectinload on the query.

Count your queries in development with Django Debug Toolbar, assertNumQueries in tests, or SQLAlchemy's echo=True. With async SQLAlchemy, lazy loading raises an error instead of running quietly, which forces you to decide up front.