N+1 queries

N+1 happens when one query loads N parent rows and each row triggers another query for related data. Latency and database load grow linearly with result size.

					1 query: SELECT * FROM orders LIMIT 100
100 queries: SELECT * FROM customers WHERE id = ?
→ 101 round trips
				

Fixes: joins, `IN (...)` preloading, dataloaders, batch loaders, narrower projections, or response-shape changes that drop unused relations.

On interviews: name detection via SQL logs or APM and explain why one giant join is not always the fix—it can duplicate rows and break pagination.

Common pitfalls: fixing N+1 with a cartesian join that explodes rows; ignoring pagination correctness; assuming typed ORMs cannot N+1.

The trade-off is batching complexity versus predictable query count.

Checklist:

• Read SQL logs for loops.
• Batch or preload relationships.
• Preserve pagination correctness.
• Match fix to cardinality and response shape.