Databasesmedium

Foreign keys: why use them and what’s a trade‑off?

Answer

Foreign keys enforce referential integrity (no orphan rows) and can simplify reasoning. The trade‑off is extra write overhead and sometimes more complex migrations. In high‑throughput systems, teams sometimes validate in the app instead of the DB.

Advanced answer

Deep dive

Expanding on the short answer — what usually matters in practice:

Context (tags): database, foreign-key, integrity, trade-offs
Data model and access patterns: dominant queries (read/write ratio, sorting, pagination).
Indexes: when they help vs hurt (write amplification, memory).
Consistency & transactions: what’s guaranteed and what can bite you.
Explain the "why", not just the "what" (intuition + consequences).
Trade-offs: what you gain/lose (time, memory, complexity, risk).
Edge cases: empty inputs, large inputs, invalid inputs, concurrency.

Examples

A tiny example (query shape):

-- Example: index + query shape
SELECT *
FROM users
WHERE email = '[email protected]'
LIMIT 1;

Common pitfalls

Too generic: no concrete trade-offs or examples.
Mixing average-case and worst-case (e.g., complexity).
Ignoring constraints: memory, concurrency, network/disk costs.

Interview follow-ups

When would you choose an alternative and why?
What production issues show up and how do you diagnose them?
How would you test edge cases?