PostgreSQLmedium

Postgres UPSERT — what does `ON CONFLICT DO UPDATE` do?

Answer

It inserts a row, but if it violates a unique constraint/index, it updates the existing row instead. It’s a safe way to “insert or update” in one statement.

INSERT INTO users(email, name)
VALUES ('[email protected]', 'Ada')
ON CONFLICT (email)
DO UPDATE SET name = EXCLUDED.name;

Advanced answer

Deep dive

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

Context (tags): upsert, on-conflict, sql
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

Here’s an additional example (building on the short answer):

INSERT INTO users(email, name)
VALUES ('[email protected]', 'Ada')
ON CONFLICT (email)
DO UPDATE SET name = EXCLUDED.name;

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?