PostgreSQLmedium

What is a GIN index and when would you use it?

Answer

GIN (Generalized Inverted Index) is great for “contains” queries on composite types like arrays and `jsonb` (e.g., `@>`, `?`). It indexes many-to-many relationships between keys and rows.

CREATE INDEX idx_events_payload ON events USING GIN (payload);

-- example query (jsonb contains)
SELECT * FROM events WHERE payload @> '{"type":"click"}';

Advanced answer

Deep dive

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

Context (tags): gin, index, jsonb, arrays
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):

CREATE INDEX idx_events_payload ON events USING GIN (payload);

-- example query (jsonb contains)
SELECT * FROM events WHERE payload @> '{"type":"click"}';

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?