Welcome to your interview.
The question is on your right: B-Tree vs BRIN for Ledgers. Take a moment with it first.
Talk your thinking through with me if you like - when you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes). Discussion and graded submissions share your five interviewer interactions, so spend them well.
In PostgreSQL, index choice can materially change both query latency and ingestion cost, especially on large append-heavy tables such as card authorisations or ledger events.
You are asked to explain how B-Tree indexes differ from BRIN indexes in PostgreSQL. Compare how they store information, what query patterns they support well, and how they affect query performance on large Revolut-style transaction tables. You should also explain write amplification: why some indexes make inserts and updates more expensive, and why BRIN often has a lower maintenance cost than B-Tree.
A strong medium-level answer should cover physical behavior at a high level, when each index is a good fit, what happens when data is naturally ordered versus randomly distributed, and the trade-off between fast point lookups and cheaper writes. You do not need to discuss every PostgreSQL index type; focus on B-Tree and BRIN with practical examples.