How do you combine lexical (BM25) and semantic (vector) retrieval, and what is Reciprocal Rank Fusion?
Pure vector search drops exact terms; pure keyword search drops meaning. The signal is understanding why each fails on its own and how to merge two rankings on incomparable scales without hand-tuning weights. Here is the answer.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
Pure vector search drops exact terms; pure keyword search drops meaning. The signal is understanding why each fails on its own and how to merge two rankings on incomparable scales without hand-tuning weights. Here is the answer.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
