Cianplan

Level 5: AI / LLM systems · L5.2 RAG & vector databases

Hybrid search, reranking and query rewriting

Combine BM25 + vectors and add a reranker.

13 min · Pro

What you take away

  • Lexical and semantic retrieval fail on opposite things, so run both and fuse the rankings — reciprocal rank fusion needs no score calibration.

And 3 more inside the class, in Pro.

How the class is laid out

  1. The problem
  2. Two retrievers, one list
  3. Check your understanding