Title: Discussion 7 — What Reranking Alone Can't Fix
Course: AIINFRA 301: Production RAG & LLMOps — Observability and Evaluation > Section: Week 07 — Cross-Encoder Rerankers and Query Rewriting/Expansion

💬 Discussion 7 — What Reranking Alone Can't Fix (20 pts)

If reranking already fixes most first-stage retrieval errors, what specific failure mode does query rewriting/expansion catch that reranking alone cannot? Think about this concretely: reranking can only reorder documents that were already retrieved in Stage 1. What happens when the right document was never in that initial candidate set at all — because the user's query used different vocabulary, was ambiguous, or was missing context the LLM could supply? Draw on this week's lecture (the two-stage cascade, HyDE, multi-query expansion) and, if you completed Lab 7, your own measured results. Post your initial response (150+ words), then respond to at least two classmates — ideally to someone who identified a different failure mode than you did, or who reached a different conclusion from their lab results.