Query Understanding and Hybrid Retrieval
~45 min read
How to process a user's raw query into a structured signal for retrieval, and why hybrid retrieval (BM25 + semantic) outperforms either method alone.
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Key points
- •Query understanding (spell correction + expansion + intent classification) is the highest-ROI investment in search quality — a misspelled query returns zero-results until corrected
- •BM25 and semantic retrieval are complementary, not competing: BM25 wins on exact/rare terms, semantic wins on vocabulary mismatch; hybrid with RRF reliably outperforms either alone
- •Query expansion improves recall but must be controlled — over-expansion ('apple' → includes 'apple fruit') creates noisy retrieval that degrades ranking precision