Week 1 of Hung-Yu Kao's NLP course at NTHU is a 91-page deck, W1_NLP_brief. It opens with 'Watch for kids', five readings of the telescope sentence, and a Chinese tongue-twister about eleven uncles to show why language is hard. Then, from an information-retrieval angle, it builds one pipeline: inverted index, tokenization, stemming, TF-IDF, BM25. The second half hits that pipeline's dead ends (synonyms, polysemy, vocabulary mismatch), moves to SVD-based LSA, and closes with a preview of dense vectors through Skip-gram, GloVe, and FastText.
An LLM will confidently answer that Oppenheimer was born in 1967 (the year he died). RAG retrieves first and generates second. The first 60 pages of Hung-Yu Kao's Fall 2025 RAG deck are all about finding the right material. Sparse vectors (bag-of-words, TF-IDF, BM25) are cheap and dependable; dense vectors catch paraphrases. BERT's [CLS] isn't a good sentence vector as-is, hence Sentence-BERT pooling and bi-encoders. A cross-encoder is accurate but needs nearly 50 million passes for 10,000 sentences, while a bi-encoder needs 20,000. SimCSE uses dropout as data augmentation, DPR beats BM25 with only 1,000 training examples, and GTR shows that scaling up a dual encoder improves out-of-domain retrieval.
The Spring 2023 edition of CS224U spends a whole unit on retrieval, because OpenQA gives you only the question and you have to find the evidence yourself, while large language models fabricate sources. The slides by Potts and Omar Khattab go from TF-IDF and BM25 through Success@K, MRR, and average precision to four neural IR designs (cross-encoder, DPR, ColBERT, SPLADE) and how each trades expressiveness against scale, and they end by asking you to count latency and cost as metrics too.
Ask AI first extracts intent, complexity, and 1–4 search terms. It then routes across metadata, BM25, Vectorize, and RRF; a retry adds Critic gaps and disables the first-pass-only BM25 short circuit.
Entering '我想找入門的ai課程' in Ask AI showed zero searched posts and triggered a refusal, while Related Reading recommended exactly the right article. The first fix addressed Chinese tokenization, the LIKE fallback, and the Vectorize data path. A second pass added short Han-and-number tokens, post metadata retrieval, and a rule that exposes sources only after both Validation and Critic pass.
Querying “認證” returned only ~10 hits while 149 files (509 occurrences) matched; 41 posts and 76 chunks were found via LIKE, but D1 chunks_fts had 0 rows and unicode61/trigram both returned 0 for 2-char CJK terms. Fix: LIKE fallback with char-level OR first, then trigram migration + pnpm sync, then pagination beyond the hard limit of 12.
Vector search handles semantics; BM25 handles keywords. Combining them with RRF is what lets you handle both fuzzy queries and exact terms at the same time.
BM25 only recognizes words that appear in the query. SPLADE infers related terms and adds them to the search, gaining partial semantic capability while preserving the precision of keyword search.