Learning track · 13 concepts

Search & IR

Lexical retrieval beyond embeddings: tokenization, inverted indexes, BM25, ranking, hybrid search, and search evaluation.

What mastery looks like

This track contains 13 connected concepts rather than an unordered reading list. Mastery means you can move from vocabulary to mechanisms, predict how the system behaves under pressure, and support a design decision with code, measurements, or a failure-recovery exercise. For Search & IR, use the track description as the boundary: learn enough detail to reason clearly about lexical retrieval beyond embeddings: tokenization, inverted indexes, bm25, ranking, hybrid search, and search evaluation.

A useful explanation names the state involved, the operation that changes it, the resource or safety constraint, and the observable signal that tells you whether the mechanism works. Avoid stopping at product names. Compare at least two approaches, state what each optimizes, and identify what breaks first as scale, concurrency, latency, or uncertainty increases.

Suggested study sequence

Start with Tokenization to establish the basic vocabulary. Continue through the core concepts by alternating explanation with an executable drill. Treat Hybrid Search, Reranking, Top-k Pruning (WAND), Learning to Rank as integration work: they should combine earlier mechanisms rather than introduce disconnected facts.

At the end of each session, record one decision you can now make, one failure mode you can now predict, and one unanswered question. Revisit that question through the linked primary sources, then prove the answer in the Playground or a real repository. The track is complete when you can transfer the reasoning to an unfamiliar system, not when every page has been opened.

Roadmaps

Concepts in this track

intro

Tokenization

Splitting text into terms: lowercasing, stemming, stop words, n-grams.

core

Inverted Index

Term → posting list mapping that powers fast keyword lookup.

core

TF-IDF

Term frequency × inverse document frequency weighting for relevance.

core

BM25

The standard lexical ranking function: TF saturation + IDF + length normalization.

core

Ranking Metrics

Precision, recall, MRR, and nDCG for measuring retrieval quality.

core

Search Evals

Building a labelled query set and harness to compare retrieval approaches.

advanced

Hybrid Search

Fusing lexical (BM25) and vector retrieval, usually via reciprocal rank fusion.

advanced

Reranking

A second-stage cross-encoder pass that reorders the top-k candidates.

core

Query Rewriting

Expanding, correcting, and normalizing queries before retrieval.

advanced

Top-k Pruning (WAND)

WAND, MaxScore, and block-max — skipping documents that cannot reach the top k.

advanced

Learning to Rank

Pointwise, pairwise, and listwise objectives — and why the metric you care about is not differentiable.