Add lesson 28: Indexes basics

[exekias]

Adds the first lesson of the new Module 7: Performance & indexing — indexes basics (roadmap item 28-indexes-basics).

What's in the lesson

• The mental model: a Seq Scan vs a B-tree index (the book-index analogy)
• CREATE INDEX, with a light before/after EXPLAIN (deep plan-reading is the next lesson)
• What B-tree covers (=, ranges, BETWEEN, IN, IS NULL, sorting, prefix LIKE)
• Selectivity: an index helps for selective predicates; the planner may (correctly) keep a Seq Scan for common values — shown both ways on the same column
• Composite indexes and the left-prefix rule
• Unique indexes (and that a PK / UNIQUE constraint creates one automatically)
• The write/space cost of indexes and when not to index

Seed

One events table populated to ~50k rows via generate_series, with a skewed status (low-selectivity demo) and ~5k distinct user_ids (high-selectivity demo), plus ANALYZE. The generate_series approach keeps the seed file tiny.

Checks

One query-returns check (user-id-index-created): confirms the learner created the user_id B-tree index by querying pg_indexes for a definition matching (user_id). It's robust to whatever the learner names the index and ignores the composite (user_id, status). Verified on Postgres 16 (0 before CREATE INDEX, 1 after).

Notes

This PR also bootstraps lessons/07-performance-and-indexing/module.yaml, byte-identical across the Module 7 lesson PRs so the shared folder merges cleanly.

npm run lessons:validate and next build both pass.

Part of #6

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Project	Deployment	Actions	Updated (UTC)
learn-postgres	Ready	Preview	Jul 2, 2026 12:23pm