Add lesson 43: Capstone: troubleshooting a slow query

[exekias]

Adds Capstone: troubleshooting a slow query (roadmap item 04-capstone-troubleshooting) — the new Module 10: Expert & operations.

Summary

Diagnose a slow query end to end — measure with EXPLAIN ANALYZE, read the plan, add the right index, and confirm it flipped.

Check

One query-returns check (orders-customer-index-added) — the learner adds the fixing index after diagnosing with EXPLAIN ANALYZE, verified via pg_indexes.

Verified end-to-end against a non-superuser Postgres 16. npm run lessons:validate (34/34) and next build both pass.

Part of #6

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Project	Deployment	Actions	Updated (UTC)
learn-postgres	Ready	Preview	Jul 15, 2026 9:14am

[exekias]

@copilot update seeding on this lesson so the query is even slower, do a 10x on number of rows

[Copilot]

@copilot update seeding on this lesson so the query is even slower, do a 10x on number of rows

Done — seed.sql now generates 3,000,000 rows (generate_series(0, 2999999)), up from 300,000. Updated all the matching row-count references in lesson.mdx too (example EXPLAIN plan, interpretation text, and the selectivity callout). Committed in 8ce3f7c.