Add lesson 29: Reading EXPLAIN#88
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Part of #6 Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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…isks The first Run block had two count(*), and Markdown paired the asterisks as emphasis, so the shell received count() and Postgres rejected it (42809). Escaped both as count(\*) per the >=2-asterisks run-block rule; verified the rendered SQL now contains count(*). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
New 'Your turn': learner uses EXPLAIN to see status='cancelled' do a Seq Scan, adds the orders(status) index, and confirms the plan flips to a Bitmap Index Scan; a query-returns check on pg_indexes verifies the index (0 before, 1 after, verified on PG16). Also corrects the earlier section, which wrongly claimed 'cancelled' already used an index-style plan — without a status index it is a Seq Scan; reworded to compare row estimates and foreshadow the capstone. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Adds the second lesson of Module 7: Performance & indexing — reading EXPLAIN (roadmap item
29-explain).What's in the lesson
EXPLAIN(plan only) vsEXPLAIN ANALYZE(actually runs the query) — with a write-safety warning (BEGIN … ROLLBACK)cost=startup..total rows width) and, with ANALYZE,actual time/rows/loopsANALYZE/ autovacuumEXPLAIN (ANALYZE, BUFFERS), plusVERBOSE/SETTINGS/FORMAT JSON; planning vs execution timeSeed
Two related tables via
generate_series(~4kcustomers, ~40korders) with an index andANALYZE. Illustrative plan output lives in fenced blocks (numbers vary per run); learners run the realEXPLAINs.Checks
One
query-returnscheck (status-index-created): a closing diagnose-fix capstone has the learner spot thestatus='cancelled'Seq Scan, add theorders(status)index, and watch the plan flip to a Bitmap Index Scan; the check confirms the index viapg_indexes(0 before, 1 after, verified on PG16). This also corrected an earlier line that wrongly claimedcancelledalready used an index-style plan (no status index existed, so it was a Seq Scan).Notes
Includes the byte-identical Module 7
module.yaml.npm run lessons:validateandnext buildboth pass.Part of #6
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