LAB: Cost Control + EXPLAIN — Cut a Query's Cost

# LAB: Cost Control + EXPLAIN — Cut a Query's Cost (60 min) > **Outcome:** Read a query plan, identify the expensive operator, and cut a > query's billed cost. Covers plan 100 wh-06. ## 1. Brief Take the provided expensive query (`cost_lab_expensive.sql`: `SELECT *` over a wide join with no partition filter). (a) Run `EXPLAIN` / dry-run and record stages + bytes; (b) rewrite it (projection pushdown, partition filter, join order / pre-aggregation); (c) show the before/after billed bytes. ## 2. Required deliverables 1. `cost_cut.sql` — original + rewritten query. 2. `evidence.md` — EXPLAIN notes (which stage dominated) + before/after bytes. ## 3. Deterministic structural checks - `no-select-star`: rewritten query has an explicit column list. - `partition-filter`: rewritten query filters on the partition column. - `cost-cut`: rewritten bytes-billed <= 50% of original. ## 4. AI-review rubric | Criterion | Weight | Pass bar | |---|---|---| | Plan reading | 40% | names the dominant stage and why it is expensive | | Rewrite quality | 40% | all three structural checks green | | Guardrail note | 20% | states a personal cost-control rule (validator-first, .dry-run, alerts) |