عرض عملي: من 4 ثوانٍ إلى 40 ms

خطوة بخطوة

1. Find the statement in pg_stat_statements; capture its EXPLAIN (ANALYZE, BUFFERS) (about 4 s, sequential scan with a sort).
2. Read the plan: the filter removes most rows, and the sort spills to disk.
3. Add (customer_id, created_at DESC); re-run and show the sort gone.
4. Planned mistake: wrap the date column in date_trunc in the WHERE; the index is skipped. Fix: use a half-open range.
5. Replace OFFSET with keyset pagination.
6. Record before/after plans and timings (about 40 ms).

Prepares MID-013-LAB1.

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