34 lines
2.6 KiB
Plaintext
Executable File
34 lines
2.6 KiB
Plaintext
Executable File
-- L14 (a) window row_number() <= 3 — cuts in the DB but scans the whole partition
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-- FeedTopNIT.l14ExplainThreeWayPlanCompareIsTheCrownJewel (seed 1000, page 20, K=3)
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-- SELECT t.feed_item_id, t.color, t.text, t.created_at FROM (
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-- SELECT h.feed_item_id, h.color, h.text, h.created_at,
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-- row_number() OVER (PARTITION BY h.feed_item_id ORDER BY h.created_at DESC) AS rn
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-- FROM highlights h WHERE h.feed_item_id IN (<page-20 ids>)) t WHERE t.rn <= 3
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-- 읽기 포인트: Hash Semi Join 이 페이지 부모들의 하이라이트 전량(rows=1509)을 읽고 Sort 한 뒤 WindowAgg 가
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-- 순번을 매긴다. PG 15+ 는 rn<=3 을 WindowAgg 의 Run Condition 으로 밀어넣지만, 파티션 정렬은
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-- 이미 1509행 전량을 훑는다. 반환은 60행이지만 buffers shared hit=430 (two-step 과 같다 = 같은 스캔).
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Subquery Scan on t (cost=531.52..543.16 rows=388 width=686) (actual time=1.376..1.486 rows=60 loops=1)
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Buffers: shared hit=430
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-> WindowAgg (cost=531.52..539.28 rows=388 width=694) (actual time=1.375..1.482 rows=60 loops=1)
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Run Condition: (row_number() OVER (?) <= 3)
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Buffers: shared hit=430
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-> Sort (cost=531.52..532.49 rows=388 width=686) (actual time=1.369..1.406 rows=1509 loops=1)
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Sort Key: h.feed_item_id, h.created_at DESC
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Sort Method: quicksort Memory: 155kB
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Buffers: shared hit=430
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-> Hash Semi Join (cost=172.47..514.84 rows=388 width=686) (actual time=0.590..0.941 rows=1509 loops=1)
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Hash Cond: (h.feed_item_id = "ANY_subquery".id)
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Buffers: shared hit=430
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-> Seq Scan on highlights h (cost=0.00..327.85 rows=3885 width=686) (actual time=0.289..0.421 rows=2917 loops=1)
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Buffers: shared hit=289
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-> Hash (cost=172.22..172.22 rows=20 width=16) (actual time=0.287..0.288 rows=20 loops=1)
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Buffers: shared hit=141
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-> Subquery Scan on "ANY_subquery" (cost=171.97..172.22 rows=20 width=16) (actual time=0.277..0.280 rows=20 loops=1)
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-> Limit (cost=171.97..172.02 rows=20 width=24) (actual time=0.277..0.279 rows=20 loops=1)
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-> Sort (cost=171.97..174.09 rows=846 width=24) (actual time=0.276..0.277 rows=20 loops=1)
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Sort Key: fi.first_highlighted_at DESC, fi.id
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-> Seq Scan on feed_items fi (cost=0.00..149.46 rows=846 width=24) (actual time=0.146..0.212 rows=1000 loops=1)
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Planning Time: 0.123 ms
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Execution Time: 1.552 ms
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