Explain planner cost model, visibility map, and covering indexes for read-heavy workloads.
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Seq Scan vs Index Scan vs Index-Only Scan — when does Postgres choose each?
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indexseq-scanperformance
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The planner estimates cost from seq_page_cost vs random_page_cost × selectivity. Highly selective (<5% rows) → Index Scan; moderately selective → Bitmap Heap Scan; all columns in index + VACUUM clean → Index-Only Scan; otherwise → Seq Scan.
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