Cookbook recipe

Index-only scans and why the visibility map matters

Applies to PostgreSQL 13–17 Last reviewed May 2026 Grounded in source

What This Is

A production experience capsule based on real incident patterns.

Why It Matters

Builds production judgment under pressure.

Next Step

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Estimated investigation4 min

Scenario

You added a covering index but EXPLAIN still shows heap fetches. Index-only scans only stay "index-only" when the visibility map says pages are all-visible. Diagnose it Look for Heap Fetches in the plan: EXPLAIN (ANALYZE) SELECT…

Investigation Path

You added a covering index but EXPLAIN still shows heap fetches. Index-only scans only stay “index-only” when the visibility map says pages are all-visible.

Diagnose it

Look for Heap Fetches in the plan:

EXPLAIN (ANALYZE)
SELECT id FROM events WHERE id BETWEEN 1 AND 1000;
-- Index Only Scan ... Heap Fetches: 942

Many heap fetches mean the visibility map is stale because VACUUM has not marked those pages all-visible.

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Career Impact

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Related & next steps

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Concepts on this page

Visibility map Transaction Autovacuum VACUUM Index
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