Cookbook recipe

Set effective_cache_size so the planner trusts the cache

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

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A production experience capsule based on real incident patterns.

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Estimated investigation4 min

Scenario

The planner avoids index scans on data that is actually cached. effective_cache_size tells it how much memory the OS + PostgreSQL likely have for caching. Diagnose it Check the current value: SHOW effective_cache_size; Why it happens…

Investigation Path

The planner avoids index scans on data that is actually cached. effective_cache_size tells it how much memory the OS + PostgreSQL likely have for caching.

Diagnose it

Check the current value:

SHOW effective_cache_size;

Why it happens

effective_cache_size does not allocate memory; it is a planner hint. Set too low, the planner assumes index pages will miss cache and over-prefers sequential scans. Set realistically, it favors index scans that pay off because pages are warm.

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