Cookbook recipe

Tune shared_buffers for your workload

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

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Estimated investigation4 min

Scenario

shared_buffers is at the tiny default and the database constantly re-reads pages from the OS cache. Right-sizing it improves cache hit rates. Diagnose it Check the buffer cache hit ratio: SELECT round(100*sum(blks_hit)/greatest(sum(blks_hit)+sum(blks_read),1),2) AS cache_hit_pct FROM pg_stat_database;…

Investigation Path

shared_buffers is at the tiny default and the database constantly re-reads pages from the OS cache. Right-sizing it improves cache hit rates.

Diagnose it

Check the buffer cache hit ratio:

SELECT round(100*sum(blks_hit)/greatest(sum(blks_hit)+sum(blks_read),1),2) AS cache_hit_pct
FROM pg_stat_database;

Why it happens

shared_buffers is PostgreSQL’s own page cache. Too small and it leans entirely on the OS cache with extra copying; too large and it competes with the OS cache and other memory. A common starting point is ~25% of RAM.

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Concepts on this page

Shared buffers
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