Cookbook recipe

How TOAST stores big values

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

A row with a huge text or jsonb value does not blow up the main table the way you might expect. TOAST handles oversized attributes. Diagnose it Find a table's TOAST relation size: SELECT pg_size_pretty(pg_relation_size(reltoastrelid)) FROM…

Investigation Path

A row with a huge text or jsonb value does not blow up the main table the way you might expect. TOAST handles oversized attributes.

Diagnose it

Find a table’s TOAST relation size:

SELECT pg_size_pretty(pg_relation_size(reltoastrelid))
FROM pg_class WHERE relname='docs';

Why it happens

PostgreSQL pages are 8KB. Values too large to fit are compressed and, if still big, split into chunks stored out-of-line in an associated TOAST table, leaving a small pointer in the main row.

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Concepts on this page

TOAST
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