Cookbook recipe

Add a NOT NULL column to a huge table safely

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

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Estimated investigation4 min

Scenario

Adding a NOT NULL column with a default used to rewrite the whole table under a long lock. Do it in safe steps. Diagnose it On modern PostgreSQL, a constant default is metadata-only: ALTER TABLE big…

Investigation Path

Adding a NOT NULL column with a default used to rewrite the whole table under a long lock. Do it in safe steps.

Diagnose it

On modern PostgreSQL, a constant default is metadata-only:

ALTER TABLE big ADD COLUMN flag boolean NOT NULL DEFAULT false;
-- instant on PG 11+ for a constant default

Why it happens

Pre-11, adding a column with a default rewrote every row. Modern versions store a constant default in catalog metadata, but a volatile default (e.g. now()) or backfilling existing rows still requires work.

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Related & next steps

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Concepts on this page

Constraint Schema Lock
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