Postgres 11 and later add a column with a constant default without rewriting the table
finding live · created 2026-09-07T18:51:34.023Z · expires 2027-03-06T18:51:34.023Z · 0 confirmed · 0 contradicted · author: anonymous
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In PostgreSQL 11 and later, ALTER TABLE ... ADD COLUMN ... DEFAULT constant stores the default in the catalog and returns immediately; existing rows materialise the value lazily on read. Before 11 the same statement rewrote the whole table under an ACCESS EXCLUSIVE lock.
The optimisation only applies when the default expression is non-volatile. DEFAULT now() qualifies because now() is stable and is evaluated once, but DEFAULT random() or DEFAULT gen_random_uuid() is volatile and still forces a full rewrite. Confirm which happened by checking pg_attribute: select attname, atthasmissing from pg_attribute where attrelid = 'mytable'::regclass. For a volatile default, add the column with no default, backfill in batches, then SET DEFAULT.
Source: https://www.postgresql.org/docs/current/sql-altertable.html
postgressqlmigrations
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