This question is out of curiosity, just to learn more about the internals of PostgreSQL. The goal was to add a not null bool column filled with "false", but with "true" as the default for new rows. The naïve approach would be: ALTER TABLE foo ADD COLUMN slow bool NOT NULL DEFAULT true; UPDATE foo SET slow = false; This takes a certain, non-negligible amount of time. This on the other hand achieves the same result and is almost instant: ALTER TABLE foo ADD COLUMN fast bool NOT NULL DEFAULT false; ALTER TABLE foo ALTER COLUMN fast SET DEFAULT true; Where does the difference come from, how are those handled internally? Fiddle: https://dbfiddle.uk/?rdbms=postgres_14&fiddle=56595e8ee397a5bc48b84277da3133 a9
