You need to add a NOT NULL column 'status' to a 500-million-row orders table in production. How do you do this without downtime?

PostgreSQL approach (zero-downtime): 1) ADD COLUMN with DEFAULT: ALTER TABLE orders ADD COLUMN status VARCHAR(20) DEFAULT 'ACTIVE'. PostgreSQL 11+ writes the default to catalog metadata only (no row rewrite) — instant. 2) Backfill if needed: UPDATE orders SET status = 'ACTIVE' WHERE status IS NULL (do in batches). 3) Add NOT NULL: ALTER TABLE orders ALTER COLUMN status SET NOT NULL (fast if no nul