In events (the pipeline log table) a processed BOOLEAN column was first added without not-null — intentionally, so a background backfill of existing rows wouldn't block writes in production. The backfill is now done, and it's time to make the column required. Sequence: build events with id (primary key) and payload (text), insert two test rows. Add processed BOOLEAN without not-null — old rows end up with NULL. Run the backfill: set FALSE for every row whose value is still empty. Only then make the column not-null — otherwise the not-null step blows up on those same old rows. Classic zero-downtime migration pattern.