Every schema change, checked against your real database first.

PROPOSED

ALTER TABLE orders ADD COLUMN status text NOT NULL DEFAULT 'pending';

SAFE ALTERNATIVE

-- 1. add the column nullable: metadata-only, no rewrite
ALTER TABLE orders ADD COLUMN status text;

-- 2. default applies to new rows only
ALTER TABLE orders ALTER COLUMN status SET DEFAULT 'pending';

-- 3. backfill in batches, off-peak, 10k at a time
UPDATE orders SET status = 'pending'
 WHERE status IS NULL AND id BETWEEN $1 AND $2;

-- 4. enforce without a full-table lock
ALTER TABLE orders ADD CONSTRAINT orders_status_present
  CHECK (status IS NOT NULL) NOT VALID;
ALTER TABLE orders VALIDATE CONSTRAINT orders_status_present;

ALTER TABLE orders DROP CONSTRAINT orders_status_present;
ALTER TABLE orders DROP COLUMN status;