数据库改字段前怎样准备迁移、备份和回滚

BEGIN;
CREATE TEMP TABLE orders (id integer PRIMARY KEY, price_cents integer) ON COMMIT DROP;
INSERT INTO orders VALUES (101,12345),(102,0),(103,NULL),(104,-250);
ALTER TABLE orders ADD COLUMN price_amount numeric(12,2);
UPDATE orders SET price_amount = price_cents / 100.0;
SELECT id, price_cents, price_amount FROM orders ORDER BY id;
SELECT count(*) AS mismatches FROM orders
WHERE price_amount IS DISTINCT FROM (price_cents / 100.0)::numeric(12,2);
ROLLBACK;