ALTER, DROP, TRUNCATE: Evolving Schemas

Sources change. Your DDL must change without breaking everything downstream.

ALTER TABLE stg_orders ADD COLUMN IF NOT EXISTS coupon_code TEXT;
ALTER TABLE stg_orders ALTER COLUMN amount_cents TYPE NUMERIC USING amount_cents::numeric;
TRUNCATE stg_orders;                       -- fast full reload of staging
DROP TABLE IF EXISTS tmp_old_extract;      -- temp cleanup

Rules

1. Additive first: new nullable columns and new tables are safe. Renames, type narrows, and drops are breaking — coordinate with dbt models and dashboards before applying.
2. IF EXISTS / IF NOT EXISTS everywhere in migration scripts so they are re-runnable (pipelines retry; migrations must survive retries).
3. TRUNCATE (not DELETE) for staging reloads: instant, minimal WAL. Never TRUNCATE a mart — rebuild via CREATE OR REPLACE / swap pattern instead.
4. DROP is forever without backups: snapshot or rename-to-archive (ALTER TABLE t RENAME TO t_retired_2026_09) before dropping anything shared.