Database migrations are easy in small applications because deploys are linear. Change the schema, deploy the code, done. In a real production system with multiple Go services, background workers, rolling deploys, and long-running jobs, schema changes need choreography. The safe pattern is expand, migrate, contract. Add the new shape while the old code still works, move traffic gradually, backfill data, then remove the old shape only after every consumer has moved. Step 1: Expand The expand migration only adds things: a nullable column, a new table, a new index, or a trigger. It should be safe to run while old code is still deployed. ALTER TABLE campaigns ADD COLUMN budget_currency VARCHAR(3) NULL; CREATE INDEX CONCURRENTLY idx_campaigns_company_currency ON campaigns (company_id, budget_currency); Avoid migrations that rewrite huge tables during business hours. Even if the database supports online operations, test the migration with realistic data volume before trusting it. Step...