A new column changes the structure of your table. Even a single field can trigger a full table rewrite, create lock contention, or break queries. On small datasets, this is invisible. On large datasets, it can be the difference between a smooth deploy and a midnight outage.
Before you run ALTER TABLE ADD COLUMN, check the size of your table. In PostgreSQL, a new nullable column with a default can rewrite the entire table. In MySQL, locks may block all writes during schema changes. For critical systems, use tools like gh-ost for MySQL or ALTER TABLE ... ADD COLUMN with DEFAULT NULL for PostgreSQL, followed by an UPDATE in controlled batches. This avoids downtime while still delivering the schema change.
Always update your ORM schema definitions in sync with your migration. A mismatched schema can send invalid queries into production. Coordinate with application code changes so the new column is only used after deployment to all nodes. Version your migrations and apply them in a staged rollout.