Adding a new column is one of the most common schema changes in production. Done wrong, it can bring down an application. Done right, it is invisible to users and fast to deploy. The key is to treat schema changes as first-class events in your release process.
A new column can mean more than extra data. It can change query performance, index usage, and application logic. Before applying it, understand the downstream effects: ORM mappings, data migrations, background jobs, caching layers. Test against realistic datasets. Check how the extra field impacts indexes and how it interacts with existing constraints.
Avoid locking the table on writes. In many databases, a blocking ALTER TABLE can halt traffic. Use tools or patterns that run migrations online. This might mean creating the new column with a default of NULL and backfilling data in small batches. Only then should you enforce defaults or constraints.