Adding a new column is simple in theory: change the schema, apply the migration, update the code. In practice, the cost grows with scale. Production databases hold millions of rows. Writes can lock tables. An unplanned migration can take down systems.
Plan the change deliberately. Start with your schema migration. Use an ALTER TABLE statement to add the new column. Decide on its data type and nullability before you run it. Adding a nullable column with a default value is often safer in live systems. Avoid backfilling large amounts of data in the same transaction—batch it to reduce load.
If the new column changes application logic, ship the change in steps. First, deploy code that can handle the absence of the column. Then, add the column. Finally, update the code to use it. This allows zero-downtime deployment and reduces rollback risk.