Adding a new column should be simple. It rarely is in production. Every database, ORM, migration tool, and API layer reacts differently. Done wrong, it breaks queries, slows reads, and fills logs with errors you do not have time to debug.
A new column is more than DDL. You must plan for schema migrations, data backfills, index changes, and application code deployment. Order matters. Run the migration before the code that uses it, or deploy feature toggles to handle nulls until the column fills with correct values. Always test in a full-scale staging environment before touching production.
For relational databases like PostgreSQL or MySQL, use ALTER TABLE ADD COLUMN with care. On large tables, this can lock writes. Consider ADD COLUMN with a default value of NULL first. Backfill in batches to reduce load. Build indexes last to avoid compounding migration time.