Adding a new column is more than an alteration to a table. It changes the shape of your data. It can reshape indexes, query plans, and application logic. Doing it right means knowing the risks and controlling them.
In PostgreSQL, ALTER TABLE ADD COLUMN is the tool. It is simple when you add a nullable column without defaults. The table metadata updates almost instantly, even on massive datasets. The moment you set NOT NULL or add a non-constant default, the database will scan every row. On large tables, that becomes an expensive lock.
MySQL behaves differently. Adding a column can trigger a table copy, blocking writes. Use ALGORITHM=INPLACE or ALGORITHM=INSTANT when supported to reduce downtime. In MySQL 8.0+, INSTANT can add a column without rewriting the table. But formats, constraints, and storage engines decide which algorithms are allowed.
For production systems, coordinate schema and code changes. First deploy application code that can handle both old and new schemas. Then run the migration during a low-traffic window, or use online schema change tools like pt-online-schema-change or gh-ost. Monitor replication lag and query performance during the change.