This doesnt really affect any advanced database that has online alters and workload management. It would just take longer but wouldn't affect any normal transactions. It's a solved problem unless the database just doesn't support it.
AFAIK, all of the common open-source databases have this problem. Postgres is better than mysql, but it still needs to do extensive locking when you alter a column.
Under heavy read/write loads and/or with large tables, this can blow up in your face.
