>a. Schema changes are considered DDL statements and hence can't be simply rolled back via transaction in most databases (as far as I know). In the database of

>a. Schema changes are considered DDL statements and hence can't be simply rolled back via transaction in most databases (as far as I know). In the database of my choice (Postgres. Since 7.1), DDL is transactional and can be rolled back. >b. If you have millions or more rows and you need to introduce a schema change, it can get painful to do so and can result in app downtime. In many databases and for many schema changes the workarounds are trivial (like adding columns without a default value).