http://adam.blog.heroku.com/past/2008/9/3/ddl_transactions/

I think the real reason there was resistance to this change is that MySQL doesn’t support DDL transactions; and wrapping migrations in a transaction is mostly pointless without this feature. Since MySQL doesn’t support it, most Rubyists don’t know what DDL transactions are – so here’s a quick primer on this incredibly useful and certainly underrated feature.

DDL = data definition language, also known as the schema. DDL commands include CREATE TABLE, DROP TABLE, ALTER TABLE, and CREATE INDEX. DDL transaction support means that the following sequence of SQL commands will not modify your database

I never even thought of that. I was fairly confident in my vision of Postgres getting faster and faster, and Mysql become a “real” database… that the two systems were converging on something resembling a common speed/feature set. But things like this still occasionally rear their ugly heads and I’m happy to be a Postgres user.