I don't know about postgres offhand, but there aren't idempotent solutions to some DDL operations in mysql. (alter table add column, off the top of my head)
Yeah in Postgres you can check for the existence of a column in a table. If it exists/doesn't exist you can drop/add the column. Obviously "ALTER TABLE x ADD COLUMN y" isn't idempotent, but the whole SQL statement around it can be idempotent.
