PostgreSQL can do the majority of migrations you would need to make instantaneously (adding columns that can be null, adding/dropping indexes etc). MySQL suffers roughly O(rows) time when changing the schema, but with PostgreSQL now at feature parity for most everything else there's little reason to go NoSQL over something like Postgres if you are worried about schema changes. With respect to fault tolerance, this is only a problem if your dataset outgrows a single master (pretty rare) and you haven't figured out a proper way to shard (most places shard on users, it's pretty hard to believe anything is un-shardable).