[gcarre]

Try ALTER TABLE ADD COLUMN on a 100 million rows table :).

Jeremy Zawodny from Craigslist explained why it helped them a lot for Craigslist archives database, where an alter table could take up to 24 hours: http://www.10gen.com/presentations/mongosf2011/craigslist

[bsg75]

OK, a MySQL problem. Not so much with PostgreSQL, where adding new columns noes not need to touch every table row.

NoSQL ~= NoMySQL