On: Ask HN: How do big web sites roll out new versions...

[brianm]

Once you are sure you are never going to roll back to the old code you can wipe out the old columns.

[gaius → brianm]

In practical terms you generally don't tho'. You might need them again, for example, for some as-yet-unplanned new feature. And doing so is of limited value anyway in many databases, you won't get the space back without a re-org and that's a costly operation to do hot, if you even can at all.