[rorymalcolm]

If you drop a 'title' column from a users table, for example, you can revert and have the title column reappear with the dropped data, new users added during this time (while the column was dropped) will not have a title.

[codingdave]

Just to clarify my understanding -

If we extend that scenario a bit to dropping a title column and at the same time adding a foo column. Then add rows with data in foo. Then revert. Do you lose the foo data?

Alternatively, can we separate those actions out? Drop columns in one migration. Add columns in another. Add rows and data. Then revert only the migration where columns were dropped, keeping the more recent adds?

[shlomi-noach]

Right. So if you're both adding one column and removing another, then the revert will lose your new column and will regain your old column. Normally, you deploy DB and app in steps. E.g. if your migration adds a new column, your app is not yet aware of the column (or else your app would break). The moment the migration completes, your app is still on the not-knowing state. It takes an app deployment to actually start utilizing the new column. If you do that, and then want to revert -- you will lose any new data you've added to the new column.

In my experience, when a schema migration goes wrong, it goes wrong with a bang. It takes seconds to maybe one minute until pagers are alarming. So I'd say in a common scenario you will not get to deploy your app with the new column awareness, because you'll have realized the migration was bad right away.

> Alternatively, can we separate those actions out? Drop columns in one migration.

If you choose to do that, then you're on safer grounds; it costs you some wall clock time, because migrations do take a while to complete on medium to large tables.

Do note that Rewind only lets you rewind your most recent deployment (PlanetScale's app will not let you run the next migration before you've committed to, or have rewinded, the previous one).