[masklinn]

> But you could get duplicate key errors when turning fk on.

Why? Fks don’t require unicity. Or do you mean that you target non-unique columns? That sounds… horrendous.

But regardless sounds like something you’d want to resolve before declaring the migration finished.

[pmontra]

You're right. I meant the troubles we get if we turn off all constraints to ease the migration, end up with duplicate keys in the target table because of mistakes, turn on constraints, errors.

[CRConrad]

> ...turn off all constraints to ease the migration, end up with duplicate keys in the target table because of mistakes, turn on constraints, errors.

[Emphasis added -- CRC.]

But that's a good thing, not a bad one -- it shows you you've made a mistake! Might even be you wouldn't have caught that mistake at all otherwise. How could it not be better to catch a mistake right as you made it and are still in the maintenance window to fix it, than to have it lingering undetected, getting ever more corrupt data, until it rears its head possibly years later, when some vital report shows your DB is full of garbage?