Boil potatoes and carrots until tender but still a little firm.
In a bowl, pour a little salt and sugar over the cucumbers (about 1 1/2 tsp kosher salt, 1/2 tsp sugar), mix with your fingers and leave to marinate for about 15-20 minutes. Drain and squeeze the excess water from the cucumber, rinse quickly under cold water and squeeze again.
In a large bowl, combine the potatoes, carrots and ginger.
With a potato masher, mash about 1/2 of the mixture, add cucumber, mayonnaise, sake, sugar, salt and pepper and mix well with a spoon or spatula.
Adjust taste and texture (by adding more mayo) to your liking. Serve.
This potato salad can keep in the fridge for about a week.