Place potatoes in a Dutch oven; cover with water. Bring to a boil. Reduce heat; cover and simmer until tender, 10-15 minutes. Drain and transfer to a large bowl., Meanwhile, combine sugar and vinegar in a small saucepan. Cook and stir over medium heat until sugar is dissolved; pour over hot potatoes. Cool to room temperature., In a large skillet, heat oil over medium-high heat. Add carrots and onion; cook and stir until crisp-tender, 6-8 minutes. Add to potatoes. Stir in beets, celery and pickles. Combine mayonnaise, salt and pepper; gently stir into potato mixture. Refrigerate, covered, until chilled.