In a 6-quart kettle bring 4 quarts salted water to a boil for spaghetti.
Peel and seed 1 cucumber and shred on large holes of grater. Squeeze shredded cucumber in a clean kitchen towel to remove excess liquid. Seed and dice remaining half cucumber. Chop garlic and mash to a paste with salt. In a large bowl stir together shredded and diced cucumber, garlic paste, yogurt, buttermilk, all but 1 tablespoon dill, and salt and pepper to taste.
Halve or quarter tomatoes and cut olives into thin slices. In a bowl stir together tomatoes, olives, and remaining tablespoon dill and season with salt and pepper.
Cook spaghetti in boiling water until al dente and drain in a colander. Rinse spaghetti under cold water to stop cooking and drain well. Add spaghetti to yogurt mixture and toss to coat.
Serve spaghetti topped with tomato mixture.
Each serving about 315 calories and 4 grams fat (12% calories from fat). Nutritional analysis provided by Gourmet