Peel and seed the cucumbers, cut them into 1/4-inch dice and toss with the salt; set aside for 15 minutes. Transfer the cucumber cubes to a sieve, rinse them under water and pat dry with towels.
In a bowl combine the cucumbers and yogurt. Gradually and slowly whisk in the olive oil, making sure yogurt absorbs it all. Add the garlic clove and mint, cover and refrigerate until well chilled. (If taking the soup on a picnic, chill it first in a bowl, then transfer it to a thermos.)
Before serving, remove the garlic cloves, give the soup a stir and thin with ice water if you wish; adjust the seasoning.
Ladle the soup into chilled soup plates and stir in the raisins, then garnish with walnuts and chives. If you wish, immediately before serving, you can drop some crushed ice into each plate.