哪个DB用于存储许多用户的地理定位（坐标）并检查它们是否实时接近？

IOS app will send from time to time coordinates to server through REST api. On server inside DB registered users are stored with their coordinates. Each user may have many friends which he want to track if they are close to him. So server after each coordinate update (from user) has to check if his friends are close to him (with a radius also stored in DB per user). It has to handle MANY users.IOS app will send from time to time coordinates