Get all retweeters of a user


I am trying to get all ids of users that retweet from a specific user. I ve store a list of user’s tweets in a dB and I want for every user to get its retweeters. First question, which is the most efficient way to do so. If for example try to get for every tweet of a user its retweeters it is going to be a very slow process. Secondly, how can I use cursor in order to get more than 100 retweeters for every tweet??