Already solved.
User should not be put into UserInfo as pointer. You should put UserInfo into User and that's it.
In other words, there is a userInfo field in the User table, and its type is Pointer. There is only the version field in UserInfo.
In this way, you can use the following code to make queries.

AVQuery *innerQuery = [AVQuery queryWithClassName:@"UserInfo"];
[innerQuery whereKeyExists:@"version"];
AVQuery *query = [AVQuery queryWithClassName:@"_User"];
[query whereKey:@"userInfo" matchesQuery:innerQuery];
[query findObjectsInBackgroundWithBlock:^(NSArray *comments, NSError *error) {
    // 
}];

If you have a better solution, please suggest it.