我做了更为详细的统计：
for u in users
collect time = left(date_iso8601(u.toc),7) into groups = u.xsf
let num_of_register_with_xsf = length(for u in groups filter u>0 return u)
let total_xsf = sum(groups)
let num_of_register = length(groups)
return {time, num_of_register_with_xsf, num_of_register,
ratio:num_of_register_with_xsf/num_of_register, xsf_per_user_that_have_xsf:total_xsf/num_of_register_with_xsf }