is it possible to using index in unix datetime colum in MySQL 5.7 when using function
using generate column and created a index on it:
ALTER TABLE report_user 
ADD COLUMN statistic_date varchar(16) 
GENERATED ALWAYS AS (date_format(FROM_UNIXTIME(statistic_time/1000), '%Y-%m-%d')) STORED;

then created index:
CREATE INDEX report_user_userid_staticdate_idx 
ON report_user(user_id,statistic_date);