PostgreSQL分组（GROUP BY子句）
select u.created_ts, count(*) from tb_user u group by u.created_ts
注意：不能缺少聚合函数
按月分组统计
select
to_char(to_timestamp(created_ts/1000), ‘YYYY-MM’) as month,
count(id) as total_call
from tb_user
where
created_ts between 1548568440000 and 1552456440000
group by month
order by month
