您可以使用generate_series()来生成first_date和last_date之间的连续日期列表：

with dates as (
    select d::date, last_date- first_date+ 1 ct
    from test, generate_series(first_date, last_date, '1d'::interval) d
    )
select subject, flag/ ct flag, d date
from dates
cross join test;

    subject     |          flag          |    date    
----------------+------------------------+------------
 this is a test | 0.50000000000000000000 | 2016-01-01
 this is a test | 0.50000000000000000000 | 2016-01-02
 this is a test | 0.50000000000000000000 | 2016-01-03
 this is a test | 0.50000000000000000000 | 2016-01-04
(4 rows)