要查看结果中缺少的行，请将LEFT JOIN添加到可能行的完整网格。网格由(req_time, customer_id, req_status)的所有可能组合构建：

SELECT d.req_time, c.customer_id, s.req_status, count(t.req_time) AS ct
FROM  (
   SELECT generate_series (min(req_time)
                         , max(req_time)
                         , '1 day')::date
   FROM   tbl
   ) d(req_time)
CROSS  JOIN (SELECT DISTINCT customer_id FROM tbl)  c(customer_id)
CROSS  JOIN (VALUES ('FAILED'::text), ('OK'))       s(req_status)
LEFT   JOIN  tbl t USING (req_time, customer_id, req_status)
GROUP  BY 1, 2, 3
ORDER  BY 1, 2, 3;

对实际表中的列进行计数，如果没有找到匹配项，则计数值为0（空值不计数）。
假设req_time是date（不是timestamp）。
相似：

• array_agg group by和null

SQL Fiddle

select
    s.req_time, s.customer_id,
    s.req_status,
    count(t.req_status is not null or null) as "count"
from
    t
    right join (
        (
            select distinct customer_id, req_time
            from t
        ) q
        cross join
        (values ('FAILED'), ('OK')) s(req_status)
    ) s on
        t.req_status = s.req_status and
        t.customer_id = s.customer_id and
        t.req_time = s.req_time
group by 1, 2, 3
order by 1, 2, 3