请使用下面的查询，

SELECT
  dt.date_seq :: DATE AS dates,
  count(r.id) AS usages
FROM reports r LEFT JOIN dispatches d ON r.dispatch_id = d.id
LEFT JOIN messages m ON d.message_id = m.id
FULL OUTER JOIN (select CURRENT_DATE + i as date_seq from generate_series(date '2020- 
05-01'- CURRENT_DATE, date '2020-05-10' - CURRENT_DATE ) i)
dt
ON d.create_at = dt.date_seq
WHERE r.status = 'SUCCESS' AND r.update_at :: DATE BETWEEN DATE '2020-05-01' AND 
'2020-05-10' AND m.client_id = 6
GROUP BY dates
ORDER BY dates;

很接近，但where子句将外部连接变回内部连接。您需要将该条件移到联接条件中。

SELECT x.day AS dates,
       count(r.id) AS usages
FROM (
   SELECT generate_series(timestamp '2020-05-01'
                        , timestamp '2020-05-10'
                        , interval '1 day')::date
) x(day)
  left join dispatches d on d.create_at = x.day
  LEFT JOIN reports r 
         ON d.id = r.dispatch_id
        and r.status = 'SUCCESS' 
        AND r.update_at::DATE BETWEEN DATE '2020-05-01' AND '2020-05-10' 
        AND m.client_id = 6
    LEFT JOIN messages m ON d.message_id = m.id
GROUP BY dates
ORDER BY dates;