PostgreSQL如果缺少则使用上一行的值

Question

我有以下查询：

WITH t as (
  SELECT date_trunc('hour', time_series) as trunc 
  FROM generate_series('2013-02-27 22:00'::timestamp, '2013-02-28 2:00', 
                       '1 hour') as time_series
  GROUP BY trunc
  ORDER BY trunc
)
SELECT DISTINCT ON(trunc) trunc, id
FROM t
LEFT JOIN (
   SELECT id, created, date_trunc('hour', created) as trunc_u
   FROM event
   ORDER BY created DESC
) u
ON trunc = trunc_u

结果如下：

"2013-02-27 22:00:00";
"2013-02-27 23:00:00";2
"2013-02-28 00:00:00";5
"2013-02-28 01:00:00";
"2013-02-28 02:00:00";

表格event具有id，created以及其他一些列，但此处仅与之相关。上面的查询为我提供了id在给定trunc时间段内生成的最后一个事件（由于DISTINCT ON每个周期都得到了很好的汇总）。

现在，NULL如果在给定的时间段内未发生任何事件，此查询将产生。我希望它返回以前的可用时间id，即使它来自不同的时间段。IE：

"2013-02-27 22:00:00";0
"2013-02-27 23:00:00";2
"2013-02-28 00:00:00";5
"2013-02-28 01:00:00";5
"2013-02-28 02:00:00";5

我确信我缺少一些简单的方法来完成此任务。有什么建议吗？

Answer 1

尝试：

WITH t as (
  SELECT time_series as trunc 
    FROM generate_series('2013-02-27 22:00'::timestamp, '2013-02-28 2:00', 
                         '1 hour') as time_series
)
SELECT DISTINCT ON(t.trunc) t.trunc, e.id
  FROM t
  JOIN event e
    ON e.created < t.trunc 
 ORDER BY t.trunc, e.created DESC

如果太慢-告诉我。我会给你一个更快的查询。