如何在Oracle中选择最密集的1分钟

Question

我有一个带有时间戳列tmstmp 的表，该表包含某些事件的日志。我需要找出在 1 分钟间隔内发生的最大事件数。

请仔细阅读！我确实不想要提取时间戳分钟分数并像这样求和：

select count(*), TO_CHAR(tmstmp,'MI') 
   from log_table 
     group by TO_CHAR(tmstmp,'MI')
     order by TO_CHAR(tmstmp,'MI');

它需要先取第一条记录，然后向前看，直到它从第一条开始1分钟内选择所有记录并求和记录数，然后取第二条并做同样的事情......

结果必须有一个（总和，起始时间戳）的记录集。

有人在某处有sn-p代码并愿意分享吗？

Answer 1

带有逻辑窗口的分析功能可以直接提供这些信息：

select l.tmstmp,
count(*) over (order by tmstmp range between current row and interval '59.999999' second following) cnt
from log_table l
order by 1
;

TMSTMP                             CNT
--------------------------- ----------
01.01.16 00:00:00,000000000          4 
01.01.16 00:00:10,000000000          4 
01.01.16 00:00:15,000000000          3 
01.01.16 00:00:20,000000000          2 
01.01.16 00:01:00,000000000          3 
01.01.16 00:01:40,000000000          2 
01.01.16 00:01:50,000000000          1

请根据您的精度调整间隔长度。它必须是低于 1 分钟的最大可能值。

要获得最大分钟数，请使用子查询（不要忘记您可能会收到一条以上的记录 - 使用 MAX 计数）：

with tst as (
select l.tmstmp,
count(*) over (order by tmstmp range between current row and interval '59.999999' second following) cnt
from log_table l)
select * from tst where cnt = (select max(cnt) from tst);

TMSTMP                             CNT
--------------------------- ----------
01.01.16 00:00:00,000000000          4 
01.01.16 00:00:10,000000000          4 

Answer 2

我认为您可以使用SELECT 语句中的子查询来实现您的目标，如下所示：

SELECT tmstmp, (
    SELECT COUNT(*) 
    FROM log_table t2 
    WHERE t2.tmstmp >= t.tmstmp AND t2.tmstmp < t.tmstmp + 1 / (24*60)
) AS events
FROM log_table t;

Answer 3

一种方法使用join 和聚合：

select t.*
from (select l.tmstmp, count(*)
      from log_table l join
           log_table l2
           on l2.tmstmp >= l.tmstmp and
              l2.tmstmp < l.tmstmp + interval '1' minute
      group by l.tmpstmp
      order by count(*) desc
     ) t
where rownum = 1;

注意：这假定tmstmp 在每一行上都是唯一的。如果这不是真的，那么子查询应该由是唯一的某个列聚合。

编辑：

对于大数据，有一种更有效的方法可以利用累积和：

select tmstamp - interval 1 minute as starttm, tmstamp as endtm, cumulative
from (select tmstamp, sum(inc) over (order by tmstamp) as cumulative
      from (select tmstamp, 1 as inc from log_table union all
            select tmstamp + interval '1' day, -1 as inc from log_table
           ) t
      order by sum(inc) over (order by tmstamp) desc
     ) t
where rownum = 1;