Question

我的表格包含first_seen（日期时间），last_seen（日期时间）和severity（整数）的事件。

我试图在谨慎的15分钟间隔内找到有多少活动：

WITH intervals AS
  (
      SELECT
        '2016-08-02 00:00:00'::TIMESTAMP
            + (n||' minutes')::INTERVAL AS start_time,
        '2016-08-02 00:00:00'::TIMESTAMP
            + ((n + 15)||' minutes')::INTERVAL AS end_time
      FROM generate_series(0, 24 * 60, 15) n
  )
SELECT
  start_time,
  (SELECT count(*) FROM event
     WHERE first_seen < end_time AND last_seen > start_time
       AND severity = 5) red,
  (SELECT count(*) FROM event
     WHERE first_seen < end_time AND last_seen > start_time
       AND severity = 4) orange,
  (SELECT count(*) FROM event
     WHERE first_seen < end_time AND last_seen > start_time
       AND severity = 3) yellow
FROM intervals;

我还有(first_seen, last_seen, severity)的索引我的问题是它看起来太慢了该表有大约10万行，并且100个间隔需要10秒。索引扫描似乎太慢了。

如何优化此查询的任何想法？

Answer 1

最好的办法是摆脱子选择。

尝试以下内容（未经测试，因此可能包含错误）：

WITH intervals AS
  (
      SELECT
        '2016-08-02 00:00:00'::TIMESTAMP
            + (n||' minutes')::INTERVAL AS start_time,
        '2016-08-02 00:00:00'::TIMESTAMP
            + ((n + 15)||' minutes')::INTERVAL AS end_time
      FROM generate_series(0, 24 * 60, 15) n
  )
SELECT
   start_time,
   sum(CASE WHEN severity = 5 THEN 1 ELSE 0 END) red,
   sum(CASE WHEN severity = 4 THEN 1 ELSE 0 END) orange,
   sum(CASE WHEN severity = 3 THEN 1 ELSE 0 END) yellow
FROM event
   RIGHT OUTER JOIN intervals
      ON first_seen < end_time AND last_seen > start_time
GROUP BY start_time;
ORDER BY start_time;

您可以通过first_seen和last_seen上的两个索引加快速度。多列索引无济于事。

使用间隔优化PostgreSQL查询时间序列

1 个答案: