PostgreSQL 动态 date_trunc 函数四舍五入到给定的时间戳

Question

我正在使用内置的 PostgreSQL 函数 date_trunc() 来聚合时间窗口，如下所示:

SELECT max(time) as time, 
COUNT(*) as ticks 
FROM sensorticks 
WHERE time between (TIMESTAMP'2019-01-04 00:15:00') AND (TIMESTAMP'2019-01-04 14:15:00') 
GROUP BY date_trunc('hour', time)
ORDER BY time desc;

结果是这样的:

time                        ticks
2019-01-04 14:14:59         892
2019-01-04 13:59:59         3575
2019-01-04 12:59:59         3552
2019-01-04 11:59:59         3560
2019-01-04 10:59:59         2671

但我不想切断第一个窗口。我需要从 14:15 开始的动态时间窗口(14:15、13:15 等等)。它应该看起来像这样:

time                        ticks
2019-01-04 14:14:59         3575
2019-01-04 13:14:59         3575
2019-01-04 12:14:59         3552
2019-01-04 11:14:59         3560
2019-01-04 10:14:59         3575

我该怎么做？

Answer 1

您查询的时间窗口似乎取决于 WHERE 子句中出现的上限范围。由于现在是整点 15 分钟，您可以尝试将时间向前移动 45 分钟进行汇总:

SELECT
    MAX(time) AS time, 
    COUNT(*) AS ticks 
FROM sensorticks 
WHERE time BETWEEN (TIMESTAMP'2019-01-04 00:15:00') AND (TIMESTAMP'2019-01-04 14:15:00') 
GROUP BY date_trunc('hour', time + interval '45 minutes')
ORDER BY time DESC;

但是上面的查询还是有问题，因为显示的时间还是有可能不正确。此外，某些时间段可能会丢失，但您仍然可能想要报告它们。要解决这个问题，我们可以尝试使用日历表:

WITH calendar AS (
    SELECT TIMESTAMP '2019-01-04 14:00:00' AS ts, TIMESTAMP '2019-01-04 14:14:59' AS display UNION ALL
    SELECT TIMESTAMP '2019-01-04 13:00:00', TIMESTAMP '2019-01-04 13:14:59' UNION ALL
    SELECT TIMESTAMP '2019-01-04 12:00:00', TIMESTAMP '2019-01-04 12:14:59' UNION ALL
    SELECT TIMESTAMP '2019-01-04 11:00:00', TIMESTAMP '2019-01-04 11:14:59' UNION ALL
    SELECT TIMESTAMP '2019-01-04 10:00:00', TIMESTAMP '2019-01-04 10:14:59'
)

然后，我们可以使用原始查询连接到这个日历表:

SELECT
    c.display AS time, 
    COUNT(*) AS ticks 
FROM calendar c
LEFT JOIN sensorticks s
    ON c.ts = date_trunc('hour', s.time + interval '45 minutes')
WHERE s.time BETWEEN (TIMESTAMP'2019-01-04 00:15:00') AND (TIMESTAMP'2019-01-04 14:15:00') 
GROUP BY c.display
ORDER BY c.display DESC;