Aggregate values over a range of hours, every hour

Question

I have a PostgreSQL 9.1 database with a table containing a timestamp and a measuring value

\'2012-10-25 01:00\'   2
\'2012-10-2         


        

Answer 1

The key is to make a virtual table against which to join your results sets. The generate_series function can help do that, in the following manner:

SELECT
    start
    , start + interval '8 hours' as end
FROM (
    SELECT generate_series(
        date'2012-01-01'
        , date'2012-02-02'
        , '1 hour'
    ) AS start
) x;

This produces output something like this:

         start          |          end           
------------------------+------------------------
 2012-01-01 00:00:00+00 | 2012-01-01 08:00:00+00
 2012-01-01 01:00:00+00 | 2012-01-01 09:00:00+00
 2012-01-01 02:00:00+00 | 2012-01-01 10:00:00+00
 2012-01-01 03:00:00+00 | 2012-01-01 11:00:00+00

This gives you something to join your data to. In this way, the following query:

SELECT
    y.start
    , round(avg(ts_val.v))
FROM
    ts_val,
    (
        SELECT
            start
            , start + interval '8 hours' as end
        FROM (
            SELECT generate_series(
                date'2012-01-01'
                , date'2012-02-02'
                , '1 hour'
            ) AS start
        ) x
    ) y
WHERE
    ts BETWEEN y.start AND y.end
GROUP BY
    y.start
ORDER BY
    y.start
;

For the following data

         ts          | v 
---------------------+---
 2012-01-01 01:00:00 | 2
 2012-01-01 09:00:00 | 2
 2012-01-01 10:00:00 | 5
(3 rows)

Will produce the following results:

         start          | round 
------------------------+-------
 2012-01-01 00:00:00+00 |   2.0
 2012-01-01 01:00:00+00 |   2.0
 2012-01-01 02:00:00+00 |   3.5
 2012-01-01 03:00:00+00 |   3.5
 2012-01-01 04:00:00+00 |   3.5
 2012-01-01 05:00:00+00 |   3.5
 2012-01-01 06:00:00+00 |   3.5
 2012-01-01 07:00:00+00 |   3.5
 2012-01-01 08:00:00+00 |   3.5
 2012-01-01 09:00:00+00 |   3.5
 2012-01-01 10:00:00+00 |   5.0
(11 rows)