Truncate timestamp to arbitrary intervals

Question

I want to plot my_values with some lower resolution on the time axis than I have in the database. I use something like the following SQL to get the average values per time interval (e.g., an hour):

SELECT DATE_TRUNC('hour', my_timestamps) AS my_time_lowres
     , AVG(my_values) 
FROM my_table 
GROUP BY my_time_lowres

---

Using date_trunc() it is possible to reduce the resolution of the timestamp to a certain degree (according to the docs):

select date_trunc('hour', timestamp '2001-02-16 20:38:40') 
-- the output is: 2001-02-16 20:00:00

This way, I can do this for the following interval sizes (and some larger/smaller sizes):

...
second
minute
hour
day
week
...

---

Is there a way to achieve this for other time intervals as well, e.g., 3 hours, 6 hours?

Answer 1

Consider this demo to bring timestamps down to a resolution of 15 minutes and aggregate resulting dupes:

WITH tbl(id, ts) AS ( VALUES
    (1::int, '2012-10-04 00:00:00'::timestamp)
   ,(2, '2012-10-04 18:23:01')
   ,(3, '2012-10-04 18:30:00')
   ,(4, '2012-10-04 18:52:33')
   ,(5, '2012-10-04 18:55:01')
   ,(6, '2012-10-04 18:59:59')
   ,(7, '2012-10-05 11:01:01')
   )
SELECT to_timestamp((extract(epoch FROM ts)::bigint / 900)*900)::timestamp
                                                            AS lower_bound
     , to_timestamp(avg(extract(epoch FROM ts)))::timestamp AS avg_ts
     , count(*) AS ct
FROM   tbl
GROUP  BY 1
ORDER  BY 1;

Result:

     lower_bound     |       avg_ts        | ct
---------------------+---------------------+----
 2012-10-04 00:00:00 | 2012-10-04 00:00:00 |  1
 2012-10-04 18:15:00 | 2012-10-04 18:23:01 |  1
 2012-10-04 18:30:00 | 2012-10-04 18:30:00 |  1
 2012-10-04 18:45:00 | 2012-10-04 18:55:51 |  3
 2012-10-05 11:00:00 | 2012-10-05 11:01:01 |  1

The trick is to extract a unix epoch like @Michael already posted. Integer division lumps them together in buckets of the chosen resolution, because fractional digits are truncated.

I divide by 900, because 15 minutes = 900 seconds.

Multiply by the same number to get the resulting lower_bound. Convert the unix epoch back to a timestamp with to_timestamp().

This works great for intervals that can be represented without fractional digits in the decimal system. For even more versatility use the often overlooked function width_bucket() like I demonstrate in this recent, closely related answer. More explanation, links and an sqlfiddle demo over there.