calculating average with grouping based on time intervals - sql

Question

In a postgres table I have store the speed of an object with a 10 seconds interval. The values are not available for every 10 seconds during the day; so it could be that there is no line for today 16:39:40
How would the query look like to get an relation containing the average of the speed for 1 minute (or 30sec or n-sec) intervals for a given day, assuming the non-existing rows mean a speed of 0.
speed_table
id (int, pk)
ts (timestamp)
speed (numeric)
I've built this query but am getting stuck on some important parts:
SELECT
date_trunc('minute', ts) AS truncated,
avg(speed)
FROM speed_table AS t
WHERE ts >= '2014-06-21 00:00:00'
AND ts <= '2014-06-21 23:59:59'
AND condition2 = 'something'
GROUP BY date_trunc('minute', ts)
ORDER BY truncated
How can I alter the interval in something other then the result of the date_trunc function eg 5 minutes of 30 seconds?
How can I add the not available rows for the remaining of the day?

Answer 1

Simple and fast solution for this particular example:
SELECT date_trunc('minute', ts) AS minute
, sum(speed)/6 AS avg_speed
FROM speed_table AS t
WHERE ts >= '2014-06-21 0:0'
AND ts < '2014-06-20 0:0' -- exclude dangling corner case
AND condition2 = 'something'
GROUP BY 1
ORDER BY 1;
You need to factor in missing rows as "0 speed". Since a minute has 6 samples, just sum and divide by 6. Missing rows evaluate to 0 implicitly.
This returns no row for minutes with no rows at all.avg_speed for missing result rows is 0.
General query for arbitrary intervals
Works for all any interval listed in the manual for date_trunc():
SELECT date_trunc('minute', g.ts) AS ts_start
, avg(COALESCE(speed, 0)) AS avg_speed
FROM (SELECT generate_series('2014-06-21 0:0'::timestamp
, '2014-06-22 0:0'::timestamp
, '10 sec'::interval) AS ts) g
LEFT JOIN speed_table t USING (ts)
WHERE (t.condition2 = 'something' OR
t.condition2 IS NULL) -- depends on actual condition!
AND g.ts <> '2014-06-22 0:0'::timestamp -- exclude dangling corner case
GROUP BY 1
ORDER BY 1;
The problematic part is the additional unknown condition. You would need to define that. And decide whether missing rows supplied by generate_series should pass the test or not (which can be tricky!).
I let them pass in my example (and all other rows with a NULL values).
Compare:
PostgreSQL: running count of rows for a query 'by minute'
Arbitrary intervals:
Truncate timestamp to arbitrary intervals
For completely arbitrary intervals consider #Clodoaldo's math based on epoch values or use the often overlooked function width_bucket(). Example:
Aggregating (x,y) coordinate point clouds in PostgreSQL
Aggregating (x,y) coordinate point clouds in PostgreSQL

Answer 2

If you had issued some data it would be possible to test so this can contain errors. Point them including the error message so I can fix.
select
to_timestamp(
(extract(epoch from ts)::integer / (60 * 2)) * (60 * 2)
) as truncated,
avg(coalesce(speed, 0)) as avg_speed
from
generate_series (
'2014-06-21 00:00:00'::timestamp,
'2014-06-22'::timestamp - interval '1 second',
'10 seconds'
) ts (ts)
left join
speed_table t on ts.ts = t.ts and condition2 = 'something'
group by 1
order by 1
The example is grouped by 30 seconds. It is number of seconds since 1970-01-01 00:00:00 (epoch) divided by 120. When you want to group by 5 minutes divide it by 12 (60 / 5).
The generate_series in the example is generating timestamps at 1 second interval. It is left outer joined to the speed table so it fills the gaps. When the speed is null then coalesce returns 0.