I have Table in Hadoop in which I have data for different sensor units with a sampling time ts of 1 mSec. I can resample the data for a single unit with a combination of different aggregate functions using the following query in Impala (Let's say I want to resample the data for each 5 minute using LAST_VALUE() as aggregate function):
SELECT DISTINCT *
from ( select ts_resample, unit,
last_value(Val1) over (partition by ts_resample order by ts rows between unbounded preceding and unbounded following) as Val1,
last_value(Val2) over (partition by ts_resample order by ts rows between unbounded preceding and unbounded following) as Val2
from (
SELECT cast(cast(unix_timestamp(cast(ts/1000 as TIMESTAMP))/300 as bigint)*300 as TIMESTAMP) as ts_resample,
ts as ts, unit as unit, Val1 as Val1, Val2 as Val2
FROM Sensor_Data.Table1 WHERE unit='Unit1') as t) as tt
If I am running this query for a single unit then I am getting the correct asnwer and there is no issue.
But if I want to resample the data for each Unit based on some aggregation function e.g. LAST_VALUE() then I am getting the wrong answer and resample result for each unit is same although I have different data for each unit. The query that I am running is given below where I am not specifying any unit in WHERE clause:
SELECT DISTINCT *
from(
select ts_resample, unit,
last_value(Val1) over (partition by ts_resample order by ts rows between unbounded preceding and unbounded following) as Val1,
last_value(Val2) over (partition by ts_resample order by ts rows between unbounded preceding and unbounded following) as Val2
from (
SELECT cast(cast(unix_timestamp(cast(ts/1000 as TIMESTAMP))/300 as bigint)*300 as TIMESTAMP) as ts_resample,
ts as ts, unit as unit, Val1 as Val1, Val2 as Val2
FROM Sensor_Data.Table1) as t) as tt
The result obtained using the above query for present three units in the data is given below:
ts_resample unit Val1 Val2
2020-12-01 00:00:00 unit1 0.8974 10.485
2020-12-01 00:00:00 unit2 0.8974 10.485
2020-12-01 00:00:00 unit3 0.8974 10.485
2020-12-01 00:05:00 unit1 0.9041 11.854
2020-12-01 00:05:00 unit2 0.9041 11.854
2020-12-01 00:05:00 unit3 0.9041 11.854
Actually I want ot get the last value for each unit that is different for each unit. Like below:
ts_resample unit Val1 Val2
2020-12-01 00:00:00 unit1 0.8974 10.485
2020-12-01 00:00:00 unit2 0.9014 11.954
2020-12-01 00:00:00 unit3 0.7854 10.821
2020-12-01 00:05:00 unit1 0.9841 11.125
2020-12-01 00:05:00 unit2 0.8742 10.963
2020-12-01 00:05:00 unit3 0.9632 11.784
Could anybody tell me that what is the problem in my query ?
Thanks
I solved this problem by giving unit information in the partition by with ts_resample. The final solution is given below:
SELECT DISTINCT *
from(
select ts_resample, unit,
last_value(Val1) over (partition by ts_resample, unit order by ts rows between unbounded preceding and unbounded following) as Val1,
last_value(Val2) over (partition by ts_resample, unit order by ts rows between unbounded preceding and unbounded following) as Val2
from (
SELECT cast(cast(unix_timestamp(cast(ts/1000 as TIMESTAMP))/300 as bigint)*300 as TIMESTAMP) as ts_resample,
ts as ts, unit as unit, Val1 as Val1, Val2 as Val2
FROM Sensor_Data.Table1) as t) as tt
After this I have the result as I wanted and showed in my question.