Method 1: Group by extracted time component

This SQL statement is computing average of the bins which created by grouping the extracted time component 

Explanation of the SQL:

"where" statement contains the filtering, it should include the stream_id (sensor_id), could include start time, end time, source .....

"group by" is to do the grouping, "by 1" meaning group by the first selection, i.e. extract(year from start_time) in this example. it can also by "by 1,2"....

"cast( data ->> 'temperature' as DOUBLE PRECISION)" is to find the 'temperature' in data and convert it to double format. Using the following SQL can make it more responsive ( just running once):

create or replace function cast_to_double(text) returns DOUBLE PRECISION as $$

begin
    -- Note the double casting to avoid infinite recursion.
    return cast($1::varchar as DOUBLE PRECISION);
exception
    when invalid_text_representation then
        return 0.0;
end;
$$ language plpgsql immutable;
create cast (text as DOUBLE PRECISION) with function cast_to_double(text);

"avg" is to get the average, it usually used alone with "group by", you can have sum, count.....

Overview

by returning the average of the datapoints, short the time for streaming. 

then we just need to convert each SQL result to a json. 

Limitation

• Monthly, daily binning will not work. For example, it will group all "December" regardless of year.
• Customized binning, such as water years, can not be used. 

Method 2: Group by pre-generated bins with join

This SQL statement pre-generate bins using "generate_series()" and tstzrange type; then it joins with datapoints table and groups by bins

with bin as (
	select tstzrange(s, s+'1 year'::interval) as r 
	from generate_series('2002-01-01 00:00:00-05'::timestamp, '2017-12-31 23:59:59-05'::timestamp, '1 year') as s
)
select 
	bin.r, 
	avg(cast( data->> 'pH' as DOUBLE PRECISION))
from datapoints
right join bin on datapoints.start_time <@ bin.r
where datapoints.stream_id = 1584
group by 1 
order by 1;

Method 3: Group by pre-generated bins with filter for aggregate function

This SQL statement pre-generate bins using "generate_series()" and tstzrange type; then it uses filter with avg function instead of join

with bin as (
	select tstzrange(s, s+'1 year'::interval) as r 
	from generate_series('2002-01-01 00:00:00-05'::timestamp, '2017-12-31 23:59:59-05'::timestamp, '1 year') as s
)
select 
	bin.r, 
	avg(cast( data->> 'pH' as DOUBLE PRECISION)) filter(where datapoints.start_time <@ bin.r)
from datapoints, bin
where datapoints.stream_id = 1584
group by 1 
order by 1;

Performance

Tested with GLGT production database. Used the stream_id 1584 which has 987,384 datapoints. Used "explain analyze"