Group by 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 .....

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

begin

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    -- Note the double casting to avoid infinite recursion.

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    return cast($1::varchar as DOUBLE PRECISION);

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exception

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exception
    when invalid_text_representation

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 then
        return 0.0;

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end;

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$$ language plpgsql immutable;

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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. 

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