Question

To compute the average angle in a table (angles in degrees [0, 360]) I use the following statement:

SELECT 
    (CASE WHEN (a < 0.0) 
    THEN a + 360.0 
    ELSE a END) as angle
FROM (
    SELECT 
        degrees(atan2(avg(sin(radians(x))), avg(cos(radians(x))))) as a
    FROM
        angle_t
    ) as t
UNION
SELECT
    x
FROM
    angle_t

when it came to testing I tried my table containing yahoo weather data:

WITH angle_t(x) AS (
    SELECT 
        cast(wind_direction as double precision)
    FROM
        weather_yahoo
    WHERE 
        time >= current_date - interval '1 days' - interval '1 hours'
    AND 
        time <= current_date - interval '1 days')

The output was:

246.670436944698
250.0
240.0

I wondered why the average angle wasn’t 245 but 246.67… so I ran another test with apparantly equal input data:

WITH angle_t(x) AS (VALUES 
    (240 :: double precision),
    (250))

The output showed the (un-)expected result:

245.0
250.0
240.0

Can anyone explain this to me? (this is PostgreSQL 8.4)

Answer 1

The UNION operator eliminates duplicate entries.

From the documentation [emphasis mine]:

UNION effectively appends the result of query2 to the result of query1 (although there is no guarantee that this is the order in which the rows are actually returned). Furthermore, it eliminates duplicate rows from its result, in the same way as DISTINCT, unless UNION ALL is used.

Therefore, if UNION ALL is used, the explanation for the unexpected outcome is obvious:

246.670436944698
250.0
240.0
250.0