Mysql – UNION is slow but both queries are fast in separate

Dunno what else to do about this one. I have one table that has has a start and a stop columns and I want to return the results of it joined both by start and by stop and I want a clear distinction between the two.
Now both queries run fast separatly:

SELECT
            UNIX_TIMESTAMP(CONVERT_TZ(start_dev, '+00:00', GetCarrierTimezone(a0.carrier_id))) AS alertStart,
            NULL AS alertStop,
            c0.name AS carrier_name,
            carrier_image,
            l0.Latitude,
            l0.Longitude
        FROM
            carriers AS c0
                INNER JOIN start_stop AS a0 ON a0.carrier_id = c0.id
                    INNER JOIN pcoarg AS l0 ON a0.startLogId = l0.id
        WHERE
                FIND_IN_SET(a0.carrier_id, '89467,1,64578,222625,45013') > 0
            AND
                start_dev > '2013-03-11 11:46:48'
            AND 
                start_dev = (SELECT MIN(start_dev) FROM start_stop AS a1 WHERE a0.carrier_id = a1.carrier_id AND DATE(a1.start_dev) = DATE(a0.start_dev))
        AND IsNotificationInSchedule(22, start_dev) > 0

So this one takes 0.063.
But if I combine it in a UNION (doesn't matter if it's UNION ALL OR DISTINCT OR WHATEVER) it just takes about 0.400 seconds.

SELECT * FROM
(
    (
        SELECT
            UNIX_TIMESTAMP(CONVERT_TZ(start_dev, '+00:00', GetCarrierTimezone(a0.carrier_id))) AS alertStart,
            NULL AS alertStop,
            c0.name AS carrier_name,
            carrier_image,
            l0.Latitude,
            l0.Longitude
        FROM
            carriers AS c0
                INNER JOIN start_stop AS a0 ON a0.carrier_id = c0.id
                    INNER JOIN pcoarg AS l0 ON a0.startLogId = l0.id
        WHERE
                FIND_IN_SET(a0.carrier_id, '89467,1,64578,222625,45013') > 0
            AND
                start_dev > '2013-03-11 11:46:48'
            AND 
                start_dev = (SELECT MIN(start_dev) FROM start_stop AS a1 WHERE a0.carrier_id = a1.carrier_id AND DATE(a1.start_dev) = DATE(a0.start_dev))
            AND IsNotificationInSchedule(22, start_dev) > 0
    ) UNION ALL (
        SELECT
            NULL AS alertStart,
            UNIX_TIMESTAMP(CONVERT_TZ(stop_dev, '+00:00', GetCarrierTimezone(a0.carrier_id))) AS alertStop,
            c0.name AS carrier_name,
            carrier_image,
            l0.Latitude,
            l0.Longitude
        FROM
            start_stop AS a0
                INNER JOIN carriers AS c0 ON a0.carrier_id = c0.id
                    INNER JOIN pcoarg AS l0 ON a0.stopLogId = l0.id
        WHERE
                FIND_IN_SET(a0.carrier_id, '89467,1,64578,222625,45013') > 0
            AND
                stop_dev > '2013-03-11 11:46:48'
            AND 
                stop_dev = (SELECT MAX(stop_dev) FROM start_stop AS a1 WHERE a0.carrier_id = a1.carrier_id AND DATE(a1.stop_dev) = DATE(a0.stop_dev))
            AND IsNotificationInSchedule(22, start_dev) > 0
    )
) AS startStops
ORDER BY IF(alertStart IS NULL, alertStop, alertStart)

Here is EXPLAIN on single query:

1   PRIMARY c0  ALL PRIMARY             17  Using where
1   PRIMARY a0  ref PRIMARY,startstop_carriers_stopdev_idx,georefidx,startstop_carriers_startdev_idx    startstop_carriers_stopdev_idx  4   test_backoffice.c0.id   72  Using where
1   PRIMARY l0  ref id ASC  id ASC  4   test_backoffice.a0.startLogId   1   Using where
2   DEPENDENT SUBQUERY  a1  ref PRIMARY,startstop_carriers_stopdev_idx,georefidx,startstop_carriers_startdev_idx    startstop_carriers_stopdev_idx  4   test_backoffice.a0.carrier_id   72  Using where; Using index

And here is the EXPLAIN for the JOIN:

1   PRIMARY <derived2>  system                  0   const row not found
2   DERIVED c0  ALL PRIMARY             17  Using where
2   DERIVED a0  ref PRIMARY,startstop_carriers_stopdev_idx,georefidx,startstop_carriers_startdev_idx    startstop_carriers_stopdev_idx  4   test_backoffice.c0.id   72  Using where
2   DERIVED l0  ref id ASC  id ASC  4   test_backoffice.a0.startLogId   1   Using where
3   DEPENDENT SUBQUERY  a1  ref PRIMARY,startstop_carriers_stopdev_idx,georefidx,startstop_carriers_startdev_idx    startstop_carriers_stopdev_idx  4   test_backoffice.a0.carrier_id   72  Using where; Using index
4   UNION   c0  ALL PRIMARY             17  Using where
4   UNION   a0  ref PRIMARY,startstop_carriers_stopdev_idx,georefidx,startstop_carriers_startdev_idx    startstop_carriers_stopdev_idx  4   test_backoffice.c0.id   72  Using where
4   UNION   l0  ref id ASC  id ASC  4   test_backoffice.a0.stopLogId    1   Using where
5   DEPENDENT SUBQUERY  a1  ref PRIMARY,startstop_carriers_stopdev_idx,georefidx,startstop_carriers_startdev_idx    startstop_carriers_stopdev_idx  4   test_backoffice.a0.carrier_id   72  Using where; Using index
    UNION RESULT    <union2,4>  ALL                     

Help on this one would be greatly appreciated. 🙂

EDIT:

I'm getting inconsistent result. If I remove the convert_tz for example and try to get the timezone outside the union I get very fast results, but If I rename the result it automatically goes down to the same underperformante query:

SELECT
    *,
    GetCarrierTimezone(carrier_id) timezone
FROM
(

this takes 0.374s

SELECT
    *,
    GetCarrierTimezone(carrier_id)
FROM
(

while this takes 0.078 (mostly the lag from the db to my machine)..