array_agg() with join question

To summarize my question at the top, why is it that when I did the JOIN,
the array_agg results reversed order?

I had a function that ran the following query:

SELECT timeslot, pollgrpid, array_agg(outval)
FROM
(SELECT * FROM rrd.a_current WHERE pollgrpid=8 ORDER BY timeslot,
dsnum) AS foo
WHERE timeslot >= now() - '1 hour'::interval AND timeslot <= now() + '1
hour'::interval
GROUP BY timeslot, pollgrpid
ORDER BY timeslot;

timeslot | pollgrpid | array_agg
------------------------+-----------+----------------------------
2013-05-14 08:58:00-04 | 8 | {1,1,0.00125217437744141}
2013-05-14 09:00:00-04 | 8 | {1,1,0.000999927520751953}
2013-05-14 09:02:00-04 | 8 | {1,1,0.00318312644958496}
2013-05-14 09:04:00-04 | 8 | {1,1,0.000761985778808594}
2013-05-14 09:06:00-04 | 8 | {1,1,0.000777959823608398}
2013-05-14 09:08:00-04 | 8 | {1,1,0.101096868515015}
2013-05-14 09:10:00-04 | 8 | {1,1,0.86168384552002}
2013-05-14 09:12:00-04 | 8 | {1,1,0.00656795501708984}
2013-05-14 09:14:00-04 | 8 | {1,1,0.102259159088135}
2013-05-14 09:16:00-04 | 8 | {1,1,0.000636100769042969}

I wanted to include missing timestamps in my results, so I joined it with
generate_series.

SELECT timeslot, pollgrpid, array_agg(outval)
FROM
( SELECT generate_series(rrd_timeslot('avail', now() - '58
minutes'::interval), now() + '1 hour'::interval, '2 minutes') AS timeslot )
AS bar
LEFT JOIN
(SELECT * FROM rrd.a_current WHERE pollgrpid=8 AND timeslot >= now()
- '1 hour'::interval AND timeslot <= now() + '1 hour'::interval ORDER BY
timeslot, dsnum) AS foo
USING(timeslot)
GROUP BY timeslot, pollgrpid
ORDER BY timeslot;

timeslot | pollgrpid | array_agg
------------------------+-----------+----------------------------
2013-05-14 09:02:00-04 | 8 | {0.00318312644958496,1,1}
2013-05-14 09:04:00-04 | 8 | {0.000761985778808594,1,1}
2013-05-14 09:06:00-04 | 8 | {0.000777959823608398,1,1}
2013-05-14 09:08:00-04 | 8 | {0.101096868515015,1,1}
2013-05-14 09:10:00-04 | 8 | {0.86168384552002,1,1}
2013-05-14 09:12:00-04 | 8 | {0.00656795501708984,1,1}
2013-05-14 09:14:00-04 | 8 | {0.102259159088135,1,1}
2013-05-14 09:16:00-04 | 8 | {0.000636100769042969,1,1}
2013-05-14 09:18:00-04 | 8 | {0.000638008117675781,1,1}
2013-05-14 09:20:00-04 | 8 | {0.174574136734009,1,1}
2013-05-14 09:22:00-04 | 8 | {0.1006920337677,1,1}
2013-05-14 09:24:00-04 | 8 | {0.00069117546081543,1,1}
2013-05-14 09:26:00-04 | 8 | {0.114289045333862,1,1}
2013-05-14 09:28:00-04 | 8 | {0.116230010986328,1,1}
2013-05-14 09:30:00-04 | 8 | {0.0349528789520264,1,1}

The array_agg results are reversed. I had to ODER BY timeslot, dsnum desc
on the right of the join to make it match. I am curious as to why this
happened. I am running 9.2.4.

Thanks,
Woody

iGLASS Networks
www.iglass.net