sub select performance

I'm trying to use a subselect and am not sure why performance suffers.

This returns instantly:

SELECT DISTINCT ON (acctsessionid) radacctid from radacct
WHERE username='cjl'
AND acctstoptime IS NOT NULL
AND date_trunc('month', now())=date_trunc('month', acctstoptime);

radacctid
-----------
244983
606131
720282
365422
1152892
949219
1125943
(7 rows)

This returns instantly as well:

SELECT count(radacctid) AS sessions,
sum(acctsessiontime) AS connecttime
FROM radacct
WHERE radacctid IN
(244983, 606131, 720282, 365422, 1152892, 949219, 1125943);

But this can take 15-30 seconds:

SELECT count(radacctid) AS sessions,
sum(acctsessiontime) AS connecttime
FROM radacct
WHERE radacctid IN
(SELECT DISTINCT ON (acctsessionid) radacctid FROM radacct
WHERE username='cjl'
AND acctstoptime IS NOT NULL
AND date_trunc('month', now())=date_trunc('month', acctstoptime));

There are probably many different ways to perform this query. My
main problem is trying to overcome the fact that try as I might,
I can't stop the occasional duplicate accounting record from being
inserted so I have to weed them out, hence the "DISTINCT ON
(acctsessionid)".

Thanks.

--
Chris Linstruth <cjl(at)qnet(dot)com>