Question about plan

Basically, I have one table A with a set of start times and duration and
another table B with just a set of times. What I want is a result that for
each time in table B count the number of intervals in table A that it is in.

The query is not so difficult but it's a large set. The plan looks like
this:

Aggregate  (cost=17365.57..17671.74 rows=3062 width=28)
  ->  Group  (cost=17365.57..17595.20 rows=30618 width=28)
        ->  Sort  (cost=17365.57..17365.57 rows=30618 width=28)
              ->  Nested Loop  (cost=0.00..14718.41 rows=30618 width=28)
                    ->  Seq Scan on internetusage  (cost=0.00..596.04 rows=191 width=16)
                    ->  Seq Scan on times  (cost=0.00..23.40 rows=1440 width=12)

The problem is that it is running out of memory (after using around 500MB).
I imagine this is because it has to build the entire result in the sort
step. Can someone tell me why it doesn't use a plan like this one?

Aggregate
  ->  Group
        ->  Merge Join
              ->  Sort
                    ->  Seq Scan on internetusage
              ->  Sort
                    ->  Seq Scan on times

This would use a fraction of the memory and in this case that would make it
faster since it could start producing output faster. Using limit 1 has no
effect. The version is 7.0.2

If later versions fix it, let me know. Workarounds would be appreciated
also. The actual query follows:

select date_trunc('day',starttime) + minute as date,
       count(iuid)
from times, internetusage
where date_trunc('month',starttime) = '2001-07-01'
and minute between starttime::time
           and starttime::time + (duration || 'seconds')::interval
group by date, starttime, minute;
--
Martijn van Oosterhout <kleptog@svana.org>
http://svana.org/kleptog/
> It would be nice if someone came up with a certification system that
> actually separated those who can barely regurgitate what they crammed over
> the last few weeks from those who command secret ninja networking powers.