yet another q

Please forgive the barrage of questions. I'm just learning how to tune
things in postgres and I've still got a bit of learning curve to get over,
apparently. I have done a lot of reading, though, I swear.

I've got two identical queries except for a change of one condition which
cuts the number of rows in half - which also has the effect of eliminating
one partition from consideration (partitioned over time and I cut the time
period in half). The query plans are considerably different as a result.
The net result is that the fast query is 5x faster than the slow query. I'm
curious if the alternate query plan is inherently faster or is it just a
case of the algorithm scaling worse than linearly with the row count, which
certainly wouldn't be surprising. The big win, for me, is that the sort
uses vastly less memory. The slow plan requires work_mem to be 1500MB to
even make it 5x worse. With a more reasonable work_mem (400MB), it drops to
something like 15x worse because it has to sort on disk.

fast plan: http://explain.depesz.com/s/iZ
slow plan: http://explain.depesz.com/s/Dv2

query:

EXPLAIN ANALYZE SELECT
t_lookup.display_name as group,
to_char(t_fact.time, 'DD/MM HH24:MI') as category,
substring(t_lookup.display_name from 1 for 20) as label,
round(sum(t_fact.total_ms)/sum(t_fact.count)) as value
FROM
portal.providers t_lookup,
day_scale_radar_performance_fact t_fact
WHERE
t_fact.probe_type_num < 3
and t_lookup.provider_id = t_fact.provider_id
and t_lookup.provider_owner_customer_id =
t_fact.provider_owner_customer_id
and t_fact.provider_owner_customer_id = 0
and t_fact.time between timezone('UTC', '2010-08-18 15:00:00') -
interval '30 day' and timezone('UTC', '2010-08-18 15:00:00')
GROUP BY
t_fact.provider_owner_customer_id, t_fact.provider_id,
t_lookup.display_name,
t_fact.time
ORDER BY
t_fact.time

table structure:

Table "perf_reporting.abstract_radar_performance_fact"
Column | Type | Modifiers
----------------------------+-----------------------------+-----------
count | bigint | not null
total_ms | bigint | not null
time | timestamp without time zone | not null
market_num | integer | not null
country_num | integer | not null
autosys_num | integer | not null
provider_owner_zone_id | integer | not null
provider_owner_customer_id | integer | not null
provider_id | integer | not null
probe_type_num | integer | not null

with individual indexes on the everything from time to the bottom on the
child tables

and

Table "portal.providers"
Column | Type | Modifiers

----------------------------+-----------------------------+------------------------
btime | timestamp without time zone | not null default
now()
mtime | timestamp without time zone | not null default
now()
version | integer | not null default
1
provider_id | integer | not null
provider_owner_zone_id | integer | not null
provider_owner_customer_id | integer | not null
provider_category_id | integer | not null
name | character varying(255) | not null
display_name | character varying(255) | not null

with indexes on every column with name ending in '_id'