Full table scan: 300 million rows

Hi,

I have the following query:

Select avg(d.amount) AS amount, y.year
>From year_ref y
Join month_ref m
On m.year_ref_id = y.id
Join daily d
On d.month_ref_id = m.id
Where y.year Between 1980 And 2000
And m.month = 12
And m.category_id = '001'
And d.daily_flag_id <> 'M'
And exists (
Select 1
From station s1
inner Join city c1
On c1.id = 10663
Where 6371.009
* SQRT(
POW(RADIANS(c1.latitude_decimal -
s1.latitude_decimal), 2)
+ (
COS(RADIANS(c1.latitude_decimal +
s1.latitude_decimal) / 2)
* POW(RADIANS(c1.longitude_decimal -
s1.longitude_decimal), 2)
)
) <= 50
And S1.station_district_id = y.station_district_id
)
Group By y.year

I posted the explain plan as an image (can't figure out how to export it
from the pgadmin3):

http://i.imgur.com/m6YIV.png

I have yet to let this query finish.

Any ideas how I can speed it up?

Thank you!