Trimming the cost of ORDER BY in a simple query

Hello,

I have a simple query with a pretty high cost (EXPLAIN ...), and I'm
wondering if I can somehow trim it.

Query (shows the last 7 dates):

=> SELECT DISTINCT date_part('year', uu.add_date), date_part('month',
uu.add_date), date_part('day', uu.add_date) FROM user_url uu WHERE
uu.user_id=1 ORDER BY date_part('year', uu.add_date) DESC,
date_part('month', uu.add_date) DESC, date_part('day', uu.add_date)
DESC LIMIT 7;

QUERY PLAN:
---------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=4510.14..4522.93 rows=2 width=8) (actual
time=19.924..20.160 rows=7 loops=1)
-> Unique (cost=4510.14..4522.93 rows=2 width=8) (actual
time=19.919..20.139 rows=7 loops=1)
-> Sort (cost=4510.14..4513.34 rows=1279 width=8) (actual
time=19.915..20.004 rows=78 loops=1)
Sort Key: date_part('year'::text, add_date),
date_part('month'::text, add_date), date_part('day'::text, add_date)
-> Index Scan using foo on user_url uu
(cost=0.00..4444.14 rows=1279 width=8) (actual time=0.095..14.761
rows=1225 loops=1)
Index Cond: (user_id = 1)
Total runtime: 20.313 ms
(7 rows)

It looks like the cost is all in ORDER BY, and if I remove ORDER BY the
execution time goes from 20-90 ms to less than 1 ms.

I do need the 7 most recent add_dates. Is there a more efficient way
of grabbing them?

Thanks,
Otis