Quoting ogjunk-pgjedan(at)yahoo(dot)com:
> 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?
The query analyzer is using the sort to detect and return distinct
values, as well. So there's not much point in trying to remove it.