Re: Trimming the cost of ORDER BY in a simple query

Quoting ogjunk-pgjedan@yahoo.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.