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Trimming the cost of ORDER BY in a simple query

From: <ogjunk-pgjedan(at)yahoo(dot)com>
To: pgsql-sql(at)postgresql(dot)org
Subject: Trimming the cost of ORDER BY in a simple query
Date: 2005-05-03 04:35:31
Message-ID: 20050503043531.23516.qmail@web31106.mail.mud.yahoo.com (view raw or flat)
Thread:
Lists: pgsql-sql
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



Responses

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Next:From: Mischa SandbergDate: 2005-05-03 06:31:05
Subject: Re: Trimming the cost of ORDER BY in a simple query
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