Re: Takes too long to fetch the data from database - Mailing list pgsql-performance

From Bruno Wolff III
Subject Re: Takes too long to fetch the data from database
Date
Msg-id 20060509073855.GA11659@wolff.to
Whole thread Raw
In response to Re: Takes too long to fetch the data from database  ("soni de" <soni.de@gmail.com>)
List pgsql-performance
On Tue, May 09, 2006 at 09:24:15 +0530,
  soni de <soni.de@gmail.com> wrote:
>
> EXPLAIN
> pdb=# EXPLAIN ANALYZE select * from wan where kname = 'pluto' order by stime
> limit 50;
> NOTICE:  QUERY PLAN:
>
> Limit  (cost=3515.32..3515.32 rows=50 width=95) (actual time=
> 230492.69..230493.07 rows=50 loops=1)
>  ->  Sort  (cost=3515.32..3515.32 rows=208 width=95) (actual time=
> 230492.68..230493.00 rows=51 loops=1)
>        ->  Seq Scan on wan  (cost=0.00..3507.32 rows=208 width=95) (actual
> time=0.44..229217.38 rows=18306 loops=1)
> Total runtime: 230631.62 msec

Unless you have an index on (kname, stime) the query is going to need to
find the records with a value for kname of 'pluto' and then get the most
recent 50 of them. It looks like there are enough estimated records
with kname = 'pluto', that a sequential scan is being prefered.
Creating an extra index will slow down inserts somewhat, but will speed
up queries like the above significantly, so may be worthwhile for you.
I think later versions of Postgres are smarter, but for sure in 7.2
you will need to write the query like:
SELECT *
  FROM wan
  WHERE kname = 'pluto'
  ORDER BY kname DESC, stime DESC
  LIMIT 50
;

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