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Re: Limit clause not using index

From: John A Meinel <john(at)arbash-meinel(dot)com>
To: Yves Vindevogel <yves(dot)vindevogel(at)implements(dot)be>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Limit clause not using index
Date: 2005-06-21 14:42:36
Message-ID: 42B8275C.8040300@arbash-meinel.com (view raw or flat)
Thread:
Lists: pgsql-performance
Yves Vindevogel wrote:

> Hi,
>
> I have a very simple query on a big table. When I issue a "limit" 
> and/or "offset" clause, the query is not using the index.
> Can anyone explain me this ?

You didn't give enough information. What does you index look like that 
you are expecting it to use?
Generally, you want to have matching columns. So you would want
CREATE INDEX blah ON tblprintjobs(loginuser, desceventdate, desceventtime);

Next, you should post EXPLAIN ANALYZE instead of regular explain, so we 
can have an idea if the planner is actually making correct estimations.

John
=:->

>
> rvponp=# explain select * from tblprintjobs order by loginuser, 
> desceventdate, desceventtime offset 25 limit 25 ;
> QUERY PLAN
> ----------------------------------------------------------------------------------- 
>
> Limit (cost=349860.62..349860.68 rows=25 width=206)
> -> Sort (cost=349860.56..351416.15 rows=622236 width=206)
> Sort Key: loginuser, desceventdate, desceventtime
> -> Seq Scan on tblprintjobs (cost=0.00..25589.36 rows=622236 width=206)
> (4 rows)
>
> rvponp=# explain select * from tblprintjobs order by loginuser, 
> desceventdate, desceventtime ;
> QUERY PLAN
> ----------------------------------------------------------------------------- 
>
> Sort (cost=349860.56..351416.15 rows=622236 width=206)
> Sort Key: loginuser, desceventdate, desceventtime
> -> Seq Scan on tblprintjobs (cost=0.00..25589.36 rows=622236 width=206)
> (3 rows)
>
> Met vriendelijke groeten,
> Bien à vous,
> Kind regards,
>
> *Yves Vindevogel*
> *Implements*



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