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Re: Wrong plan sequential scan instead of an index one

From: Michael Fuhr <mike(at)fuhr(dot)org>
To: Gaetano Mendola <mendola(at)bigfoot(dot)com>
Cc: "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>, Claus Guttesen <kometen(at)gmail(dot)com>
Subject: Re: Wrong plan sequential scan instead of an index one
Date: 2007-03-30 10:46:11
Message-ID: 20070330104611.GA1540@winnie.fuhr.org (view raw or flat)
Thread:
Lists: pgsql-performance
On Fri, Mar 30, 2007 at 12:08:26PM +0200, Gaetano Mendola wrote:
> Claus Guttesen wrote:
> > Try changing random_page_cost from the default 4 to 2 in postgresql.conf:
> > 
> > random_page_cost = 2
> 
> I have tuned that number already at 2.5, lowering it to 2 doesn't change
> the plan.

The following 19-fold overestimate is influencing the rest of the
plan:

  ->  Seq Scan on l_pvcp  (cost=0.00..2.17 rows=19 width=4) (actual time=0.066..0.081 rows=1 loops=1)
        Filter: (value ~~* '%pi%'::text)

Have you tried increasing the statistics target on l_pvcp.value?
I ran your queries against canned data in 8.2.3 and better statistics
resulted in more accurate row count estimates for this and other
parts of the plan.  I don't recall if estimates for non-leading-character
matches in earlier versions can benefit from better statistics.

-- 
Michael Fuhr

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Subject: Re: Wrong plan sequential scan instead of an index one
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