Date: Fri, 11 Jun 2010 07:44:24 -0500
From: Kenneth Marshall <ktm@rice.edu>
To: Anj Adu <fotographs@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>,
	Andy Colson <andy@squeakycode.net>, pgsql-performance@postgresql.org
Subject: Re: slow query performance
Message-ID: <20100611124424.GO3063@aart.is.rice.edu>
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Hi Anj,

That is an indication that your system was less correctly
modeled with a random_page_cost=2 which means that the system
will assume that random I/O is cheaper than it is and will
choose plans based on that model. If this is not the case,
the plan chosen will almost certainly be slower for any
non-trivial query. You can put a 200mph speedometer in a
VW bug but it will never go 200mph.

Regards,
Ken

On Thu, Jun 10, 2010 at 07:54:01PM -0700, Anj Adu wrote:
> I changed random_page_cost=4 (earlier 2) and the performance issue is gone
> 
> I am not clear why a page_cost of 2 on really fast disks would perform badly.
> 
> Thank you for all your help and time.
> 
> On Thu, Jun 10, 2010 at 8:32 AM, Anj Adu <fotographs@gmail.com> wrote:
> > Attached
> >
> > Thank you
> >
> >
> > On Thu, Jun 10, 2010 at 6:28 AM, Robert Haas <robertmhaas@gmail.com> wrote:
> >> On Wed, Jun 9, 2010 at 11:17 PM, Anj Adu <fotographs@gmail.com> wrote:
> >>> The plan is unaltered . There is a separate index on theDate as well
> >>> as one on node_id
> >>>
> >>> I have not specifically disabled sequential scans.
> >>
> >> Please do "SHOW ALL" and attach the results as a text file.
> >>
> >>> This query performs much better on 8.1.9 on a similar sized
> >>> table.(althought the random_page_cost=4 on 8.1.9 and 2 on 8.4.0 )
> >>
> >> Well that could certainly matter...
> >>
> >> --
> >> Robert Haas
> >> EnterpriseDB: http://www.enterprisedb.com
> >> The Enterprise Postgres Company
> >>
> >
> 
> -- 
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