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Re: How to change query planner configuration paramerters

From: Melaka Gunasekara <donmelaka(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Craig Ringer <ringerc(at)ringerc(dot)id(dot)au>, Raghavendra <raghavendra(dot)rao(at)enterprisedb(dot)com>, pgsql-admin(at)postgresql(dot)org
Subject: Re: How to change query planner configuration paramerters
Date: 2011-09-18 16:42:37
Message-ID: CAM4Dy=3bGvoT_9oi--6b0r4=5UHiXmym9mV-dejC5+teTEoLqw@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-admin
Thanks for the information Tom.
It solved my confusion.

On Sun, Sep 18, 2011 at 10:03 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Craig Ringer <ringerc(at)ringerc(dot)id(dot)au> writes:
> > On 18/09/2011 5:51 PM, Melaka Gunasekara wrote:
> >> Merge Full Join (cost=10000000074.40..10000000093.69 rows=1159
> width=286)
>
> >> Can you suggest why the merge join is being suggested when I have
> >> turned it off ?
>
> > AFAIK SETting a join type to "off" really just increases the cost
> > estimate so high that the planner will avoid using it where it has any
> > alternative. In this case, it doesn't seem to think it has any other way
> > to execute the query, or it thinks that any other way will be so
> > incredibly, insanely slow that the merge join is still better.
>
> It's the first of those --- FULL joins are only implemented in the
> mergejoin logic, not in hash or nestloop joins, so there is no other way
> to do this query.  (But as of 9.1, hash joins can do them too.)
>
>                        regards, tom lane
>



-- 
Best Regards,
Melaka

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