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Re: TB-sized databases

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: TB-sized databases
Date: 2008-03-17 23:47:12
Message-ID: 200803172347.m2HNlCD29489@momjian.us (view raw or flat)
Thread:
Lists: pgsql-performance
I have _not_ added a TODO for this item.  Let me know if one is needed.

---------------------------------------------------------------------------

Tom Lane wrote:
> Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com> writes:
> > Tom Lane wrote:
> >> There's something fishy about this --- given that that plan has a lower
> >> cost estimate, it should've picked it without any artificial
> >> constraints.
> 
> > I think the reason it's not picking it was discussed back in this thread
> > too.
> > http://archives.postgresql.org/pgsql-performance/2005-03/msg00675.php
> > http://archives.postgresql.org/pgsql-performance/2005-03/msg00684.php
> > My offset 0 is forcing the outer join.
> > [Edit: Ugh - meant cartesian join - which helps this kind of query.]
> 
> Ah; I missed the fact that the two relations you want to join first
> don't have any connecting WHERE clause.
> 
> The concern I mentioned in the above thread was basically that I don't
> want the planner to go off chasing Cartesian join paths in general ---
> they're usually useless and would result in an exponential explosion
> in the number of join paths considered in many-table queries.
> 
> However, in this case the reason that the Cartesian join might be
> relevant is that both of them are needed in order to form an inner
> indexscan on the big table.  I wonder if we could drive consideration
> of the Cartesian join off of noticing that.  It'd take some rejiggering
> around best_inner_indexscan(), or somewhere in that general vicinity.
> 
> Way too late for 8.3, but something to think about for next time.
> 
> 			regards, tom lane
> 
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-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>        http://momjian.us
  EnterpriseDB                             http://postgres.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +

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