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Re: Planner creating ineffective plans on LEFT OUTER joins

From: Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Planner creating ineffective plans on LEFT OUTER joins
Date: 2008-06-26 21:33:48
Message-ID: 48640B3C.3010401@cheapcomplexdevices.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Tom Lane wrote:
> Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com> writes:
>> Couldn't the planner itself make a good guess if it should
>> keep trying based on the estimated cost?
> 
>> if (the_best_plan_I_found_so_far_looks_like_itll_take_an_hour)
>>    keep_optimizing_for_a_few_minutes
>> if (the_best_plan_I_found_so_far_looks_like_itll_take_0.01ms)
>>    stop_planning_and_run_with_it
> 
> You're operating on a mistaken assumption, which is that we generate a
> complete plan and then generate another.  The places where we'd actually
> be doing something with an effort variable are usually dealing with
> small parts of plans, or even with preparatory calculations before we've
> got anything plan-like at all.  They haven't got a sufficiently holistic
> view of what's happening to apply a rule like the above.

Then could the logic wait until the final plan is computed;
and if that final plan looks very expensive (compared with
the plan time so far), try again with the effort variable
automatically increased?

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