Re: "Constraint exclusion" is not general enough

On Mon, 2006-08-07 at 22:01 -0400, Tom Lane wrote:
> "Florian G. Pflug" <fgp@phlo.org> writes:
> > Tom Lane wrote:
> >> But you don't have any cost numbers until after you've done the plan.
> 
> > Couldn't this work similar to geqo_effort? The planner could
> > try planning the query using only cheap algorithmns, and if
> > the cost exceeds a certain value, it'd restart, and use
> > more sophisticated methods.
> 
> AFAICS this would be a net loss on average.  Most of the time, the
> constraint exclusion code doesn't win, and so throwing away all your
> planning work to try it is going to be a loser most of the time.

If constraint exclusion does not make any changes, mark the plan as
invalid, then there is no need to replan.
    1. Generate plan cheaply    2. If under $threshold, execute query. The cost of further planning       is
significantcompared to executing this potentially       non-optimal plan.    3. Run constraint exclusion. If it changes
theclauses due to       constraint exclusion, mark the plan as invalid. I assume       constraint exclusion is
relativelyself contained.    4. Invalid plan is replanned. Still valid plan (no potential       improvements can be
made)is executed.
 

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