Re: "Constraint exclusion" is not general enough

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.

On the other hand, if the "consider-replanning" threshold is high enough,
than that additional time really doesn't matter - If a query runs for minutes,
or even hours, a few wasted cycles during planning don't hurt.

greetings, Florian Pflug