Re: Improve planner cost estimations for alternative subplans

On Fri, Jun 5, 2020 at 9:08 AM Alexey Bashtanov <bashtanov(at)imap(dot)cc> wrote:

>
> In [1] we found a situation where it leads to a suboptimal plan,
> as it bloats the overall cost into large figures,
> a decision related to an outer part of the plan look negligible to the
> planner,
> and as a result it doesn't elaborate on choosing the optimal one.
>
>
Did this geometric average method result in choosing the desired plan for
this case?

> The patch is to fix it. Our linear model for costs cannot quite accommodate
> the piecewise linear matter of alternative subplans,
> so it is based on ugly heuristics and still cannot be very precise,
> but I think it's better than the current one.
>
> Thoughts?
>
>
Is there another place in planner where two alternatives are averaged
together and that cost is used?

To me, it feels a little bit weird that we are averaging together the
startup cost of a plan which will always have a 0 startup cost and a
plan that will always have a non-zero startup cost and the per tuple
cost of a plan that will always have a negligible per tuple cost and one
that might have a very large per tuple cost.

I guess it feels different because instead of comparing alternatives you
are blending them.

I don't have any academic basis for saying that the alternatives costs
shouldn't be averaged together for use in the rest of the plan, so I
could definitely be wrong.

--
Melanie Plageman