On Fri, Jun 5, 2020 at 9:08 AM Alexey Bashtanov <bashta...@imap.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
