Re: disfavoring unparameterized nested loops

On Thu, Sep 29, 2022 at 07:46:18PM -0400, Tom Lane wrote:
> Bruce Momjian <bruce(at)momjian(dot)us> writes:
> > I think the point the original poster as making, and I have made in the
> > past, is that even of two optimizer costs are the same, one might be
> > more penalized by misestimation than the other, and we don't have a good
> > way of figuring that into our plan choices.
>
> Agreed, but dealing with uncertainty in those numbers is an enormous
> task if you want to do it right. "Doing it right", IMV, would start
> out by extending all the selectivity estimation functions to include
> error bars; then we could have error bars on rowcount estimates and
> then costs; then we could start adding policies about avoiding plans
> with too large a possible upper-bound cost. Trying to add such
> policy with no data to go on is not going to work well.
>
> I think Peter's point is that a quick-n-dirty patch is likely to make
> as many cases worse as it makes better. That's certainly my opinion
> about the topic.

Agreed on all points --- I was thinking error bars too.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EDB https://enterprisedb.com

Indecision is a decision. Inaction is an action. Mark Batterson