Re: disfavoring unparameterized nested loops

On Thu, Sep 29, 2022 at 07:51:47PM -0400, Bruce Momjian wrote:
> On Thu, Sep 29, 2022 at 07:46:18PM -0400, Tom Lane wrote:
> > 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.

Actually, if we wanted to improve things in this area, we should have a
set of queries that don't chose optimal plans we can test with. We used
to see them a lot before we had extended statistics, but I don't
remember seeing many recently, let alone a collection of them. I guess
that is good.

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

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