Re: disfavoring unparameterized nested loops

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.

regards, tom lane