RE: Improve selectivity estimate for range queries

Hi,

Thanks for the comments, and I'm sorry for the late reply.

> From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
> Sent: Friday, January 11, 2019 7:04 AM
> > Robert Haas <robertmhaas@gmail.com> writes:
> > On Fri, Dec 21, 2018 at 11:50 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >> A smaller-footprint way to fix the immediate problem might be to
> >> change the values of DEFAULT_INEQ_SEL and friends so that they're
> >> even less likely to be matched by accident.  That is, instead of
> >> 0.3333333333333333, use 0.333333333333342 or some such.
> 
> > That's not a dumb idea, but it seems pretty unprincipled to me, and to
> > be honest I'm kind of surprised that you're not proposing something
> > cleaner.
> 
> The problem is the invasiveness of such a change (large) vs the benefit (not so large).  The
upthread
> patch attempted to add a separate signaling path, but it was very incomplete --- and yet both I
and
> Horiguchi-san thought it was already too messy.
> 
> Maybe at some point we'll go over to something reasonably principled, like adding confidence
intervals
> to all selectivity estimates.  That would be *really* invasive but perhaps would bring enough
benefit
> to justify itself.  But the current patch is just attempting to fix one extremely narrow pain
point,
> and if that is all it's doing it should have a commensurately small footprint.  So I don't think
the
> submitted patch looks good from a cost/benefit standpoint.
> 
Yes, I agree with you.  Indeed the patch I attached is insufficient in cost-effectiveness.
However, I want to solve problems of that real estimates happened to equal to the default 
values such as this case, even though it's a narrow pain point.  So I tried distinguishing
explicitly between real estimates and otherwise as Robert said.

The idea Tom proposed and Horiguchi-san tried seems reasonable, but I'm concerned whether
any range queries really cannot match 0.333333333333342 (or some such) by accident in any 
environments.  Is the way which Horiguchi-san did enough to prove that?


Best regards,
Yuzuko Hosoya
NTT Open Source Software Center