From: Tom Lane <tgl@sss.pgh.pa.us>
To: Kuntal Ghosh <kuntalghosh.2007@gmail.com>
cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: WIP patch: distinguish selectivity of < from <= and > from >=
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Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Tue, 04 Jul 2017 11:50:21 -0400"
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Date: Tue, 04 Jul 2017 13:24:08 -0400
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I wrote:
> No, the thing that is bothering me is why it seems to be correct to
> apply a positive correction for ">=", a negative correction for "<",
> and no correction for "<=" or ">".  That seems weird and I can't
> construct a plausible explanation for it.

After further thought, I can put a little more clarity to this, but
it's still not really resolved.  It's easily shown by experiment that
the existing code correctly computes the probability that "x <= p"
where p is the given probe value.  It uses that value as-is for the <
and <= cases, and 1 minus that value for > and >=.  From this statement,
it's clear why the above is the right way to correct matters.  What I
find remarkable is that this is what the loop computes regardless of
which of the four operators is used to probe, and regardless of whether
the probe value p is exactly equal to some histogram boundary value.
That doesn't seem intuitive at all --- when p does match a histogram
entry, you'd think it would matter which operator you probe with.

(Pokes at it some more...) Oh, interesting: it behaves that way except
when p is exactly the lowest histogram entry.  The code is probably
blowing off that edge case without enough thought.

			regards, tom lane


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