Re: doc: Make selectivity example match wording

From: "Dian M Fay" <dian(dot)m(dot)fay(at)gmail(dot)com>
To: "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com>
Cc: "PostgreSQL Hackers" <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: doc: Make selectivity example match wording
Date: 2022-07-17 23:07:15
Message-ID: CLIB4H644MFM.1Y9ADT3YL7IZ3@medusa
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On Sat Jul 16, 2022 at 11:23 PM EDT, David G. Johnston wrote:
> Thanks for the review. I generally like everything you said but it made me
> realize that I still didn't really understand the intent behind the
> formula. I spent way too much time working that out for myself, then
> turned what I found useful into this v2 patch.
> It may need some semantic markup still but figured I'd see if the idea
> makes sense.
> I basically rewrote, in a bit different style, the same material into the
> code comments, then proceeded to rework the proof that was already present
> there.
> I did do this in somewhat of a vacuum. I'm not inclined to learn this all
> start-to-end though. If the abrupt style change is unwanted so be it. I'm
> not really sure how much benefit the proof really provides. The comments
> in the docs are probably sufficient for the code as well - just define why
> the three pieces of the formula exist and are packaged into a single
> multiplier called selectivity as an API choice. I suspect once someone
> gets to that comment it is fair to assume some prior knowledge.
> Admittedly, I didn't really come into this that way...

Fair enough, I only know what I can glean from the comments in
eqjoinsel_inner and friends myself. I do think even this smaller change
is valuable because the current example talks about using an algorithm
based on the number of distinct values immediately after showing
n_distinct == -1, so making it clear that this case uses num_rows
instead is helpful.

"This value does get scaled in the non-unique case" again could be more
specific ("since here all values are unique, otherwise the calculation
uses num_distinct" perhaps?). But past that quibble I'm good.

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