Re: doc: Make selectivity example match wording

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Dian M Fay <dian(dot)m(dot)fay(at)gmail(dot)com>
Cc: "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: doc: Make selectivity example match wording
Date: 2023-10-31 15:42:12
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On Sun, Jul 17, 2022 at 07:07:15PM -0400, Dian M Fay wrote:
> 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.

Patch applied to master.

Bruce Momjian <bruce(at)momjian(dot)us>

Only you can decide what is important to you.

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