Re: Should heapam_estimate_rel_size consider fillfactor?

On 7/3/23 11:40, Tomas Vondra wrote:
> ...
>
> FWIW the reason why the integer division is intentional is most likely
> that we want "floor" semantics - if there's 10.23 rows per page, that
> really means 10 rows per page.
>
> I doubt it makes a huge difference in this particular place, considering
> we're calculating the estimate from somewhat unreliable values, and then
> use it for rough estimate of relation size.
>
> But from this POV, I think it's more correct to do it "my" way:
>
> density = (usable_bytes_per_page * fillfactor / 100) / tuple_width;
>
> because that's doing *two* separate integer divisions, with floor
> semantics. First we calculate "usable bytes" (rounded down), then
> average number of rows per page (also rounded down).
>
> Corey's formula would do just one integer division. I don't think it
> makes a huge difference, though. I mean, it's just an estimate and so we
> can't expect to be 100% accurate.
>

Pushed, using the formula with two divisions (as in the original patch).

regards

--
Tomas Vondra
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