Re: Bitmap table scan cost per page formula

On Tue, Dec 19, 2017 at 7:25 PM, Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:

>
> I started playing with this weeks ago (probably during Vitaliy's problem
> report). Is there any reason cost_bitmap_heap_scan shouldn't interpolate
> based
> on correlation from seq_page_cost to rand_page_cost, same as cost_index ?
>

I think that doing something like that is a good idea in general, but
someone has to implement the code, and so far no one seems enthused to do
so. You seem pretty interested in the topic, so....

It is not obvious to me how to pass the correlation from the cost_index up
to the bitmap heap scan, especially not if it has to go through a BitmapAnd
or a BitmapOr to get there. Maybe the optimization would only be used in
the case where there are no BitmapAnd or BitmapOr, at least for a proof of
concept?

I haven't been able to reproduce your test case, but I have not had the
hardware or the time to try very hard. I think next year I'll have more
time, but I don't know about the hardware.

Cheers,

Jeff