Re: Bitmap table scan cost per page formula

On Tue, Dec 19, 2017 at 2:55 PM, Haisheng Yuan <hyuan(at)pivotal(dot)io> wrote:
>
> Below is the graph (credit to Heikki) that plots the total estimated cost
> of a bitmap heap scan, where table size is 10000 pages, and
> random_page_cost=10 and seq_page_cost=1. X axis is the number of pages
> fetche. I.e. on the left, no pages are fetched, and on the right end, at
> 10000, all pages are fetched. The original formula is in black, the new
> formula in red, and the horizontal line, in blue, shows the cost of a Seq
> Scan.
> [image: Inline image 3]
>
>
> Thoughts? Any better ideas?
>

The parabola-shape curve we're getting at present is clearly wrong;
approaching a horizontal line as an asymptote seems much better. However,
shouldn't the red line level off at some level *above* the blue line rather
than *at* the blue line? Reading the index pages isn't free, so a
sequential scan should be preferred when we're going to read the whole
table anyway.

--
Robert Haas
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