Re: Should we update the random_page_cost default value?

Hi,

On Mon, Oct 06, 2025 at 02:59:16AM +0200, Tomas Vondra wrote:
> I started looking at how we calculated the 4.0 default back in 2000.
> Unfortunately, there's a lot of info, as Tom pointed out in 2024 [2].
> But he outlined how the experiment worked:
> 
> - generate large table (much bigger than RAM)
> - measure runtime of seq scan
> - measure runtime of full-table index scan
> - calculate how much more expensive a random page access is

Ok, but I also read somewhere (I think it might have been Bruce in a
recent (last few years) discussion of random_page_cost) that on top of
that, we assumed 90% (or was it 95%?) of the queries were cached in
shared_buffers (probably preferably the indexes), so that while random
access is massively slower than sequential access (surely not 4x by
2000) is offset by that. I only quickly read your mail, but I didn't see
any discussion of caching on first glance, or do you think it does not
matter much?


Michael