Re: Should we update the random_page_cost default value?

Hi,

On 2025-10-07 14:08:27 +0200, Tomas Vondra wrote:
> On 10/7/25 01:56, Andres Freund wrote:
> > A correlated index scan today will not do IO combining, despite being
> > accounted as seq_page_cost. So just doing individual 8kB IOs actually seems to
> > be the appropriate comparison.  Even with table fetches in index scans doing
> > IO combining as part by your work, the reads of the index data itself won't be
> > combined. And I'm sure other things won't be either.
> > 
> 
> But that's the point. If the sequential reads do I/O combining and index
> scans don't (and I don't think that will change anytime soon), then that
> makes sequential I/O much more efficient / cheaper. And we better
> reflect that in the cost somehow. Maybe increasing the random_page_cost
> is not the right/best solution? That's possible.

The table fetch portion of an index scan uses seq_page_cost too, with the
degree of it being used determined by the correlation (c.f. cost_index()).
Given that we use random page cost and sequential page cost both for index
scan and non-index scan related costs, I just don't see how it can make sense
to include index related overheads in random_page_cost but not seq_page_cost.

Greetings,

Andres Freund