Re: Dynamic gathering the values for seq_page_cost/xxx_cost

On Tue, Sep 22, 2020 at 10:57 AM Andy Fan <zhihui.fan1213@gmail.com> wrote:
>
>
> My tools set the random_page_cost to 8.6, but based on the fio data, it should be
> set to 12.3 on the same hardware.  and I do see the better plan as well with 12.3.
> Looks too smooth to believe it is true..
>
> The attached result_fio_mytool.tar.gz is my test result.   You can use git show HEAD^^
> is the original plan with 8.6.  git show HEAD^  show the plan changes after we changed
> the random_page_cost. git show HEAD shows the run time statistics changes for these queries.
> I also uploaded the test tool [1] for this for your double check.

The scripts seem to start and stop the server, drop caches for every
query. That's where you are seeing that setting random_page_cost to
fio based ratio provides better plans. But in practice, these costs
need to be set on a server where the queries are run concurrently and
repeatedly. That's where the caching behaviour plays an important
role. Can we write a tool which can recommend costs for that scenario?
How do the fio based cost perform when the queries are run repeatedly?

-- 
Best Wishes,
Ashutosh Bapat