Re: On disable_cost

On Tue, Mar 12, 2024 at 4:55 PM David Rowley <dgrowleyml@gmail.com> wrote:
> The primary place I see issues with disabled_cost is caused by
> STD_FUZZ_FACTOR.  When you add 1.0e10 to a couple of modestly costly
> paths, it makes them appear fuzzily the same in cases where one could
> be significantly cheaper than the other. If we were to bump up the
> disable_cost it would make this problem worse.

Hmm, good point.

> So maybe the fix could be to set disable_cost to something like
> 1.0e110 and adjust compare_path_costs_fuzzily to not apply the
> fuzz_factor for paths >= disable_cost.   However, I wonder if that
> risks the costs going infinite after a couple of cartesian joins.

Yeah, I think the disabled flag is a better answer if we can make it
work. No matter what value we pick for disable_cost, it's bound to
skew the planning sometimes.

--
Robert Haas
EDB: http://www.enterprisedb.com