Re: On disable_cost

Robert Haas <robertmhaas@gmail.com> writes:
> On Fri, Aug 2, 2024 at 9:13 AM David Rowley <dgrowleyml@gmail.com> wrote:
>> ... That way you maintain the
>> existing behaviour of not optimising for disabled node types and don't
>> risk plan changes if the final cost comes out cheaper than the initial
>> cost.

> All three initial_cost_XXX functions have a comment that says "This
> must quickly produce lower-bound estimates of the path's startup and
> total costs," i.e. the final cost should never be cheaper. I'm pretty
> sure that it was the design intention here that no path ever gets
> rejected at the initial cost stage that would have been accepted at
> the final cost stage.

That absolutely is the expectation, and we'd better be careful not
to break it.

            regards, tom lane