Re: On disable_cost

On Mon, Apr 1, 2024 at 7:54 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> What I think we're mostly doing in the regression tests is shutting
> off every relevant type of plan except one. I theorize that what we
> actually want to do is tell the planner what we do want to happen,
> rather than what we don't want to happen, but we've got this weird set
> of GUCs that do the opposite of that and we're super-attached to them
> because they've existed forever.

So rather than listing all the things we don't want to happen, we need a
way to force (nay, highly encourage) a particular solution. As our costing
is a based on positive numbers, what if we did something like this in
costsize.c?

Cost disable_cost = 1.0e10;
Cost promotion_cost = 1.0e10; // or higher or lower, depending on
how strongly we want to "beat" disable_costs effects.
...

if (!enable_seqscan)
startup_cost += disable_cost;
else if (promote_seqscan)
startup_cost -= promotion_cost; // or replace "promote" with
"encourage"?

Cheers,
Greg