Re: On disable_cost

On Thu, Aug 22, 2024 at 8:07 AM Jelte Fennema-Nio <postgres(at)jeltef(dot)nl> wrote:
> Are the disabled node counts still expected to be stable even with
> GEQO? If not, maybe we should have a way to turn them off after all.
> Although I agree that always disabling them when COSTS OFF is set is
> probably also undesirable. How about a new option, e.g. EXPLAIN
> (DISABLED OFF)

Hmm, I hadn't thought about that. There are no GEQO-specific changes
in this patch, which AFAIK is OK, because I think GEQO just relies on
the core planning machinery to decide everything about the cost of
paths, and is really only experimenting with different join orders. So
I think if it picks the same join order, it should get the same count
of disabled nodes everywhere. If it doesn't pick the same order,
you'll get a different plan entirely.

I don't think I quite want to jump into inventing a new EXPLAIN option
right this minute. I'm not against the idea, but I don't want to jump
into engineering solutions before I understand what the problems are,
so I think we should give this a little time. I'll be a bit surprised
if this doesn't elicit a few strong reactions, but I want to see what
people are actually sad (or, potentially, happy) about.

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