Re: pg_plan_advice

> Does the plan end up disabled in that case?

I understand that except for join order advice which might not be
honored(due to GEQO's randomness), other forms of advice—like scan types or
join methods—remain effective since those operations are inevitable parts
of the plan regardless of the specific tree structure.
Thanks for the explanation regarding the design philosophy. It clarifies
that the primary goal is stabilizing plans within the optimizer's valid
search space rather than forcing impossible paths. I will keep the
geqo_threshold adjustment in mind if strict structure enforcement is ever
needed.
Regards
Haibo

On Thu, Jan 8, 2026 at 11:53 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Wed, Jan 7, 2026 at 5:47 PM Haibo Yan <tristan(dot)yim(at)gmail(dot)com> wrote:
> > Instead, the planner seems to silently ignore the structural constraint
> of the advice and falls back to a path GEQO can actually find.
>
> Does the plan end up disabled in that case?
>
> > I believe this behavior is acceptable because pg_plan_advice is intended
> to stabilize plans that the optimizer can generate. Since GEQO cannot
> generate Bushy plans, users should not be supplying them.
>
> Right, I agree. A core principal here is that you can only nudge the
> planner towards a plan it would have considered anyway. In the case of
> GEQO, there is some randomness to which plans are considered. Your
> advice will only be reliably take into account if it applies to
> elements that must be part of the final plan. For instance, if you
> advise the use of a sequential scan or an index scan, that should
> work, because that relation has to be scanned somehow. Advice on a
> join method should almost always work, since it can apply to any
> non-leading table. Of course, you also won't be able to advise an
> infeasible join method, but that would be true without GEQO, too.
> Advice on the join order is going to be iffy when using GEQO -- if a
> compatible join order is highly likely to be considered, e.g. because
> you specify something like JOIN_ORDER(just_one) that only sets the
> driving table -- then it'll probably work, but if you give a complete
> join order specification, it probably won't. If you want to avoid
> that, you can adjust geqo_threshold.
>
> Thanks for looking into this.
>
> --
> Robert Haas
> EDB: http://www.enterprisedb.com
>