Re: BUG #19037: Planner fails on estimating array length with "no relation entry" error

Richard Guo <guofenglinux(at)gmail(dot)com> writes:
> On Sun, Aug 31, 2025 at 6:51 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> The most expedient solution is probably to hack examine_variable
>> so that it doesn't fail if root->simple_rel_array isn't there yet.
>> That seems mighty ugly though.

> As an alternative, I wonder if we could pass root as NULL to
> cost_subplan() when it's called from build_subplan(), and possibly
> also from SS_process_ctes(). At those points, the root does not yet
> contain enough information to safely consult statistics. Meanwhile,
> cost_qual_eval() and the functions it calls are already well-equipped
> to handle a NULL root.

Good suggestion, that does seem more in keeping with established
hacks^H^H^Hpractice. It'll need some commentary about why.

I'm slightly tempted to do the dirty work in cost_subplan() itself,
ie pass a NULL down to cost_qual_eval from there. Right now it
doesn't matter, but maybe in future there would be some other
safer use for the parent root in cost_subplan()? This approach
would force the same behavior for the third caller of cost_subplan,
SS_make_initplan_from_plan. But I don't think it matters there,
since an initplan will by definition not be consulting anything from
the parent plan.

regards, tom lane