Re: JIT compilation per plan node

On Tue, 20 Feb 2024 at 06:38, David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
>
> On Tue, 20 Feb 2024 at 18:31, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > FWIW, I seriously doubt that an extra walk of the plan tree is even
> > measurable compared to the number of cycles JIT compilation will
> > expend if it's called. So I don't buy your argument here.
> > We would be better off to do this in a way that's clean and doesn't
> > add overhead for non-JIT-enabled builds.
>
> The extra walk of the tree would need to be done for every plan, not
> just the ones where we do JIT. I'd rather find a way to not add this
> extra plan tree walk, especially since the vast majority of cases on
> an average instance won't be doing any JIT.

I'm not saying I'd prefer the extra walk, but I don't think you'd need
to do this extra walk for all plans. Afaict you could skip the extra
walk when top_plan->total_cost < jit_above_cost. i.e. only doing the
extra walk to determine which exact nodes to JIT for cases where we
currently JIT all nodes. That would limit the extra walk overhead to
cases where we currently already spend significant resources on JITing
stuff.