Re: Hybrid Hash/Nested Loop joins and caching results from subplans

On Fri, 12 Mar 2021 at 14:59, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> David Rowley <dgrowleyml(at)gmail(dot)com> writes:
> > On Tue, 23 Feb 2021 at 18:43, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> I doubt it's that bad. We could cache such info in RestrictInfo
> >> for quals, or PathTarget for tlists, without much new notational
> >> overhead. That doesn't cover everything the planner deals with
> >> of course, but it would cover enough that you'd be chasing pretty
> >> small returns to worry about more.
>
> > This seems like a pretty good idea. So I coded it up.
>
> > The 0001 patch adds a has_volatile bool field to RestrictInfo and sets
> > it when building the RestrictInfo.
>
> I'm -1 on doing it exactly that way, because you're expending
> the cost of those lookups without certainty that you need the answer.
> I had in mind something more like the way that we cache selectivity
> estimates in RestrictInfo, in which the value is cached when first
> demanded and then re-used on subsequent checks --- see in
> clause_selectivity_ext, around line 750. You do need a way for the
> field to have a "not known yet" value, but that's not hard. Moreover,
> this sort of approach can be less invasive than what you did here,
> because the caching behavior can be hidden inside
> contain_volatile_functions, rather than having all the call sites
> know about it explicitly.

I coded up something more along the lines of what I think you had in
mind for the 0001 patch.

Updated patches attached.

David