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

On Wed, 19 Aug 2020 at 16:23, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> David Rowley <dgrowleyml(at)gmail(dot)com> writes:
> > I don't object to making the change. I just object to making it only
> > to put it back again later when someone else speaks up that they'd
> > prefer to keep nodes modular and not overload them in obscure ways.
>
> > So other input is welcome. Is it too weird to overload SubPlan and
> > Nested Loop this way? Or okay to do that if it squeezes out a dozen
> > or so nanoseconds per tuple?
>
> If you need somebody to blame it on, blame it on me - but I agree
> that that is an absolutely horrid abuse of NestLoop. We might as
> well reduce explain.c to a one-liner that prints "Here Be Dragons",
> because no one will understand what this display is telling them.

Thanks for chiming in. I'm relieved it's not me vs everyone else anymore.

> I'm also quite skeptical that adding overhead to nodeNestloop.c
> to support this would actually be a net win once you account for
> what happens in plans where the caching is of no value.

Agreed.

David