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

On Tue, 11 Aug 2020 at 12:21, Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> On 2020-07-09 10:25:14 +1200, David Rowley wrote:
> > On Thu, 9 Jul 2020 at 04:53, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > > I'm not convinced it's a good idea to introduce a separate executor node
> > > for this. There's a fair bit of overhead in them, and they will only be
> > > below certain types of nodes afaict. It seems like it'd be better to
> > > pull the required calls into the nodes that do parametrized scans of
> > > subsidiary nodes. Have you considered that?
> >
> > I see 41 different node types mentioned in ExecReScan(). I don't
> > really think it would be reasonable to change all those.
>
> But that's because we dispatch ExecReScan mechanically down to every
> single executor node. That doesn't determine how many nodes would need
> to modify to include explicit caching? What am I missing?
>
> Wouldn't we need roughly just nodeNestloop.c and nodeSubplan.c
> integration?

hmm, I think you're right there about those two node types. I'm just
not sure you're right about overloading these node types to act as a
cache. How would you inform users via EXPLAIN ANALYZE of how many
cache hits/misses occurred? What would you use to disable it for an
escape hatch for when the planner makes a bad choice about caching?

David