Re: Foreign join pushdown vs EvalPlanQual

On Thu, Nov 12, 2015 at 12:54 AM, Etsuro Fujita
<fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
> Really? I think there would be not a little burden on an FDW author; when
> postgres_fdw delegates to the subplan to the remote server, for example, it
> would need to create a remote join query by looking at tuples possibly
> fetched and stored in estate->es_epqTuple[], send the query and receive the
> result during the callback routine. Furthermore, what I'm most concerned
> about is that wouldn't be efficient. So, my question about that approach is
> whether FDWs really do some thing like that during the callback routine,
> instead of performing a secondary join plan locally. As I said before, I
> know that KaiGai-san considers that that approach would be useful for custom
> joins. But I see zero evidence that there is a good use-case for an FDW.

It could do that. But it could also just invoke a subplan as you are
proposing. Or at least, I think we should set it up so that such a
thing is possible. In which case I don't see the problem.

--
Robert Haas
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