Re: Foreign join pushdown vs EvalPlanQual

On 2015/11/18 3:19, Robert Haas wrote:
> 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.

I suppose you (and KaiGai-san) are probably right, but I really fail to
see it actually doing that.

Best regards,
Etsuro Fujita