Re: Foreign join pushdown vs EvalPlanQual

On Fri, Jul 3, 2015 at 6:25 AM, Etsuro Fujita
<fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
> Can't FDWs get the join information through the root, which I think we would
> pass to the API as the argument?

This is exactly what Tom suggested originally, and it has some appeal,
but neither KaiGai nor I could see how to make it work . Do you have
an idea? It's not too late to go back and change the API.

The problem that was bothering us (or at least what was bothering me)
is that the PlannerInfo provides only a list of SpecialJoinInfo
structures, which don't directly give you the original join order. In
fact, min_righthand and min_lefthand are intended to constraint the
*possible* join orders, and are deliberately designed *not* to specify
a single join order. If you're sending a query to a remote PostgreSQL
node, you don't want to know what all the possible join orders are;
it's the remote side's job to plan the query. You do, however, need
an easy way to identify one join order that you can use to construct a
query. It didn't seem easy to do that without duplicating
make_join_rel(), which seemed like a bad idea.

But maybe there's a good way to do it. Tom wasn't crazy about this
hook both because of the frequency of calls and also because of the
long argument list. I think those concerns are legitimate; I just
couldn't see how to make the other way work.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company