Re: pgsql: Code review for foreign/custom join pushdown patch.

On 2015-05-10 18:36:45 +0000, Tom Lane wrote:
> * Consider foreign joins only between foreign tables on the same server,
> rather than between any two foreign tables with the same underlying FDW
> handler function.  In most if not all cases, the FDW would simply have had
> to apply the same-server restriction itself (far more expensively, both for
> lack of caching and because it would be repeated for each combination of
> input sub-joins), or else risk nasty bugs.  Anyone who's really intent on
> doing something outside this restriction can always use the
> set_join_pathlist_hook.

Hm. I'm not sure I can agree with that judgement. It does not seem that
far fetched to want to push joins between two different FDW servers to
one FDW.  Especially with inheritance now being allowed it's quite
possible people will come up with complicated partitioning scheme where
it's good where the server the user is interacting with does as little
as possible.  Possibly it'll also allow to reduce slow network traffic.

Greetings,

Andres Freund