Re: Re: [COMMITTERS] pgsql: Only try to push down foreign joins if the user mapping OIDs mat

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Wed, Mar 16, 2016 at 4:10 AM, Ashutosh Bapat <
> ashutosh(dot)bapat(at)enterprisedb(dot)com> wrote:
>> In 9.5, postgres_fdw allowed to prepare statements involving foreign
>> tables without an associated user mapping as long as planning did not
>> require the user mapping. Remember, planner would require user mapping in
>> case use_remote_estimate is ON for that foreign table. The user mapping
>> would be certainly required at the time of execution. So executing such a
>> prepared statement would throw error. If somebody created a user mapping
>> between prepare and execute, execute would not throw an error. A join can
>> be pushed down only when user mappings associated with the joining
>> relations are known and they match. But given the behavior of 9.5 we should
>> let the prepare succeed, even if the join can not be pushed down because of
>> unknown user mapping. Before this fix, the test was not letting the prepare
>> succeed, which is not compliant with 9.5.

> If a query against a single table with no user mapping is legal, I don't
> see why pushing down a join between two tables neither of which has a user
> mapping shouldn't also be legal.

The key point here is that Ashutosh is arguing on the basis of the
behavior of postgres_fdw, which is not representative of all FDWs.
The core code has no business assuming that all FDWs require user
mappings; file_fdw is a counterexample.

I think the behavior Robert suggests is just fine.

regards, tom lane