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

On Wed, Mar 16, 2016 at 4:10 AM, Ashutosh Bapat <
ashutosh(dot)bapat(at)enterprisedb(dot)com> wrote:

>
> Now what's going on here? It seems to me that either postgres_fdw
>> requires a user mapping (in which case this ought to fail) or it
>> doesn't (in which case this ought to push the join down). I don't
>> understand how working but not pushing the join down can ever be the
>> right behavior.
>>
> 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.

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