Re: [HACKERS] postgres_fdw bug in 9.6

(2018/01/16 11:17), Tom Lane wrote:
> Etsuro Fujita<fujita.etsuro@lab.ntt.co.jp>  writes:
>> (2018/01/16 1:47), Robert Haas wrote:
>>> Hmm, I was thinking that bar and baz wouldn't be constrained to return
>>> just one tuple in that case, but I'm wrong: there would just be one
>>> tuple per relation in that case.  However, that would also be true for
>>> a full join, wouldn't it?
>
>> Consider:
>
>> postgres=# create table bar (a int, b text);
>> postgres=# create table baz (a int, b text);
>> postgres=# insert into bar values (1, 'bar');
>> postgres=# insert into baz values (2, 'baz');
>> postgres=# select * from bar full join baz on bar.a = baz.a;
>>    a |  b  | a |  b
>> ---+-----+---+-----
>>    1 | bar |   |
>>      |     | 2 | baz
>> (2 rows)
>
>> Both relations have one tuple, but the full join produces two join
>> tuples.  I think it would be possible that something like this happens
>> when executing a local join plan for a foreign join that performs a full
>> join remotely.
>
> Doesn't really matter though, does it?  Each of those join rows will
> be processed as a separate EPQ event.

I assume that such a local join plan is executed as part of a FOR UPDATE 
query like the one shown by Robert (the bar/baz foreign join part in 
that query), so I am thinking that those join rows will be processed as 
a single event.

Best regards,
Etsuro Fujita