From: | Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, David Steele <david(at)pgmasters(dot)net>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: [HACKERS] postgres_fdw bug in 9.6 |
Date: | 2018-01-16 03:00:53 |
Message-ID: | 5A5D6AE5.8070101@lab.ntt.co.jp |
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Lists: | pgsql-hackers |
(2018/01/16 11:17), Tom Lane wrote:
> Etsuro Fujita<fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)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
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