Re: Foreign join pushdown vs EvalPlanQual

On 2015/10/20 13:11, Etsuro Fujita wrote:
> On 2015/10/20 5:34, Robert Haas wrote:
>> On Mon, Oct 19, 2015 at 3:45 AM, Etsuro Fujita
>> <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>>> As Tom mentioned, just recomputing the original join tuple is not good
>>> enough. We would need to rejoin the test tuples for the baserels
>>> even if
>>> ROW_MARK_COPY is in use. Consider:
>>>
>>> A=# BEGIN;
>>> A=# UPDATE t SET a = a + 1 WHERE b = 1;
>>> B=# SELECT * from t, ft1, ft2
>>> WHERE t.a = ft1.a AND t.b = ft2.b AND ft1.c = ft2.c FOR UPDATE;
>>> A=# COMMIT;
>>>
>>> where the plan for the SELECT FOR UPDATE is
>>>
>>> LockRows
>>> -> Nested Loop
>>> -> Seq Scan on t
>>> -> Foreign Scan on <ft1, ft2>
>>> Remote SQL: SELECT * FROM ft1 JOIN ft2 WHERE ft1.c = ft2.c
>>> AND ft1.a
>>> = $1 AND ft2.b = $2
>>>
>>> If an EPQ recheck is invoked by the A's UPDATE, just recomputing the
>>> original join tuple from the whole-row image that you proposed would
>>> output
>>> an incorrect result in the EQP recheck since the value a in the updated
>>> version of a to-be-joined tuple in t would no longer match the value
>>> ft1.a
>>> extracted from the whole-row image if the A's UPDATE has committed
>>> successfully. So I think we would need to rejoin the tuples
>>> populated from
>>> the whole-row images for the baserels ft1 and ft2, by executing the
>>> secondary plan with the new parameter values for a and b.

>> No. You just need to populate fdw_recheck_quals correctly, same as
>> for the scan case.

> Yeah, I think we can probably do that for the case where a pushed-down
> join clause is an inner-join one, but I'm not sure that we can do that
> for the case where that clause is an outer-join one. Maybe I'm missing
> something, though.

As I said yesterday, that opinion of me is completely wrong. Sorry for
the incorrectness. Let me explain a little bit more. I still think
that even if ROW_MARK_COPY is in use, we would need to locally rejoin
the tuples populated from the whole-row images for the foreign tables
involved in a remote join, using a secondary plan. Consider eg,

SELECT localtab.*, ft2 from localtab, ft1, ft2
WHERE ft1.x = ft2.x AND ft1.y = localtab.y FOR UPDATE

In this case, since the output of the foreign join would not include any
ft1 columns, I don't think we could do the same thing as for the scan
case, even if populating fdw_recheck_quals correctly. And I think we
would need to rejoin the tuples, using a local join execution plan,
which would have the parameterization for the to-be-pushed-down clause
ft1.y = localtab.y. I'm still missing something, though.

Best regards,
Etsuro Fujita