Re: Foreign join pushdown vs EvalPlanQual

On 2015/10/20 5:34, Robert Haas wrote:
> On Mon, Oct 19, 2015 at 3:45 AM, Etsuro Fujita
> <fujita.etsuro@lab.ntt.co.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.

Best regards,
Etsuro Fujita