On Tue, Sep 8, 2015 at 5:35 AM, Etsuro Fujita
<fujita.etsuro@lab.ntt.co.jp> wrote:
> On 2015/09/04 0:33, Robert Haas wrote:
>> I'm worried that trawling through that
>> SpecialJoinInfo data will end up needing to duplicate much of
>> make_join_rel and add_paths_to_joinrel. For example, consider:
>>
>> SELECT * FROM verysmall v JOIN (bigft1 FULL JOIN bigft2 ON bigft1.x =
>> bigft2.x) ON v.q = bigft1.q AND v.r = bigft2.r;
>>
>> The best path for this plan is presumably something like this:
>>
>> Nested Loop
>> -> Seq Scan on verysmall v
>> -> Foreign Scan on bigft1 and bigft2
>> Remote SQL: SELECT * FROM bigft1 FULL JOIN bigft2 ON bigft1.x =
>> bigft2.x AND bigft1.q = $1 AND bigft2.r = $2
>>
>> Now, how is the FDW going to figure out that it needs to generate this
>> parameterized path without duplicating this code from
>> add_paths_to_joinrel?
>>
>> /*
>> * Decide whether it's sensible to generate parameterized paths for
>> this
>> * joinrel, and if so, which relations such paths should require.
>> There
>> * is usually no need to create a parameterized result path unless
>> there
>> ...
>>
>> Maybe there's a very simple answer to this question and I'm just not
>> seeing it, but I really don't see how that's going to work.
>
>
> Why don't you look at the "regular" (local join execution) paths that were
> already generated. I think that if we called the FDW at a proper hook
> location, the FDW could probably find a regular path in rel->pathlist of the
> join rel (bigft1, bigft2) that possibly generates something like:
>
> Nested Loop
> -> Seq Scan on verysmall v
> -> Nested Loop
> Join Filter: (bigft1.a = bigft2.a)
> -> Foreign Scan on bigft1
> Remote SQL: SELECT * FROM bigft1 WHERE bigft1.q = $1
> -> Foreign Scan on bigft2
> Remote SQL: SELECT * FROM bigft2 WHERE bigft2.r = $2
>
> From the parameterization of the regular nestloop path for joining bigft1
> and bigft2 locally, I think that the FDW could find that it's sensible to
> generate the foreign-join path for (bigft1, bigft2) with the
> parameterization.
But that path might have already been discarded on the basis of cost.
I think Tom's idea is better: let the FDW consult some state cached
for this purpose in the RelOptInfo.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
