Re: postgres_fdw join pushdown (was Re: Custom/Foreign-Join-APIs)

On Tue, Feb 16, 2016 at 12:26 PM, Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp
> wrote:

> On 2016/02/16 15:22, Ashutosh Bapat wrote:
>
>> During join planning, the planner tries multiple combinations of joining
>> relations, thus the same base or join relation can be part of multiple
>> of combination. Hence remote_conds or joinclauses will get linked
>> multiple times as they are bidirectional lists, thus breaking linkages
>> of previous join combinations tried. E.g. while planning A join B join C
>> join D planner will come up with combinations like A(B(CD)) or (AB)(CD)
>> or ((AB)C)D etc. and remote_conds from A will first be linked into
>> A(B(CD)), then AB breaking the first linkages.
>>
>
> Exactly, but I don't think that that needs to be considered because we
> have this at the beginning of postgresGetGForeignJoinPaths:
>
> /*
> * Skip if this join combination has been considered already.
> */
> if (joinrel->fdw_private)
> return;
>
>
There will be different joinrels for A(B(CD)) and (AB) where A's
remote_conds need to be pulled up. The check you have mentioned above only
protects us from adding paths multiple times to (AB) when we encounter it
for (AB)(CD) and ((AB)C)D.

--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company