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

On 2016/02/16 16:02, Ashutosh Bapat wrote:
> On Tue, Feb 16, 2016 at 12:26 PM, Etsuro Fujita
> <fujita.etsuro@lab.ntt.co.jp <mailto:fujita.etsuro@lab.ntt.co.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.

Agreed.

> 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.

Sorry, I don't understand this fully.

Best regards,
Etsuro Fujita