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(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp <mailto: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.

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