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