On 2016/03/14 16:42, Ashutosh Bapat wrote:
On Mon, Mar 14, 2016 at 9:05 AM, Etsuro Fujita
<fujita.ets...@lab.ntt.co.jp <mailto:fujita.ets...@lab.ntt.co.jp>> wrote:

    Here is the comments for foreign_join_ok in postgres_fdw.c:

      * Assess whether the join between inner and outer relations can be
    pushed down
      * to the foreign server. As a side effect, save information we obtain
    in this
      * function to PgFdwRelationInfo passed in.
      * Joins that satisfy conditions below are safe to push down.
      * 1) Join type is INNER or OUTER (one of LEFT/RIGHT/FULL)
      * 2) Both outer and inner portions are safe to push-down
      * 3) All foreign tables in the join belong to the same foreign server
    and use
      *    the same user mapping.
      * 4) All join conditions are safe to push down
      * 5) No relation has local filter (this can be relaxed for INNER JOIN,
    if we
      *    can move unpushable clauses upwards in the join tree).

    The condition 3 is now checked by the core, so I'd like to remove that
    condition from the above comments.

It was left there intentionally to document all the conditions in one
place (some from the core and some from the FDW itself), for a ready
reference. In case tomorrow core thinks that matching user mapping is
not required, postgres_fdw would still require it to be incorporated.

Thank you for the explanation! I understand the reason, but that seems confusing to me.

    In addition, I'd like to update some related comments in
    src/include/nodes/relation.h and src/backend/optimizer/path/joinpath.c.

Those look fine. Sorry for missing those in the commit and thanks for
providing a patch for the same.

No problem.

Best regards,
Etsuro Fujita

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