One of the tests in CFBot has failed at [1] with: - Relations: (public.ft1 t1) SEMI JOIN (public.ft2 t2) - Remote SQL: SELECT r1."C 1", r1.c2, r1.c3, r1.c4, r1.c5, r1.c6, r1.c7, r1.c8 FROM "S 1"."T 1" r1 WHERE ((r1."C 1" < 20)) AND EXISTS (SELECT NULL FROM "S 1"."T 1" r3 WHERE ((r3."C 1" > 10)) AND ((date(r3.c5) = '1970-01-17'::date)) AND ((r3.c3 = r1.c3))) ORDER BY r1."C 1" ASC NULLS LAST -(4 rows) + Sort Key: t1.c1 + -> Foreign Scan + Output: t1.c1, t1.c2, t1.c3, t1.c4, t1.c5, t1.c6, t1.c7, t1.c8 + Relations: (public.ft1 t1) SEMI JOIN (public.ft2 t2) + Remote SQL: SELECT r1."C 1", r1.c2, r1.c3, r1.c4, r1.c5, r1.c6, r1.c7, r1.c8 FROM "S 1"."T 1" r1 WHERE ((r1."C 1" < 20)) AND EXISTS (SELECT NULL FROM "S 1"."T 1" r3 WHERE ((r3."C 1" > 10)) AND ((date(r3.c5) = '1970-01-17'::date)) AND ((r3.c3 = r1.c3))) +(7 rows)
Thanks. I looked into it and have figured out why the plan differs. With this patch the SEMI JOIN that is pushed down to the remote server is now implemented using JOIN_RIGHT_SEMI, whereas previously it was implemented using JOIN_SEMI. Consequently, this leads to changes in the costs of the paths: path with the sort pushed down to remote server, and path with the sort added atop the foreign join. And at last the latter one wins by a slim margin.
I think we can simply update the expected file to fix this plan diff, as attached.