Re: Support "Right Semi Join" plan shapes

On Sun, Jan 7, 2024 at 3:03 PM vignesh C <vignesh21(at)gmail(dot)com> wrote:

> 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.

Thanks
Richard