pgsql: pg_plan_advice: Don't generate FOREIGN_JOIN advice for a single

pg_plan_advice: Don't generate FOREIGN_JOIN advice for a single relation.

A foreign scan can target a single relation while still reaching the
fs_relids branch of pgpa_build_scan() -- for example, when postgres_fdw
pushes an aggregate down over one foreign table. In that case, no
advice should be emitted.

Author: Mahendra Singh Thalor <mahi6run(at)gmail(dot)com>
Co-authored-by: Robert Haas <rhaas(at)postgresql(dot)org>
Discussion: http://postgr.es/m/CAKYtNAofuAJBz6++SeikpCb=Y=MO1QgEuZNJ+KZOP2johF1r4Q@mail.gmail.com

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/89f5f860cc584cf4c531dbc38b614e4db5c61e24

Modified Files
--------------
contrib/pg_plan_advice/Makefile | 4 +-
contrib/pg_plan_advice/meson.build | 5 ++
contrib/pg_plan_advice/pgpa_scan.c | 8 ++-
contrib/pg_plan_advice/t/001_foreign_scan.pl | 83 ++++++++++++++++++++++++++++
4 files changed, 97 insertions(+), 3 deletions(-)