Don't pass grouped relations to FDWs

build_grouped_rel() makes a flat copy of the input relation, including
its FDW fields.  As a result, when building a grouped join relation
for eager aggregation, add_paths_to_joinrel() called the FDW's
GetForeignJoinPaths for it.  postgres_fdw then failed with "Aggref
found where not expected" on the partial Aggrefs in the target list,
and in any case it would have treated a partially aggregated input as
a plain foreign scan.

FDWs are not prepared to handle grouped relations, so clear serverid,
fdwroutine and fdw_private in build_grouped_rel().

Reported-by: Robert Haas <robertmhaas@gmail.com>
Author: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/CA+Tgmob7iSM9YkRM44VjUDuaCchW-fY54MV5njpTZTL9uNyV4w@mail.gmail.com
Backpatch-through: 19

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -4632,6 +4632,32 @@ select sum(q.a), count(q.b) from ft4 left join (select 13, avg(ft1.c1), sum(ft2.
  650 |    50
 (1 row)
 
+-- Eager aggregation: a partially grouped rel must not be joined remotely
+explain (verbose, costs off)
+select t1.c2, sum(t2.c1) from ft1 t1 inner join ft2 t2 on (t1.c2 = t2.c2) where t1.c1 < 20 group by t1.c2 order by 1;
+                                                                                   QUERY PLAN                                                                                    
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ Foreign Scan
+   Output: t1.c2, (sum(t2.c1))
+   Relations: Aggregate on ((public.ft1 t1) INNER JOIN (public.ft2 t2))
+   Remote SQL: SELECT r1.c2, sum(r2."C 1") FROM ("S 1"."T 1" r1 INNER JOIN "S 1"."T 1" r2 ON (((r2.c2 = r1.c2)) AND ((r1."C 1" < 20)))) GROUP BY 1 ORDER BY r1.c2 ASC NULLS LAST
+(4 rows)
+
+select t1.c2, sum(t2.c1) from ft1 t1 inner join ft2 t2 on (t1.c2 = t2.c2) where t1.c1 < 20 group by t1.c2 order by 1;
+ c2 |  sum   
+----+--------
+  0 |  50500
+  1 |  99200
+  2 |  99400
+  3 |  99600
+  4 |  99800
+  5 | 100000
+  6 | 100200
+  7 | 100400
+  8 | 100600
+  9 | 100800
+(10 rows)
+
 -- Not supported cases
 -- Grouping sets
 explain (verbose, costs off)

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -1440,6 +1440,11 @@ explain (verbose, costs off)
 select sum(q.a), count(q.b) from ft4 left join (select 13, avg(ft1.c1), sum(ft2.c1) from ft1 right join ft2 on (ft1.c1 = ft2.c1)) q(a, b, c) on (ft4.c1 <= q.b);
 select sum(q.a), count(q.b) from ft4 left join (select 13, avg(ft1.c1), sum(ft2.c1) from ft1 right join ft2 on (ft1.c1 = ft2.c1)) q(a, b, c) on (ft4.c1 <= q.b);
 
+-- Eager aggregation: a partially grouped rel must not be joined remotely
+explain (verbose, costs off)
+select t1.c2, sum(t2.c1) from ft1 t1 inner join ft2 t2 on (t1.c2 = t2.c2) where t1.c1 < 20 group by t1.c2 order by 1;
+select t1.c2, sum(t2.c1) from ft1 t1 inner join ft2 t2 on (t1.c2 = t2.c2) where t1.c1 < 20 group by t1.c2 order by 1;
+
 
 -- Not supported cases
 -- Grouping sets

src/backend/optimizer/util/relnode.c

@@ -531,6 +531,13 @@ build_grouped_rel(PlannerInfo *root, RelOptInfo *rel)
 	grouped_rel->nullable_partexprs = NULL;
 	grouped_rel->consider_partitionwise_join = false;
 
+	/*
+	 * clear FDW info; FDWs don't know how to handle grouped relations
+	 */
+	grouped_rel->serverid = InvalidOid;
+	grouped_rel->fdwroutine = NULL;
+	grouped_rel->fdw_private = NULL;
+
 	/*
 	 * clear size estimates
 	 */