postgres_fdw: Push down aggregates to remote servers.

Now that the upper planner uses paths, and now that we have proper hooks
to inject paths into the upper planning process, it's possible for
foreign data wrappers to arrange to push aggregates to the remote side
instead of fetching all of the rows and aggregating them locally.  This
figures to be a massive win for performance, so teach postgres_fdw to
do it.

Jeevan Chalke and Ashutosh Bapat.  Reviewed by Ashutosh Bapat with
additional testing by Prabhat Sahu.  Various mostly cosmetic changes
by me.

contrib/postgres_fdw/postgres_fdw.h

@@ -92,6 +92,9 @@ typedef struct PgFdwRelationInfo
 	RelOptInfo *innerrel;
 	JoinType	jointype;
 	List	   *joinclauses;
+
+	/* Grouping information */
+	List	   *grouped_tlist;
 } PgFdwRelationInfo;
 
 /* in postgres_fdw.c */
@@ -155,7 +158,7 @@ extern void deparseAnalyzeSql(StringInfo buf, Relation rel,
 				  List **retrieved_attrs);
 extern void deparseStringLiteral(StringInfo buf, const char *val);
 extern Expr *find_em_expr_for_rel(EquivalenceClass *ec, RelOptInfo *rel);
-extern List *build_tlist_to_deparse(RelOptInfo *foreign_rel);
+extern List *build_tlist_to_deparse(RelOptInfo *foreignrel);
 extern void deparseSelectStmtForRel(StringInfo buf, PlannerInfo *root,
 						RelOptInfo *foreignrel, List *tlist,
 						List *remote_conds, List *pathkeys,

src/backend/optimizer/plan/createplan.c

@@ -3243,7 +3243,14 @@ create_foreignscan_plan(PlannerInfo *root, ForeignPath *best_path,
 	/* Copy foreign server OID; likewise, no need to make FDW do this */
 	scan_plan->fs_server = rel->serverid;
 
-	/* Likewise, copy the relids that are represented by this foreign scan */
+	/*
+	 * Likewise, copy the relids that are represented by this foreign scan. An
+	 * upper rel doesn't have relids set, but it covers all the base relations
+	 * participating in the underlying scan, so use root's all_baserels.
+	 */
+	if (rel->reloptkind == RELOPT_UPPER_REL)
+		scan_plan->fs_relids = root->all_baserels;
+	else
 		scan_plan->fs_relids = best_path->path.parent->relids;
 
 	/*