postgres_fdw: Avoid possible misbehavior when RETURNING tableoid column only.

deparseReturningList ended up adding up RETURNING NULL to the code, but
code elsewhere saw an empty list of attributes and concluded that it
should not expect tuples from the remote side.

Etsuro Fujita and Robert Haas, reviewed by Thom Brown

contrib/postgres_fdw/deparse.c

@@ -112,6 +112,7 @@ static void deparseTargetList(StringInfo buf,
 				  PlannerInfo *root,
 				  Index rtindex,
 				  Relation rel,
+				  bool is_returning,
 				  Bitmapset *attrs_used,
 				  List **retrieved_attrs);
 static void deparseReturningList(StringInfo buf, PlannerInfo *root,
@@ -776,7 +777,7 @@ deparseSelectSql(Bitmapset *attrs_used, List **retrieved_attrs,
 	 * Construct SELECT list
 	 */
 	appendStringInfoString(buf, "SELECT ");
-	deparseTargetList(buf, root, foreignrel->relid, rel, attrs_used,
+	deparseTargetList(buf, root, foreignrel->relid, rel, false, attrs_used,
 					  retrieved_attrs);
 
 	/*
@@ -790,7 +791,8 @@ deparseSelectSql(Bitmapset *attrs_used, List **retrieved_attrs,
 
 /*
  * Emit a target list that retrieves the columns specified in attrs_used.
- * This is used for both SELECT and RETURNING targetlists.
+ * This is used for both SELECT and RETURNING targetlists; the is_returning
+ * parameter is true only for a RETURNING targetlist.
  *
  * The tlist text is appended to buf, and we also create an integer List
  * of the columns being retrieved, which is returned to *retrieved_attrs.
@@ -800,6 +802,7 @@ deparseTargetList(StringInfo buf,
 				  PlannerInfo *root,
 				  Index rtindex,
 				  Relation rel,
+				  bool is_returning,
 				  Bitmapset *attrs_used,
 				  List **retrieved_attrs)
 {
@@ -829,6 +832,8 @@ deparseTargetList(StringInfo buf,
 		{
 			if (!first)
 				appendStringInfoString(buf, ", ");
+			else if (is_returning)
+				appendStringInfoString(buf, " RETURNING ");
 			first = false;
 
 			deparseColumnRef(buf, rtindex, i, root);
@@ -846,6 +851,8 @@ deparseTargetList(StringInfo buf,
 	{
 		if (!first)
 			appendStringInfoString(buf, ", ");
+		else if (is_returning)
+			appendStringInfoString(buf, " RETURNING ");
 		first = false;
 
 		appendStringInfoString(buf, "ctid");
@@ -855,7 +862,7 @@ deparseTargetList(StringInfo buf,
 	}
 
 	/* Don't generate bad syntax if no undropped columns */
-	if (first)
+	if (first && !is_returning)
 		appendStringInfoString(buf, "NULL");
 }
 
@@ -1113,11 +1120,8 @@ deparseReturningList(StringInfo buf, PlannerInfo *root,
 	}
 
 	if (attrs_used != NULL)
-	{
-		appendStringInfoString(buf, " RETURNING ");
-		deparseTargetList(buf, root, rtindex, rel, attrs_used,
+		deparseTargetList(buf, root, rtindex, rel, true, attrs_used,
 						  retrieved_attrs);
-	}
 	else
 		*retrieved_attrs = NIL;
 }

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -2408,6 +2408,59 @@ SELECT c1,c2,c3,c4 FROM ft2 ORDER BY c1;
  1104 | 204 | ddd                | 
 (819 rows)
 
+EXPLAIN (verbose, costs off)
+INSERT INTO ft2 (c1,c2,c3) VALUES (9999,999,'foo') RETURNING tableoid::regclass;
+                                                                                           QUERY PLAN                                                                                            
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ Insert on public.ft2
+   Output: (tableoid)::regclass
+   Remote SQL: INSERT INTO "S 1"."T 1"("C 1", c2, c3, c4, c5, c6, c7, c8) VALUES ($1, $2, $3, $4, $5, $6, $7, $8)
+   ->  Result
+         Output: 9999, 999, NULL::integer, 'foo'::text, NULL::timestamp with time zone, NULL::timestamp without time zone, NULL::character varying, 'ft2       '::character(10), NULL::user_enum
+(5 rows)
+
+INSERT INTO ft2 (c1,c2,c3) VALUES (9999,999,'foo') RETURNING tableoid::regclass;
+ tableoid 
+----------
+ ft2
+(1 row)
+
+EXPLAIN (verbose, costs off)
+UPDATE ft2 SET c3 = 'bar' WHERE c1 = 9999 RETURNING tableoid::regclass;
+                                                    QUERY PLAN                                                     
+-------------------------------------------------------------------------------------------------------------------
+ Update on public.ft2
+   Output: (tableoid)::regclass
+   Remote SQL: UPDATE "S 1"."T 1" SET c3 = $2 WHERE ctid = $1
+   ->  Foreign Scan on public.ft2
+         Output: c1, c2, NULL::integer, 'bar'::text, c4, c5, c6, c7, c8, ctid
+         Remote SQL: SELECT "C 1", c2, c4, c5, c6, c7, c8, ctid FROM "S 1"."T 1" WHERE (("C 1" = 9999)) FOR UPDATE
+(6 rows)
+
+UPDATE ft2 SET c3 = 'bar' WHERE c1 = 9999 RETURNING tableoid::regclass;
+ tableoid 
+----------
+ ft2
+(1 row)
+
+EXPLAIN (verbose, costs off)
+DELETE FROM ft2 WHERE c1 = 9999 RETURNING tableoid::regclass;
+                                     QUERY PLAN                                     
+------------------------------------------------------------------------------------
+ Delete on public.ft2
+   Output: (tableoid)::regclass
+   Remote SQL: DELETE FROM "S 1"."T 1" WHERE ctid = $1
+   ->  Foreign Scan on public.ft2
+         Output: ctid
+         Remote SQL: SELECT ctid FROM "S 1"."T 1" WHERE (("C 1" = 9999)) FOR UPDATE
+(6 rows)
+
+DELETE FROM ft2 WHERE c1 = 9999 RETURNING tableoid::regclass;
+ tableoid 
+----------
+ ft2
+(1 row)
+
 -- Test that trigger on remote table works as expected
 CREATE OR REPLACE FUNCTION "S 1".F_BRTRIG() RETURNS trigger AS $$
 BEGIN

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -413,6 +413,15 @@ EXPLAIN (verbose, costs off)
 DELETE FROM ft2 USING ft1 WHERE ft1.c1 = ft2.c2 AND ft1.c1 % 10 = 2;
 DELETE FROM ft2 USING ft1 WHERE ft1.c1 = ft2.c2 AND ft1.c1 % 10 = 2;
 SELECT c1,c2,c3,c4 FROM ft2 ORDER BY c1;
+EXPLAIN (verbose, costs off)
+INSERT INTO ft2 (c1,c2,c3) VALUES (9999,999,'foo') RETURNING tableoid::regclass;
+INSERT INTO ft2 (c1,c2,c3) VALUES (9999,999,'foo') RETURNING tableoid::regclass;
+EXPLAIN (verbose, costs off)
+UPDATE ft2 SET c3 = 'bar' WHERE c1 = 9999 RETURNING tableoid::regclass;
+UPDATE ft2 SET c3 = 'bar' WHERE c1 = 9999 RETURNING tableoid::regclass;
+EXPLAIN (verbose, costs off)
+DELETE FROM ft2 WHERE c1 = 9999 RETURNING tableoid::regclass;
+DELETE FROM ft2 WHERE c1 = 9999 RETURNING tableoid::regclass;
 
 -- Test that trigger on remote table works as expected
 CREATE OR REPLACE FUNCTION "S 1".F_BRTRIG() RETURNS trigger AS $$