pgsql: Only try to push down foreign joins if the user mapping OIDs mat

diff --git a/contrib/file_fdw/input/file_fdw.source b/contrib/file_fdw/input/file_fdw.source
index 416753d..35db4af 100644
--- a/contrib/file_fdw/input/file_fdw.source
+++ b/contrib/file_fdw/input/file_fdw.source
@@ -166,20 +166,23 @@ DROP TABLE agg;
 SET ROLE file_fdw_superuser;
 SELECT * FROM agg_text ORDER BY a;
 SET ROLE file_fdw_user;
 SELECT * FROM agg_text ORDER BY a;
 SET ROLE no_priv_user;
 SELECT * FROM agg_text ORDER BY a;   -- ERROR
 SET ROLE file_fdw_user;
 \t on
 EXPLAIN (VERBOSE, COSTS FALSE) SELECT * FROM agg_text WHERE a > 0;
 \t off
+-- file FDW allows foreign tables to be accessed without user mapping
+DROP USER MAPPING FOR file_fdw_user SERVER file_server;
+SELECT * FROM agg_text ORDER BY a;
 
 -- privilege tests for object
 SET ROLE file_fdw_superuser;
 ALTER FOREIGN TABLE agg_text OWNER TO file_fdw_user;
 ALTER FOREIGN TABLE agg_text OPTIONS (SET format 'text');
 SET ROLE file_fdw_user;
 ALTER FOREIGN TABLE agg_text OPTIONS (SET format 'text');
 SET ROLE file_fdw_superuser;
 
 -- cleanup
diff --git a/contrib/file_fdw/output/file_fdw.source b/contrib/file_fdw/output/file_fdw.source
index 8719694..26f4999 100644
--- a/contrib/file_fdw/output/file_fdw.source
+++ b/contrib/file_fdw/output/file_fdw.source
@@ -315,31 +315,41 @@ SELECT * FROM agg_text ORDER BY a;   -- ERROR
 ERROR:  permission denied for relation agg_text
 SET ROLE file_fdw_user;
 \t on
 EXPLAIN (VERBOSE, COSTS FALSE) SELECT * FROM agg_text WHERE a > 0;
  Foreign Scan on public.agg_text
    Output: a, b
    Filter: (agg_text.a > 0)
    Foreign File: @abs_srcdir@/data/agg.data
 
 \t off
+-- file FDW allows foreign tables to be accessed without user mapping
+DROP USER MAPPING FOR file_fdw_user SERVER file_server;
+SELECT * FROM agg_text ORDER BY a;
+  a  |    b    
+-----+---------
+   0 | 0.09561
+  42 |  324.78
+  56 |     7.8
+ 100 |  99.097
+(4 rows)
+
 -- privilege tests for object
 SET ROLE file_fdw_superuser;
 ALTER FOREIGN TABLE agg_text OWNER TO file_fdw_user;
 ALTER FOREIGN TABLE agg_text OPTIONS (SET format 'text');
 SET ROLE file_fdw_user;
 ALTER FOREIGN TABLE agg_text OPTIONS (SET format 'text');
 ERROR:  only superuser can change options of a file_fdw foreign table
 SET ROLE file_fdw_superuser;
 -- cleanup
 RESET ROLE;
 DROP EXTENSION file_fdw CASCADE;
-NOTICE:  drop cascades to 8 other objects
+NOTICE:  drop cascades to 7 other objects
 DETAIL:  drop cascades to server file_server
-drop cascades to user mapping for file_fdw_user on server file_server
 drop cascades to user mapping for file_fdw_superuser on server file_server
 drop cascades to user mapping for no_priv_user on server file_server
 drop cascades to foreign table agg_text
 drop cascades to foreign table agg_csv
 drop cascades to foreign table agg_bad
 drop cascades to foreign table text_csv
 DROP ROLE file_fdw_superuser, file_fdw_user, no_priv_user;
diff --git a/contrib/postgres_fdw/expected/postgres_fdw.out b/contrib/postgres_fdw/expected/postgres_fdw.out
index 48bdbef..48a7afa 100644
--- a/contrib/postgres_fdw/expected/postgres_fdw.out
+++ b/contrib/postgres_fdw/expected/postgres_fdw.out
@@ -1699,27 +1699,44 @@ EXECUTE join_stmt;
  30 | 30
  32 |   
  34 |   
  36 | 36
  38 |   
  40 |   
 (10 rows)
 
 -- change the session user to view_owner and execute the statement. Because of
 -- change in session user, the plan should get invalidated and created again.
--- While creating the plan, it should throw error since there is no user mapping
--- available for view_owner.
+-- The join will not be pushed down since the joining relations do not have a
+-- valid user mapping. 
 SET SESSION ROLE view_owner;
 EXPLAIN (COSTS OFF, VERBOSE) EXECUTE join_stmt;
-ERROR:  user mapping not found for "view_owner"
-EXECUTE join_stmt;
-ERROR:  user mapping not found for "view_owner"
+                            QUERY PLAN                            
+------------------------------------------------------------------
+ Limit
+   Output: t1.c1, t2.c1
+   ->  Sort
+         Output: t1.c1, t2.c1
+         Sort Key: t1.c1, t2.c1
+         ->  Hash Left Join
+               Output: t1.c1, t2.c1
+               Hash Cond: (t1.c1 = t2.c1)
+               ->  Foreign Scan on public.ft4 t1
+                     Output: t1.c1, t1.c2, t1.c3
+                     Remote SQL: SELECT c1 FROM "S 1"."T 3"
+               ->  Hash
+                     Output: t2.c1
+                     ->  Foreign Scan on public.ft5 t2
+                           Output: t2.c1
+                           Remote SQL: SELECT c1 FROM "S 1"."T 4"
+(16 rows)
+
 RESET ROLE;
 DEALLOCATE join_stmt;
 CREATE VIEW v_ft5 AS SELECT * FROM ft5;
 -- change owner of v_ft5 to view_owner so that the effective user for scan on
 -- ft5 is view_owner and not the current user.
 ALTER VIEW v_ft5 OWNER TO view_owner;
 -- create a public user mapping for loopback server
 -- drop user mapping for current_user.
 DROP USER MAPPING FOR CURRENT_USER SERVER loopback;
 CREATE USER MAPPING FOR PUBLIC SERVER loopback;
@@ -1762,20 +1779,54 @@ EXPLAIN (COSTS false, VERBOSE) EXECUTE join_stmt;
                ->  Foreign Scan on public.ft5
                      Output: ft5.c1, ft5.c2, ft5.c3
                      Remote SQL: SELECT c1 FROM "S 1"."T 4" ORDER BY c1 ASC NULLS LAST
 (13 rows)
 
 EXECUTE join_stmt;
  c1 | c1 
 ----+----
 (0 rows)
 
+-- Joins involving joins, which can't be pushed down, should not be pushed down
+EXPLAIN (COSTS false, VERBOSE)
+SELECT t1.c1, t2.c1 FROM (ft5 t1 JOIN v_ft5 t2 ON (t1.c1 = t2.c1)) left join (ft5 t3 JOIN v_ft5 t4 ON (t3.c1 = t4.c1)) ON (t1.c1 = t3.c1);
+                            QUERY PLAN                            
+------------------------------------------------------------------
+ Hash Join
+   Output: t1.c1, ft5.c1
+   Hash Cond: (t1.c1 = ft5.c1)
+   ->  Hash Right Join
+         Output: t1.c1
+         Hash Cond: (t3.c1 = t1.c1)
+         ->  Hash Join
+               Output: t3.c1
+               Hash Cond: (t3.c1 = ft5_1.c1)
+               ->  Foreign Scan on public.ft5 t3
+                     Output: t3.c1, t3.c2, t3.c3
+                     Remote SQL: SELECT c1 FROM "S 1"."T 4"
+               ->  Hash
+                     Output: ft5_1.c1
+                     ->  Foreign Scan on public.ft5 ft5_1
+                           Output: ft5_1.c1
+                           Remote SQL: SELECT c1 FROM "S 1"."T 4"
+         ->  Hash
+               Output: t1.c1
+               ->  Foreign Scan on public.ft5 t1
+                     Output: t1.c1
+                     Remote SQL: SELECT c1 FROM "S 1"."T 4"
+   ->  Hash
+         Output: ft5.c1
+         ->  Foreign Scan on public.ft5
+               Output: ft5.c1
+               Remote SQL: SELECT c1 FROM "S 1"."T 4"
+(27 rows)
+
 -- recreate the dropped user mapping for further tests
 CREATE USER MAPPING FOR CURRENT_USER SERVER loopback;
 DROP USER MAPPING FOR PUBLIC SERVER loopback;
 -- ===================================================================
 -- parameterized queries
 -- ===================================================================
 -- simple join
 PREPARE st1(int, int) AS SELECT t1.c3, t2.c3 FROM ft1 t1, ft2 t2 WHERE t1.c1 = $1 AND t2.c1 = $2;
 EXPLAIN (VERBOSE, COSTS false) EXECUTE st1(1, 2);
                                                                QUERY PLAN                                                                
diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index 40bffd6..45873c3 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -3361,20 +3361,30 @@ foreign_join_ok(PlannerInfo *root, RelOptInfo *joinrel, JoinType jointype,
 				JoinPathExtraData *extra)
 {
 	PgFdwRelationInfo *fpinfo;
 	PgFdwRelationInfo *fpinfo_o;
 	PgFdwRelationInfo *fpinfo_i;
 	ListCell   *lc;
 	List	   *joinclauses;
 	List	   *otherclauses;
 
 	/*
+	 * Core code may call GetForeignJoinPaths hook, even when the join relation
+	 * doesn't have a valid user mapping associated with it. See
+	 * build_join_rel() for details. We can not push down such join, since there 
+	 * doesn't exist a user mapping which can be used to connect to the foreign
+	 * server.
+	 */
+	if (!OidIsValid(joinrel->umid))
+		return false;
+	 
+	/*
 	 * We support pushing down INNER, LEFT, RIGHT and FULL OUTER joins.
 	 * Constructing queries representing SEMI and ANTI joins is hard, hence
 	 * not considered right now.
 	 */
 	if (jointype != JOIN_INNER && jointype != JOIN_LEFT &&
 		jointype != JOIN_RIGHT && jointype != JOIN_FULL)
 		return false;
 
 	/*
 	 * If either of the joining relations is marked as unsafe to pushdown, the
diff --git a/contrib/postgres_fdw/sql/postgres_fdw.sql b/contrib/postgres_fdw/sql/postgres_fdw.sql
index 4b88a30..8487318 100644
--- a/contrib/postgres_fdw/sql/postgres_fdw.sql
+++ b/contrib/postgres_fdw/sql/postgres_fdw.sql
@@ -435,25 +435,24 @@ SELECT t1."C 1" FROM "S 1"."T 1" t1, LATERAL (SELECT DISTINCT t2.c1, t3.c1 FROM
 CREATE USER view_owner;
 -- grant privileges on ft4 and ft5 to view_owner
 GRANT ALL ON ft4 TO view_owner;
 GRANT ALL ON ft5 TO view_owner;
 -- prepare statement with current session user
 PREPARE join_stmt AS SELECT t1.c1, t2.c1 FROM ft4 t1 LEFT JOIN ft5 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c1, t2.c1 OFFSET 10 LIMIT 10;
 EXPLAIN (COSTS OFF, VERBOSE) EXECUTE join_stmt;
 EXECUTE join_stmt;
 -- change the session user to view_owner and execute the statement. Because of
 -- change in session user, the plan should get invalidated and created again.
--- While creating the plan, it should throw error since there is no user mapping
--- available for view_owner.
+-- The join will not be pushed down since the joining relations do not have a
+-- valid user mapping. 
 SET SESSION ROLE view_owner;
 EXPLAIN (COSTS OFF, VERBOSE) EXECUTE join_stmt;
-EXECUTE join_stmt;
 RESET ROLE;
 DEALLOCATE join_stmt;
 
 CREATE VIEW v_ft5 AS SELECT * FROM ft5;
 -- change owner of v_ft5 to view_owner so that the effective user for scan on
 -- ft5 is view_owner and not the current user.
 ALTER VIEW v_ft5 OWNER TO view_owner;
 -- create a public user mapping for loopback server
 -- drop user mapping for current_user.
 DROP USER MAPPING FOR CURRENT_USER SERVER loopback;
@@ -463,20 +462,24 @@ CREATE USER MAPPING FOR PUBLIC SERVER loopback;
 PREPARE join_stmt AS SELECT t1.c1, t2.c1 FROM ft5 t1 JOIN v_ft5 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c1, t2.c1 OFFSET 100 LIMIT 10;
 EXPLAIN (COSTS false, VERBOSE) EXECUTE join_stmt;
 EXECUTE join_stmt;
 -- create user mapping for view_owner and execute the prepared statement
 -- the join should not be pushed down since joining relations now use two
 -- different user mappings
 CREATE USER MAPPING FOR view_owner SERVER loopback;
 EXPLAIN (COSTS false, VERBOSE) EXECUTE join_stmt;
 EXECUTE join_stmt;
 
+-- Joins involving joins, which can't be pushed down, should not be pushed down
+EXPLAIN (COSTS false, VERBOSE)
+SELECT t1.c1, t2.c1 FROM (ft5 t1 JOIN v_ft5 t2 ON (t1.c1 = t2.c1)) left join (ft5 t3 JOIN v_ft5 t4 ON (t3.c1 = t4.c1)) ON (t1.c1 = t3.c1);
+
 -- recreate the dropped user mapping for further tests
 CREATE USER MAPPING FOR CURRENT_USER SERVER loopback;
 DROP USER MAPPING FOR PUBLIC SERVER loopback;
 
 -- ===================================================================
 -- parameterized queries
 -- ===================================================================
 -- simple join
 PREPARE st1(int, int) AS SELECT t1.c3, t2.c3 FROM ft1 t1, ft2 t2 WHERE t1.c1 = $1 AND t2.c1 = $2;
 EXPLAIN (VERBOSE, COSTS false) EXECUTE st1(1, 2);
diff --git a/src/backend/foreign/foreign.c b/src/backend/foreign/foreign.c
index 239849b..f1feb85 100644
--- a/src/backend/foreign/foreign.c
+++ b/src/backend/foreign/foreign.c
@@ -24,21 +24,21 @@
 #include "miscadmin.h"
 #include "utils/builtins.h"
 #include "utils/memutils.h"
 #include "utils/rel.h"
 #include "utils/syscache.h"
 
 
 extern Datum pg_options_to_table(PG_FUNCTION_ARGS);
 extern Datum postgresql_fdw_validator(PG_FUNCTION_ARGS);
 
-static HeapTuple find_user_mapping(Oid userid, Oid serverid);
+static HeapTuple find_user_mapping(Oid userid, Oid serverid, bool missing_ok);
 
 /*
  * GetForeignDataWrapper -	look up the foreign-data wrapper by OID.
  */
 ForeignDataWrapper *
 GetForeignDataWrapper(Oid fdwid)
 {
 	Form_pg_foreign_data_wrapper fdwform;
 	ForeignDataWrapper *fdw;
 	Datum		datum;
@@ -216,21 +216,21 @@ GetUserMappingById(Oid umid)
  * PUBLIC mappings (userid == InvalidOid).
  */
 UserMapping *
 GetUserMapping(Oid userid, Oid serverid)
 {
 	Datum		datum;
 	HeapTuple	tp;
 	bool		isnull;
 	UserMapping *um;
 
-	tp = find_user_mapping(userid, serverid);
+	tp = find_user_mapping(userid, serverid, false);
 
 	um = (UserMapping *) palloc(sizeof(UserMapping));
 	um->umid = HeapTupleGetOid(tp);
 	um->userid = userid;
 	um->serverid = serverid;
 
 	/* Extract the umoptions */
 	datum = SysCacheGetAttr(USERMAPPINGUSERSERVER,
 							tp,
 							Anum_pg_user_mapping_umoptions,
@@ -243,66 +243,84 @@ GetUserMapping(Oid userid, Oid serverid)
 	ReleaseSysCache(tp);
 
 	return um;
 }
 
 /*
  * GetUserMappingId - look up the user mapping, and return its OID
  *
  * If no mapping is found for the supplied user, we also look for
  * PUBLIC mappings (userid == InvalidOid).
+ *
+ * If missing_ok is true, the function returns InvalidOid when it does not find
+ * required user mapping. Otherwise, find_user_mapping() throws error if it
+ * does not find required user mapping. 
  */
 Oid
-GetUserMappingId(Oid userid, Oid serverid)
+GetUserMappingId(Oid userid, Oid serverid, bool missing_ok)
 {
 	HeapTuple	tp;
 	Oid			umid;
 
-	tp = find_user_mapping(userid, serverid);
+	tp = find_user_mapping(userid, serverid, missing_ok);
+
+	Assert(missing_ok || tp);
+
+	if (!tp && missing_ok)
+		return InvalidOid;
 
 	/* Extract the Oid */
 	umid = HeapTupleGetOid(tp);
 
 	ReleaseSysCache(tp);
 
 	return umid;
 }
 
 
 /*
  * find_user_mapping - Guts of GetUserMapping family.
  *
  * If no mapping is found for the supplied user, we also look for
  * PUBLIC mappings (userid == InvalidOid).
+ *
+ * If missing_ok is true, the function returns NULL, if it does not find
+ * the required user mapping. Otherwise, it throws error if it does not
+ * find the required user mapping.
  */
 static HeapTuple
-find_user_mapping(Oid userid, Oid serverid)
+find_user_mapping(Oid userid, Oid serverid, bool missing_ok)
 {
 	HeapTuple	tp;
 
 	tp = SearchSysCache2(USERMAPPINGUSERSERVER,
 						 ObjectIdGetDatum(userid),
 						 ObjectIdGetDatum(serverid));
 
 	if (HeapTupleIsValid(tp))
 		return tp;
 
 	/* Not found for the specific user -- try PUBLIC */
 	tp = SearchSysCache2(USERMAPPINGUSERSERVER,
 						 ObjectIdGetDatum(InvalidOid),
 						 ObjectIdGetDatum(serverid));
 
 	if (!HeapTupleIsValid(tp))
-		ereport(ERROR,
-				(errcode(ERRCODE_UNDEFINED_OBJECT),
-				 errmsg("user mapping not found for \"%s\"",
-						MappingUserName(userid))));
+	{
+		if (missing_ok)
+			return NULL;
+		else
+			ereport(ERROR,
+					(errcode(ERRCODE_UNDEFINED_OBJECT),
+					 errmsg("user mapping not found for \"%s\"",
+							MappingUserName(userid))));
+	}
 
 	return tp;
 }
 
 
 /*
  * GetForeignTable - look up the foreign table definition by relation oid.
  */
 ForeignTable *
 GetForeignTable(Oid relid)
diff --git a/src/backend/optimizer/util/relnode.c b/src/backend/optimizer/util/relnode.c
index 763d39d..aea69b0 100644
--- a/src/backend/optimizer/util/relnode.c
+++ b/src/backend/optimizer/util/relnode.c
@@ -176,25 +176,30 @@ build_simple_rel(PlannerInfo *root, int relid, RelOptKind reloptkind)
 	if (rte->relkind == RELKIND_FOREIGN_TABLE)
 	{
 		/*
 		 * This should match what ExecCheckRTEPerms() does.
 		 *
 		 * Note that if the plan ends up depending on the user OID in any
 		 * way - e.g. if it depends on the computed user mapping OID - we must
 		 * ensure that it gets invalidated in the case of a user OID change.
 		 * See RevalidateCachedQuery and more generally the hasForeignJoin
 		 * flags in PlannerGlobal and PlannedStmt.
+		 *
+		 * An FDW may choose not to enforce user mapping, in which case there
+		 * may not be a valid user mapping available for the given foreign
+		 * relation. In such a case rel->umid is set to InvalidOid and
+		 * rel->serverid will be valid.
 		 */
 		Oid		userid;
 
 		userid = OidIsValid(rte->checkAsUser) ? rte->checkAsUser : GetUserId();
-		rel->umid = GetUserMappingId(userid, rel->serverid);
+		rel->umid = GetUserMappingId(userid, rel->serverid, true);
 	}
 	else
 		rel->umid = InvalidOid;
 
 	/* Save the finished struct in the query's simple_rel_array */
 	root->simple_rel_array[relid] = rel;
 
 	/*
 	 * If this rel is an appendrel parent, recurse to build "other rel"
 	 * RelOptInfos for its children.  They are "other rels" because they are
@@ -435,26 +440,30 @@ build_join_rel(PlannerInfo *root,
 	joinrel->joininfo = NIL;
 	joinrel->has_eclass_joins = false;
 
 	/*
 	 * Set up foreign-join fields if outer and inner relation are foreign
 	 * tables (or joins) belonging to the same server and using the same
 	 * user mapping.
 	 *
 	 * Otherwise those fields are left invalid, so FDW API will not be called
 	 * for the join relation.
+	 *
+	 * For FDWs, like file_fdw, which ignore user mapping, user mapping id
+	 * associated with the joining relation may be invalid. A valid serverid
+	 * differntiates between a pushded down join with invalid user mapping and
+	 * a join which can not be pushed down because of user mapping mismatch.
 	 */
 	if (OidIsValid(outer_rel->serverid) &&
 		inner_rel->serverid == outer_rel->serverid &&
 		inner_rel->umid == outer_rel->umid)
 	{
-		Assert(OidIsValid(outer_rel->umid));
 		joinrel->serverid = outer_rel->serverid;
 		joinrel->umid = outer_rel->umid;
 		joinrel->fdwroutine = outer_rel->fdwroutine;
 	}
 
 	/*
 	 * Create a new tlist containing just the vars that need to be output from
 	 * this join (ie, are needed for higher joinclauses or final output).
 	 *
 	 * NOTE: the tlist order for a join rel will depend on which pair of outer
diff --git a/src/include/foreign/foreign.h b/src/include/foreign/foreign.h
index 71f8e55..fb945e9 100644
--- a/src/include/foreign/foreign.h
+++ b/src/include/foreign/foreign.h
@@ -65,21 +65,21 @@ typedef struct ForeignTable
 {
 	Oid			relid;			/* relation Oid */
 	Oid			serverid;		/* server Oid */
 	List	   *options;		/* ftoptions as DefElem list */
 } ForeignTable;
 
 
 extern ForeignServer *GetForeignServer(Oid serverid);
 extern ForeignServer *GetForeignServerByName(const char *name, bool missing_ok);
 extern UserMapping *GetUserMapping(Oid userid, Oid serverid);
-extern Oid GetUserMappingId(Oid userid, Oid serverid);
+extern Oid GetUserMappingId(Oid userid, Oid serverid, bool missing_ok);
 extern UserMapping *GetUserMappingById(Oid umid);
 extern ForeignDataWrapper *GetForeignDataWrapper(Oid fdwid);
 extern ForeignDataWrapper *GetForeignDataWrapperByName(const char *name,
 							bool missing_ok);
 extern ForeignTable *GetForeignTable(Oid relid);
 
 extern List *GetForeignColumnOptions(Oid relid, AttrNumber attnum);
 
 extern Oid	get_foreign_data_wrapper_oid(const char *fdwname, bool missing_ok);
 extern Oid	get_foreign_server_oid(const char *servername, bool missing_ok);

diff --git a/contrib/file_fdw/input/file_fdw.source b/contrib/file_fdw/input/file_fdw.source
index 416753d..35db4af 100644
--- a/contrib/file_fdw/input/file_fdw.source
+++ b/contrib/file_fdw/input/file_fdw.source
@@ -166,20 +166,23 @@ DROP TABLE agg;
 SET ROLE file_fdw_superuser;
 SELECT * FROM agg_text ORDER BY a;
 SET ROLE file_fdw_user;
 SELECT * FROM agg_text ORDER BY a;
 SET ROLE no_priv_user;
 SELECT * FROM agg_text ORDER BY a;   -- ERROR
 SET ROLE file_fdw_user;
 \t on
 EXPLAIN (VERBOSE, COSTS FALSE) SELECT * FROM agg_text WHERE a > 0;
 \t off
+-- file FDW allows foreign tables to be accessed without user mapping
+DROP USER MAPPING FOR file_fdw_user SERVER file_server;
+SELECT * FROM agg_text ORDER BY a;
 
 -- privilege tests for object
 SET ROLE file_fdw_superuser;
 ALTER FOREIGN TABLE agg_text OWNER TO file_fdw_user;
 ALTER FOREIGN TABLE agg_text OPTIONS (SET format 'text');
 SET ROLE file_fdw_user;
 ALTER FOREIGN TABLE agg_text OPTIONS (SET format 'text');
 SET ROLE file_fdw_superuser;
 
 -- cleanup
diff --git a/contrib/file_fdw/output/file_fdw.source b/contrib/file_fdw/output/file_fdw.source
index 8719694..26f4999 100644
--- a/contrib/file_fdw/output/file_fdw.source
+++ b/contrib/file_fdw/output/file_fdw.source
@@ -315,31 +315,41 @@ SELECT * FROM agg_text ORDER BY a;   -- ERROR
 ERROR:  permission denied for relation agg_text
 SET ROLE file_fdw_user;
 \t on
 EXPLAIN (VERBOSE, COSTS FALSE) SELECT * FROM agg_text WHERE a > 0;
  Foreign Scan on public.agg_text
    Output: a, b
    Filter: (agg_text.a > 0)
    Foreign File: @abs_srcdir@/data/agg.data
 
 \t off
+-- file FDW allows foreign tables to be accessed without user mapping
+DROP USER MAPPING FOR file_fdw_user SERVER file_server;
+SELECT * FROM agg_text ORDER BY a;
+  a  |    b    
+-----+---------
+   0 | 0.09561
+  42 |  324.78
+  56 |     7.8
+ 100 |  99.097
+(4 rows)
+
 -- privilege tests for object
 SET ROLE file_fdw_superuser;
 ALTER FOREIGN TABLE agg_text OWNER TO file_fdw_user;
 ALTER FOREIGN TABLE agg_text OPTIONS (SET format 'text');
 SET ROLE file_fdw_user;
 ALTER FOREIGN TABLE agg_text OPTIONS (SET format 'text');
 ERROR:  only superuser can change options of a file_fdw foreign table
 SET ROLE file_fdw_superuser;
 -- cleanup
 RESET ROLE;
 DROP EXTENSION file_fdw CASCADE;
-NOTICE:  drop cascades to 8 other objects
+NOTICE:  drop cascades to 7 other objects
 DETAIL:  drop cascades to server file_server
-drop cascades to user mapping for file_fdw_user on server file_server
 drop cascades to user mapping for file_fdw_superuser on server file_server
 drop cascades to user mapping for no_priv_user on server file_server
 drop cascades to foreign table agg_text
 drop cascades to foreign table agg_csv
 drop cascades to foreign table agg_bad
 drop cascades to foreign table text_csv
 DROP ROLE file_fdw_superuser, file_fdw_user, no_priv_user;
diff --git a/contrib/postgres_fdw/expected/postgres_fdw.out b/contrib/postgres_fdw/expected/postgres_fdw.out
index 48bdbef..48a7afa 100644
--- a/contrib/postgres_fdw/expected/postgres_fdw.out
+++ b/contrib/postgres_fdw/expected/postgres_fdw.out
@@ -1699,27 +1699,44 @@ EXECUTE join_stmt;
  30 | 30
  32 |   
  34 |   
  36 | 36
  38 |   
  40 |   
 (10 rows)
 
 -- change the session user to view_owner and execute the statement. Because of
 -- change in session user, the plan should get invalidated and created again.
--- While creating the plan, it should throw error since there is no user mapping
--- available for view_owner.
+-- The join will not be pushed down since the joining relations do not have a
+-- valid user mapping. 
 SET SESSION ROLE view_owner;
 EXPLAIN (COSTS OFF, VERBOSE) EXECUTE join_stmt;
-ERROR:  user mapping not found for "view_owner"
-EXECUTE join_stmt;
-ERROR:  user mapping not found for "view_owner"
+                            QUERY PLAN                            
+------------------------------------------------------------------
+ Limit
+   Output: t1.c1, t2.c1
+   ->  Sort
+         Output: t1.c1, t2.c1
+         Sort Key: t1.c1, t2.c1
+         ->  Hash Left Join
+               Output: t1.c1, t2.c1
+               Hash Cond: (t1.c1 = t2.c1)
+               ->  Foreign Scan on public.ft4 t1
+                     Output: t1.c1, t1.c2, t1.c3
+                     Remote SQL: SELECT c1 FROM "S 1"."T 3"
+               ->  Hash
+                     Output: t2.c1
+                     ->  Foreign Scan on public.ft5 t2
+                           Output: t2.c1
+                           Remote SQL: SELECT c1 FROM "S 1"."T 4"
+(16 rows)
+
 RESET ROLE;
 DEALLOCATE join_stmt;
 CREATE VIEW v_ft5 AS SELECT * FROM ft5;
 -- change owner of v_ft5 to view_owner so that the effective user for scan on
 -- ft5 is view_owner and not the current user.
 ALTER VIEW v_ft5 OWNER TO view_owner;
 -- create a public user mapping for loopback server
 -- drop user mapping for current_user.
 DROP USER MAPPING FOR CURRENT_USER SERVER loopback;
 CREATE USER MAPPING FOR PUBLIC SERVER loopback;
@@ -1762,20 +1779,54 @@ EXPLAIN (COSTS false, VERBOSE) EXECUTE join_stmt;
                ->  Foreign Scan on public.ft5
                      Output: ft5.c1, ft5.c2, ft5.c3
                      Remote SQL: SELECT c1 FROM "S 1"."T 4" ORDER BY c1 ASC NULLS LAST
 (13 rows)
 
 EXECUTE join_stmt;
  c1 | c1 
 ----+----
 (0 rows)
 
+-- Joins involving joins, which can't be pushed down, should not be pushed down
+EXPLAIN (COSTS false, VERBOSE)
+SELECT t1.c1, t2.c1 FROM (ft5 t1 JOIN v_ft5 t2 ON (t1.c1 = t2.c1)) left join (ft5 t3 JOIN v_ft5 t4 ON (t3.c1 = t4.c1)) ON (t1.c1 = t3.c1);
+                            QUERY PLAN                            
+------------------------------------------------------------------
+ Hash Join
+   Output: t1.c1, ft5.c1
+   Hash Cond: (t1.c1 = ft5.c1)
+   ->  Hash Right Join
+         Output: t1.c1
+         Hash Cond: (t3.c1 = t1.c1)
+         ->  Hash Join
+               Output: t3.c1
+               Hash Cond: (t3.c1 = ft5_1.c1)
+               ->  Foreign Scan on public.ft5 t3
+                     Output: t3.c1, t3.c2, t3.c3
+                     Remote SQL: SELECT c1 FROM "S 1"."T 4"
+               ->  Hash
+                     Output: ft5_1.c1
+                     ->  Foreign Scan on public.ft5 ft5_1
+                           Output: ft5_1.c1
+                           Remote SQL: SELECT c1 FROM "S 1"."T 4"
+         ->  Hash
+               Output: t1.c1
+               ->  Foreign Scan on public.ft5 t1
+                     Output: t1.c1
+                     Remote SQL: SELECT c1 FROM "S 1"."T 4"
+   ->  Hash
+         Output: ft5.c1
+         ->  Foreign Scan on public.ft5
+               Output: ft5.c1
+               Remote SQL: SELECT c1 FROM "S 1"."T 4"
+(27 rows)
+
 -- recreate the dropped user mapping for further tests
 CREATE USER MAPPING FOR CURRENT_USER SERVER loopback;
 DROP USER MAPPING FOR PUBLIC SERVER loopback;
 -- ===================================================================
 -- parameterized queries
 -- ===================================================================
 -- simple join
 PREPARE st1(int, int) AS SELECT t1.c3, t2.c3 FROM ft1 t1, ft2 t2 WHERE t1.c1 = $1 AND t2.c1 = $2;
 EXPLAIN (VERBOSE, COSTS false) EXECUTE st1(1, 2);
                                                                QUERY PLAN                                                                
diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index 40bffd6..45873c3 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -3361,20 +3361,30 @@ foreign_join_ok(PlannerInfo *root, RelOptInfo *joinrel, JoinType jointype,
 				JoinPathExtraData *extra)
 {
 	PgFdwRelationInfo *fpinfo;
 	PgFdwRelationInfo *fpinfo_o;
 	PgFdwRelationInfo *fpinfo_i;
 	ListCell   *lc;
 	List	   *joinclauses;
 	List	   *otherclauses;
 
 	/*
+	 * Core code may call GetForeignJoinPaths hook, even when the join relation
+	 * doesn't have a valid user mapping associated with it. See
+	 * build_join_rel() for details. We can not push down such join, since there 
+	 * doesn't exist a user mapping which can be used to connect to the foreign
+	 * server.
+	 */
+	if (!OidIsValid(joinrel->umid))
+		return false;
+	 
+	/*
 	 * We support pushing down INNER, LEFT, RIGHT and FULL OUTER joins.
 	 * Constructing queries representing SEMI and ANTI joins is hard, hence
 	 * not considered right now.
 	 */
 	if (jointype != JOIN_INNER && jointype != JOIN_LEFT &&
 		jointype != JOIN_RIGHT && jointype != JOIN_FULL)
 		return false;
 
 	/*
 	 * If either of the joining relations is marked as unsafe to pushdown, the
diff --git a/contrib/postgres_fdw/sql/postgres_fdw.sql b/contrib/postgres_fdw/sql/postgres_fdw.sql
index 4b88a30..8487318 100644
--- a/contrib/postgres_fdw/sql/postgres_fdw.sql
+++ b/contrib/postgres_fdw/sql/postgres_fdw.sql
@@ -435,25 +435,24 @@ SELECT t1."C 1" FROM "S 1"."T 1" t1, LATERAL (SELECT DISTINCT t2.c1, t3.c1 FROM
 CREATE USER view_owner;
 -- grant privileges on ft4 and ft5 to view_owner
 GRANT ALL ON ft4 TO view_owner;
 GRANT ALL ON ft5 TO view_owner;
 -- prepare statement with current session user
 PREPARE join_stmt AS SELECT t1.c1, t2.c1 FROM ft4 t1 LEFT JOIN ft5 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c1, t2.c1 OFFSET 10 LIMIT 10;
 EXPLAIN (COSTS OFF, VERBOSE) EXECUTE join_stmt;
 EXECUTE join_stmt;
 -- change the session user to view_owner and execute the statement. Because of
 -- change in session user, the plan should get invalidated and created again.
--- While creating the plan, it should throw error since there is no user mapping
--- available for view_owner.
+-- The join will not be pushed down since the joining relations do not have a
+-- valid user mapping. 
 SET SESSION ROLE view_owner;
 EXPLAIN (COSTS OFF, VERBOSE) EXECUTE join_stmt;
-EXECUTE join_stmt;
 RESET ROLE;
 DEALLOCATE join_stmt;
 
 CREATE VIEW v_ft5 AS SELECT * FROM ft5;
 -- change owner of v_ft5 to view_owner so that the effective user for scan on
 -- ft5 is view_owner and not the current user.
 ALTER VIEW v_ft5 OWNER TO view_owner;
 -- create a public user mapping for loopback server
 -- drop user mapping for current_user.
 DROP USER MAPPING FOR CURRENT_USER SERVER loopback;
@@ -463,20 +462,24 @@ CREATE USER MAPPING FOR PUBLIC SERVER loopback;
 PREPARE join_stmt AS SELECT t1.c1, t2.c1 FROM ft5 t1 JOIN v_ft5 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c1, t2.c1 OFFSET 100 LIMIT 10;
 EXPLAIN (COSTS false, VERBOSE) EXECUTE join_stmt;
 EXECUTE join_stmt;
 -- create user mapping for view_owner and execute the prepared statement
 -- the join should not be pushed down since joining relations now use two
 -- different user mappings
 CREATE USER MAPPING FOR view_owner SERVER loopback;
 EXPLAIN (COSTS false, VERBOSE) EXECUTE join_stmt;
 EXECUTE join_stmt;
 
+-- Joins involving joins, which can't be pushed down, should not be pushed down
+EXPLAIN (COSTS false, VERBOSE)
+SELECT t1.c1, t2.c1 FROM (ft5 t1 JOIN v_ft5 t2 ON (t1.c1 = t2.c1)) left join (ft5 t3 JOIN v_ft5 t4 ON (t3.c1 = t4.c1)) ON (t1.c1 = t3.c1);
+
 -- recreate the dropped user mapping for further tests
 CREATE USER MAPPING FOR CURRENT_USER SERVER loopback;
 DROP USER MAPPING FOR PUBLIC SERVER loopback;
 
 -- ===================================================================
 -- parameterized queries
 -- ===================================================================
 -- simple join
 PREPARE st1(int, int) AS SELECT t1.c3, t2.c3 FROM ft1 t1, ft2 t2 WHERE t1.c1 = $1 AND t2.c1 = $2;
 EXPLAIN (VERBOSE, COSTS false) EXECUTE st1(1, 2);
diff --git a/src/backend/foreign/foreign.c b/src/backend/foreign/foreign.c
index 239849b..f1feb85 100644
--- a/src/backend/foreign/foreign.c
+++ b/src/backend/foreign/foreign.c
@@ -24,21 +24,21 @@
 #include "miscadmin.h"
 #include "utils/builtins.h"
 #include "utils/memutils.h"
 #include "utils/rel.h"
 #include "utils/syscache.h"
 
 
 extern Datum pg_options_to_table(PG_FUNCTION_ARGS);
 extern Datum postgresql_fdw_validator(PG_FUNCTION_ARGS);
 
-static HeapTuple find_user_mapping(Oid userid, Oid serverid);
+static HeapTuple find_user_mapping(Oid userid, Oid serverid, bool missing_ok);
 
 /*
  * GetForeignDataWrapper -	look up the foreign-data wrapper by OID.
  */
 ForeignDataWrapper *
 GetForeignDataWrapper(Oid fdwid)
 {
 	Form_pg_foreign_data_wrapper fdwform;
 	ForeignDataWrapper *fdw;
 	Datum		datum;
@@ -216,21 +216,21 @@ GetUserMappingById(Oid umid)
  * PUBLIC mappings (userid == InvalidOid).
  */
 UserMapping *
 GetUserMapping(Oid userid, Oid serverid)
 {
 	Datum		datum;
 	HeapTuple	tp;
 	bool		isnull;
 	UserMapping *um;
 
-	tp = find_user_mapping(userid, serverid);
+	tp = find_user_mapping(userid, serverid, false);
 
 	um = (UserMapping *) palloc(sizeof(UserMapping));
 	um->umid = HeapTupleGetOid(tp);
 	um->userid = userid;
 	um->serverid = serverid;
 
 	/* Extract the umoptions */
 	datum = SysCacheGetAttr(USERMAPPINGUSERSERVER,
 							tp,
 							Anum_pg_user_mapping_umoptions,
@@ -243,66 +243,84 @@ GetUserMapping(Oid userid, Oid serverid)
 	ReleaseSysCache(tp);
 
 	return um;
 }
 
 /*
  * GetUserMappingId - look up the user mapping, and return its OID
  *
  * If no mapping is found for the supplied user, we also look for
  * PUBLIC mappings (userid == InvalidOid).
+ *
+ * If missing_ok is true, the function returns InvalidOid when it does not find
+ * required user mapping. Otherwise, find_user_mapping() throws error if it
+ * does not find required user mapping. 
  */
 Oid
-GetUserMappingId(Oid userid, Oid serverid)
+GetUserMappingId(Oid userid, Oid serverid, bool missing_ok)
 {
 	HeapTuple	tp;
 	Oid			umid;
 
-	tp = find_user_mapping(userid, serverid);
+	tp = find_user_mapping(userid, serverid, missing_ok);
+
+	Assert(missing_ok || tp);
+
+	if (!tp && missing_ok)
+		return InvalidOid;
 
 	/* Extract the Oid */
 	umid = HeapTupleGetOid(tp);
 
 	ReleaseSysCache(tp);
 
 	return umid;
 }
 
 
 /*
  * find_user_mapping - Guts of GetUserMapping family.
  *
  * If no mapping is found for the supplied user, we also look for
  * PUBLIC mappings (userid == InvalidOid).
+ *
+ * If missing_ok is true, the function returns NULL, if it does not find
+ * the required user mapping. Otherwise, it throws error if it does not
+ * find the required user mapping.
  */
 static HeapTuple
-find_user_mapping(Oid userid, Oid serverid)
+find_user_mapping(Oid userid, Oid serverid, bool missing_ok)
 {
 	HeapTuple	tp;
 
 	tp = SearchSysCache2(USERMAPPINGUSERSERVER,
 						 ObjectIdGetDatum(userid),
 						 ObjectIdGetDatum(serverid));
 
 	if (HeapTupleIsValid(tp))
 		return tp;
 
 	/* Not found for the specific user -- try PUBLIC */
 	tp = SearchSysCache2(USERMAPPINGUSERSERVER,
 						 ObjectIdGetDatum(InvalidOid),
 						 ObjectIdGetDatum(serverid));
 
 	if (!HeapTupleIsValid(tp))
-		ereport(ERROR,
-				(errcode(ERRCODE_UNDEFINED_OBJECT),
-				 errmsg("user mapping not found for \"%s\"",
-						MappingUserName(userid))));
+	{
+		if (missing_ok)
+			return NULL;
+		else
+			ereport(ERROR,
+					(errcode(ERRCODE_UNDEFINED_OBJECT),
+					 errmsg("user mapping not found for \"%s\"",
+							MappingUserName(userid))));
+	}
 
 	return tp;
 }
 
 
 /*
  * GetForeignTable - look up the foreign table definition by relation oid.
  */
 ForeignTable *
 GetForeignTable(Oid relid)
diff --git a/src/backend/optimizer/util/relnode.c b/src/backend/optimizer/util/relnode.c
index 763d39d..aea69b0 100644
--- a/src/backend/optimizer/util/relnode.c
+++ b/src/backend/optimizer/util/relnode.c
@@ -176,25 +176,30 @@ build_simple_rel(PlannerInfo *root, int relid, RelOptKind reloptkind)
 	if (rte->relkind == RELKIND_FOREIGN_TABLE)
 	{
 		/*
 		 * This should match what ExecCheckRTEPerms() does.
 		 *
 		 * Note that if the plan ends up depending on the user OID in any
 		 * way - e.g. if it depends on the computed user mapping OID - we must
 		 * ensure that it gets invalidated in the case of a user OID change.
 		 * See RevalidateCachedQuery and more generally the hasForeignJoin
 		 * flags in PlannerGlobal and PlannedStmt.
+		 *
+		 * An FDW may choose not to enforce user mapping, in which case there
+		 * may not be a valid user mapping available for the given foreign
+		 * relation. In such a case rel->umid is set to InvalidOid and
+		 * rel->serverid will be valid.
 		 */
 		Oid		userid;
 
 		userid = OidIsValid(rte->checkAsUser) ? rte->checkAsUser : GetUserId();
-		rel->umid = GetUserMappingId(userid, rel->serverid);
+		rel->umid = GetUserMappingId(userid, rel->serverid, true);
 	}
 	else
 		rel->umid = InvalidOid;
 
 	/* Save the finished struct in the query's simple_rel_array */
 	root->simple_rel_array[relid] = rel;
 
 	/*
 	 * If this rel is an appendrel parent, recurse to build "other rel"
 	 * RelOptInfos for its children.  They are "other rels" because they are
@@ -435,26 +440,30 @@ build_join_rel(PlannerInfo *root,
 	joinrel->joininfo = NIL;
 	joinrel->has_eclass_joins = false;
 
 	/*
 	 * Set up foreign-join fields if outer and inner relation are foreign
 	 * tables (or joins) belonging to the same server and using the same
 	 * user mapping.
 	 *
 	 * Otherwise those fields are left invalid, so FDW API will not be called
 	 * for the join relation.
+	 *
+	 * For FDWs, like file_fdw, which ignore user mapping, user mapping id
+	 * associated with the joining relation may be invalid. A valid serverid
+	 * differntiates between a pushded down join with invalid user mapping and
+	 * a join which can not be pushed down because of user mapping mismatch.
 	 */
 	if (OidIsValid(outer_rel->serverid) &&
 		inner_rel->serverid == outer_rel->serverid &&
 		inner_rel->umid == outer_rel->umid)
 	{
-		Assert(OidIsValid(outer_rel->umid));
 		joinrel->serverid = outer_rel->serverid;
 		joinrel->umid = outer_rel->umid;
 		joinrel->fdwroutine = outer_rel->fdwroutine;
 	}
 
 	/*
 	 * Create a new tlist containing just the vars that need to be output from
 	 * this join (ie, are needed for higher joinclauses or final output).
 	 *
 	 * NOTE: the tlist order for a join rel will depend on which pair of outer
diff --git a/src/include/foreign/foreign.h b/src/include/foreign/foreign.h
index 71f8e55..fb945e9 100644
--- a/src/include/foreign/foreign.h
+++ b/src/include/foreign/foreign.h
@@ -65,21 +65,21 @@ typedef struct ForeignTable
 {
 	Oid			relid;			/* relation Oid */
 	Oid			serverid;		/* server Oid */
 	List	   *options;		/* ftoptions as DefElem list */
 } ForeignTable;
 
 
 extern ForeignServer *GetForeignServer(Oid serverid);
 extern ForeignServer *GetForeignServerByName(const char *name, bool missing_ok);
 extern UserMapping *GetUserMapping(Oid userid, Oid serverid);
-extern Oid GetUserMappingId(Oid userid, Oid serverid);
+extern Oid GetUserMappingId(Oid userid, Oid serverid, bool missing_ok);
 extern UserMapping *GetUserMappingById(Oid umid);
 extern ForeignDataWrapper *GetForeignDataWrapper(Oid fdwid);
 extern ForeignDataWrapper *GetForeignDataWrapperByName(const char *name,
 							bool missing_ok);
 extern ForeignTable *GetForeignTable(Oid relid);
 
 extern List *GetForeignColumnOptions(Oid relid, AttrNumber attnum);
 
 extern Oid	get_foreign_data_wrapper_oid(const char *fdwname, bool missing_ok);
 extern Oid	get_foreign_server_oid(const char *servername, bool missing_ok);