diff --git a/src/backend/foreign/foreign.c b/src/backend/foreign/foreign.c
index 763ee7c..407dbf0 100644
--- a/src/backend/foreign/foreign.c
+++ b/src/backend/foreign/foreign.c
@@ -31,6 +31,7 @@
 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);
 
 /*
  * GetForeignDataWrapper -	look up the foreign-data wrapper by OID.
@@ -174,23 +175,7 @@ GetUserMapping(Oid userid, Oid serverid)
 	bool		isnull;
 	UserMapping *um;
 
-	tp = SearchSysCache2(USERMAPPINGUSERSERVER,
-						 ObjectIdGetDatum(userid),
-						 ObjectIdGetDatum(serverid));
-
-	if (!HeapTupleIsValid(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))));
+	tp = find_user_mapping(userid, serverid);
 
 	um = (UserMapping *) palloc(sizeof(UserMapping));
 	um->userid = userid;
@@ -213,6 +198,62 @@ GetUserMapping(Oid userid, Oid serverid)
 
 
 /*
+ * 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).
+ */
+Oid
+GetUserMappingId(Oid userid, Oid serverid)
+{
+	HeapTuple	tp;
+	Oid			umid;
+
+	tp = find_user_mapping(userid, serverid);
+
+	/* 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).
+ */
+static HeapTuple
+find_user_mapping(Oid userid, Oid serverid)
+{
+	HeapTuple	tp;
+
+	tp = SearchSysCache2(USERMAPPINGUSERSERVER,
+						 ObjectIdGetDatum(userid),
+						 ObjectIdGetDatum(serverid));
+
+	if (!HeapTupleIsValid(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))));
+
+	return tp;
+}
+
+
+/*
  * GetForeignTable - look up the foreign table definition by relation oid.
  */
 ForeignTable *
diff --git a/src/backend/optimizer/util/plancat.c b/src/backend/optimizer/util/plancat.c
index b04dc2e..6afbcea 100644
--- a/src/backend/optimizer/util/plancat.c
+++ b/src/backend/optimizer/util/plancat.c
@@ -28,6 +28,7 @@
 #include "catalog/dependency.h"
 #include "catalog/heap.h"
 #include "foreign/fdwapi.h"
+#include "foreign/foreign.h"
 #include "miscadmin.h"
 #include "nodes/makefuncs.h"
 #include "optimizer/clauses.h"
@@ -383,12 +384,20 @@ get_relation_info(PlannerInfo *root, Oid relationObjectId, bool inhparent,
 	/* Grab foreign-table info using the relcache, while we have it */
 	if (relation->rd_rel->relkind == RELKIND_FOREIGN_TABLE)
 	{
+		RangeTblEntry *rte;
+		Oid		userid;
+
 		rel->serverid = GetForeignServerIdByRelId(RelationGetRelid(relation));
 		rel->fdwroutine = GetFdwRoutineForRelation(relation, true);
+
+		rte = planner_rt_fetch(rel->relid, root);
+		userid = OidIsValid(rte->checkAsUser) ? rte->checkAsUser : GetUserId();
+		rel->umid = GetUserMappingId(userid, rel->serverid);
 	}
 	else
 	{
 		rel->serverid = InvalidOid;
+		rel->umid = InvalidOid;
 		rel->fdwroutine = NULL;
 	}
 
diff --git a/src/backend/optimizer/util/relnode.c b/src/backend/optimizer/util/relnode.c
index 1d635cd..f8abab4 100644
--- a/src/backend/optimizer/util/relnode.c
+++ b/src/backend/optimizer/util/relnode.c
@@ -122,6 +122,7 @@ build_simple_rel(PlannerInfo *root, int relid, RelOptKind reloptkind)
 	rel->subroot = NULL;
 	rel->subplan_params = NIL;
 	rel->serverid = InvalidOid;
+	rel->umid = InvalidOid;
 	rel->fdwroutine = NULL;
 	rel->fdw_private = NULL;
 	rel->baserestrictinfo = NIL;
@@ -385,6 +386,7 @@ build_join_rel(PlannerInfo *root,
 	joinrel->subroot = NULL;
 	joinrel->subplan_params = NIL;
 	joinrel->serverid = InvalidOid;
+	joinrel->umid = InvalidOid;
 	joinrel->fdwroutine = NULL;
 	joinrel->fdw_private = NULL;
 	joinrel->baserestrictinfo = NIL;
@@ -395,12 +397,19 @@ build_join_rel(PlannerInfo *root,
 
 	/*
 	 * Set up foreign-join fields if outer and inner relation are foreign
-	 * tables (or joins) belonging to the same server.
+	 * 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.
 	 */
 	if (OidIsValid(outer_rel->serverid) &&
-		inner_rel->serverid == outer_rel->serverid)
+		inner_rel->serverid == outer_rel->serverid &&
+		OidIsValid(outer_rel->umid) &&
+		inner_rel->umid == outer_rel->umid)
 	{
 		joinrel->serverid = outer_rel->serverid;
+		joinrel->umid = outer_rel->umid;
 		joinrel->fdwroutine = outer_rel->fdwroutine;
 	}
 
diff --git a/src/include/foreign/foreign.h b/src/include/foreign/foreign.h
index c820e09..3e450d6 100644
--- a/src/include/foreign/foreign.h
+++ b/src/include/foreign/foreign.h
@@ -71,6 +71,7 @@ typedef struct 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 ForeignDataWrapper *GetForeignDataWrapper(Oid fdwid);
 extern ForeignDataWrapper *GetForeignDataWrapperByName(const char *name,
 							bool missing_ok);
diff --git a/src/include/nodes/relation.h b/src/include/nodes/relation.h
index 279051e..2d6c313 100644
--- a/src/include/nodes/relation.h
+++ b/src/include/nodes/relation.h
@@ -469,6 +469,7 @@ typedef struct RelOptInfo
 
 	/* Information about foreign tables and foreign joins */
 	Oid			serverid;		/* identifies server for the table or join */
+	Oid			umid;			/* identifies user mapping for the table or join */
 	/* use "struct FdwRoutine" to avoid including fdwapi.h here */
 	struct FdwRoutine *fdwroutine;
 	void	   *fdw_private;