Update more comments for 96198d94.

Etsuro Fujita, reviewed (though not completely endorsed) by Ashutosh
Bapat, and slightly expanded by me.

contrib/postgres_fdw/postgres_fdw.c

@@ -3349,10 +3349,8 @@ postgresImportForeignSchema(ImportForeignSchemaStmt *stmt, Oid serverOid)
  *
  * 1) Join type is INNER or OUTER (one of LEFT/RIGHT/FULL)
  * 2) Both outer and inner portions are safe to push-down
- * 3) All foreign tables in the join belong to the same foreign server and use
- *	  the same user mapping.
- * 4) All join conditions are safe to push down
- * 5) No relation has local filter (this can be relaxed for INNER JOIN, if we
+ * 3) All join conditions are safe to push down
+ * 4) No relation has local filter (this can be relaxed for INNER JOIN, if we
  *	  can move unpushable clauses upwards in the join tree).
  */
 static bool

src/backend/optimizer/path/joinpath.c

@@ -213,7 +213,8 @@ add_paths_to_joinrel(PlannerInfo *root,
 
 	/*
 	 * 5. If inner and outer relations are foreign tables (or joins) belonging
-	 * to the same server, give the FDW a chance to push down joins.
+	 * to the same server and using the same user mapping, give the FDW a
+	 * chance to push down joins.
 	 */
 	if (joinrel->fdwroutine &&
 		joinrel->fdwroutine->GetForeignJoinPaths)

src/include/nodes/relation.h

@@ -448,9 +448,11 @@ typedef struct PlannerInfo
  *		in just as for a baserel, except we don't bother with lateral_vars.
  *
  * If the relation is either a foreign table or a join of foreign tables that
- * all belong to the same foreign server, these fields will be set:
+ * all belong to the same foreign server and use the same user mapping, these
+ * fields will be set:
  *
  *		serverid - OID of foreign server, if foreign table (else InvalidOid)
+ *		umid - OID of user mapping, if foreign table (else InvalidOid)
  *		fdwroutine - function hooks for FDW, if foreign table (else NULL)
  *		fdw_private - private state for FDW, if foreign table (else NULL)
  *