diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 8fbef434f36..1c84756639d 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -26634,7 +26634,8 @@ postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
         <function>pg_relation_filepath</function>.  For a relation in the
         database's default tablespace, the tablespace can be specified as zero.
         Returns <literal>NULL</literal> if no relation in the current database
-        is associated with the given values.
+        is associated with the given values, or if dealing with a temporary
+        relation.
        </para></entry>
       </row>
      </tbody>
diff --git a/src/backend/utils/adt/dbsize.c b/src/backend/utils/adt/dbsize.c
index 9de2ed09d99..9f42a46f4fc 100644
--- a/src/backend/utils/adt/dbsize.c
+++ b/src/backend/utils/adt/dbsize.c
@@ -900,6 +900,9 @@ pg_relation_filenode(PG_FUNCTION_ARGS)
  *
  * We don't fail but return NULL if we cannot find a mapping.
  *
+ * Temporary relations are not detected, returning NULL (see
+ * RelidByRelfilenumber() for the reasons).
+ *
  * InvalidOid can be passed instead of the current database's default
  * tablespace.
  */
diff --git a/src/backend/utils/cache/relfilenodemap.c b/src/backend/utils/cache/relfilenodemap.c
index 56d7c73d339..6658f9bc473 100644
--- a/src/backend/utils/cache/relfilenodemap.c
+++ b/src/backend/utils/cache/relfilenodemap.c
@@ -132,6 +132,11 @@ InitializeRelfilenodeMap(void)
  * Map a relation's (tablespace, filenode) to a relation's oid and cache the
  * result.
  *
+ * A temporary relation may share its relfilenumber with a permanent relation
+ * or temporary relations created in other backends.  Being able to uniquely
+ * identify a temporary relation would require a backend's proc number, which
+ * we do not know about.  Hence, this function ignores this case.
+ *
  * Returns InvalidOid if no relation matching the criteria could be found.
  */
 Oid
@@ -211,6 +216,9 @@ RelidByRelfilenode(Oid reltablespace, Oid relfilenode)
 		{
 			Form_pg_class classform = (Form_pg_class) GETSTRUCT(ntp);
 
+			if (classform->relpersistence == RELPERSISTENCE_TEMP)
+				continue;
+
 			if (found)
 				elog(ERROR,
 					 "unexpected duplicate for tablespace %u, relfilenode %u",
diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out
index 3305c6ed24a..fcae98a7aff 100644
--- a/src/test/regress/expected/alter_table.out
+++ b/src/test/regress/expected/alter_table.out
@@ -3460,12 +3460,15 @@ SELECT conname as constraint, obj_description(oid, 'pg_constraint') as comment F
 -- filenode function call can return NULL for a relation dropped concurrently
 -- with the call's surrounding query, so ignore a NULL mapped_oid for
 -- relations that no longer exist after all calls finish.
+-- Temporary relations are ignored, as not supported by pg_filenode_relation().
 CREATE TEMP TABLE filenode_mapping AS
 SELECT
     oid, mapped_oid, reltablespace, relfilenode, relname
 FROM pg_class,
     pg_filenode_relation(reltablespace, pg_relation_filenode(oid)) AS mapped_oid
-WHERE relkind IN ('r', 'i', 'S', 't', 'm') AND mapped_oid IS DISTINCT FROM oid;
+WHERE relkind IN ('r', 'i', 'S', 't', 'm')
+  AND relpersistence != 't'
+  AND mapped_oid IS DISTINCT FROM oid;
 SELECT m.* FROM filenode_mapping m LEFT JOIN pg_class c ON c.oid = m.oid
 WHERE c.oid IS NOT NULL OR m.mapped_oid IS NOT NULL;
  oid | mapped_oid | reltablespace | relfilenode | relname 
diff --git a/src/test/regress/expected/create_table.out b/src/test/regress/expected/create_table.out
index a958b849799..a89a84bbb76 100644
--- a/src/test/regress/expected/create_table.out
+++ b/src/test/regress/expected/create_table.out
@@ -297,6 +297,18 @@ ERROR:  tables declared WITH OIDS are not supported
 -- but explicitly not adding oids is still supported
 CREATE TEMP TABLE withoutoid() WITHOUT OIDS; DROP TABLE withoutoid;
 CREATE TEMP TABLE withoutoid() WITH (oids = false); DROP TABLE withoutoid;
+-- temporary tables are ignored by pg_filenode_relation().
+CREATE TEMP TABLE relation_filenode_check(c1 int);
+SELECT relpersistence,
+  pg_filenode_relation (reltablespace, pg_relation_filenode(oid))
+  FROM pg_class
+  WHERE relname = 'relation_filenode_check';
+ relpersistence | pg_filenode_relation 
+----------------+----------------------
+ t              | 
+(1 row)
+
+DROP TABLE relation_filenode_check;
 -- check restriction with default expressions
 -- invalid use of column reference in default expressions
 CREATE TABLE default_expr_column (id int DEFAULT (id));
diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql
index b7aa3ee179f..fe59ff42546 100644
--- a/src/test/regress/sql/alter_table.sql
+++ b/src/test/regress/sql/alter_table.sql
@@ -2180,13 +2180,15 @@ SELECT conname as constraint, obj_description(oid, 'pg_constraint') as comment F
 -- filenode function call can return NULL for a relation dropped concurrently
 -- with the call's surrounding query, so ignore a NULL mapped_oid for
 -- relations that no longer exist after all calls finish.
+-- Temporary relations are ignored, as not supported by pg_filenode_relation().
 CREATE TEMP TABLE filenode_mapping AS
 SELECT
     oid, mapped_oid, reltablespace, relfilenode, relname
 FROM pg_class,
     pg_filenode_relation(reltablespace, pg_relation_filenode(oid)) AS mapped_oid
-WHERE relkind IN ('r', 'i', 'S', 't', 'm') AND mapped_oid IS DISTINCT FROM oid;
-
+WHERE relkind IN ('r', 'i', 'S', 't', 'm')
+  AND relpersistence != 't'
+  AND mapped_oid IS DISTINCT FROM oid;
 SELECT m.* FROM filenode_mapping m LEFT JOIN pg_class c ON c.oid = m.oid
 WHERE c.oid IS NOT NULL OR m.mapped_oid IS NOT NULL;
 
diff --git a/src/test/regress/sql/create_table.sql b/src/test/regress/sql/create_table.sql
index cc41f58ba22..fa198339ae9 100644
--- a/src/test/regress/sql/create_table.sql
+++ b/src/test/regress/sql/create_table.sql
@@ -304,6 +304,14 @@ CREATE TABLE withoid() WITH (oids = true);
 CREATE TEMP TABLE withoutoid() WITHOUT OIDS; DROP TABLE withoutoid;
 CREATE TEMP TABLE withoutoid() WITH (oids = false); DROP TABLE withoutoid;
 
+-- temporary tables are ignored by pg_filenode_relation().
+CREATE TEMP TABLE relation_filenode_check(c1 int);
+SELECT relpersistence,
+  pg_filenode_relation (reltablespace, pg_relation_filenode(oid))
+  FROM pg_class
+  WHERE relname = 'relation_filenode_check';
+DROP TABLE relation_filenode_check;
+
 -- check restriction with default expressions
 -- invalid use of column reference in default expressions
 CREATE TABLE default_expr_column (id int DEFAULT (id));