diff --git a/src/backend/utils/cache/lsyscache.c b/src/backend/utils/cache/lsyscache.c
index 0c116b32ef..12b2532d95 100644
--- a/src/backend/utils/cache/lsyscache.c
+++ b/src/backend/utils/cache/lsyscache.c
@@ -1846,6 +1846,30 @@ get_rel_relkind(Oid relid)
 		return '\0';
 }
 
+/*
+ * get_rel_relispartition
+ *
+ *		Returns the relispartition flag associated with a given relation.
+ */
+bool
+get_rel_relispartition(Oid relid)
+{
+	HeapTuple	tp;
+
+	tp = SearchSysCache1(RELOID, ObjectIdGetDatum(relid));
+	if (HeapTupleIsValid(tp))
+	{
+		Form_pg_class reltup = (Form_pg_class) GETSTRUCT(tp);
+		bool		result;
+
+		result = reltup->relispartition;
+		ReleaseSysCache(tp);
+		return result;
+	}
+	else
+		return false;
+}
+
 /*
  * get_rel_tablespace
  *
diff --git a/src/backend/utils/cache/partcache.c b/src/backend/utils/cache/partcache.c
index e35a43405e..5757301d05 100644
--- a/src/backend/utils/cache/partcache.c
+++ b/src/backend/utils/cache/partcache.c
@@ -797,31 +797,38 @@ RelationGetPartitionQual(Relation rel)
  * get_partition_qual_relid
  *
  * Returns an expression tree describing the passed-in relation's partition
- * constraint. If there is no partition constraint returns NULL; this can
- * happen if the default partition is the only partition.
+ * constraint.
+ *
+ * If the relation is not found, or is not a partition, or there is no
+ * partition constraint, return NULL.  We must guard against the first two
+ * cases because this supports a SQL function that could be passed any OID.
+ * The last case can happen even if relispartition is true, when a default
+ * partition is the only partition.
  */
 Expr *
 get_partition_qual_relid(Oid relid)
 {
-	Relation	rel = heap_open(relid, AccessShareLock);
 	Expr	   *result = NULL;
-	List	   *and_args;
 
-	/* Do the work only if this relation is a partition. */
-	if (rel->rd_rel->relispartition)
+	/* Do the work only if this relation exists and is a partition. */
+	if (get_rel_relispartition(relid))
 	{
+		Relation	rel = relation_open(relid, AccessShareLock);
+		List	   *and_args;
+
 		and_args = generate_partition_qual(rel);
 
+		/* Convert implicit-AND list format to boolean expression */
 		if (and_args == NIL)
 			result = NULL;
 		else if (list_length(and_args) > 1)
 			result = makeBoolExpr(AND_EXPR, and_args, -1);
 		else
 			result = linitial(and_args);
-	}
 
-	/* Keep the lock. */
-	heap_close(rel, NoLock);
+		/* Keep the lock, to allow safe deparsing against the rel by caller. */
+		relation_close(rel, NoLock);
+	}
 
 	return result;
 }
@@ -845,7 +852,6 @@ generate_partition_qual(Relation rel)
 	MemoryContext oldcxt;
 	Datum		boundDatum;
 	bool		isnull;
-	PartitionBoundSpec *bound;
 	List	   *my_qual = NIL,
 			   *result = NIL;
 	Relation	parent;
@@ -859,8 +865,8 @@ generate_partition_qual(Relation rel)
 		return copyObject(rel->rd_partcheck);
 
 	/* Grab at least an AccessShareLock on the parent table */
-	parent = heap_open(get_partition_parent(RelationGetRelid(rel)),
-					   AccessShareLock);
+	parent = relation_open(get_partition_parent(RelationGetRelid(rel)),
+						   AccessShareLock);
 
 	/* Get pg_class.relpartbound */
 	tuple = SearchSysCache1(RELOID, RelationGetRelid(rel));
@@ -871,13 +877,17 @@ generate_partition_qual(Relation rel)
 	boundDatum = SysCacheGetAttr(RELOID, tuple,
 								 Anum_pg_class_relpartbound,
 								 &isnull);
-	if (isnull)
-		elog(ERROR, "null relpartbound for relation %u", RelationGetRelid(rel));
-	bound = castNode(PartitionBoundSpec,
-					 stringToNode(TextDatumGetCString(boundDatum)));
-	ReleaseSysCache(tuple);
+	if (!isnull)
+	{
+		PartitionBoundSpec *bound;
 
-	my_qual = get_qual_from_partbound(rel, parent, bound);
+		bound = castNode(PartitionBoundSpec,
+						 stringToNode(TextDatumGetCString(boundDatum)));
+
+		my_qual = get_qual_from_partbound(rel, parent, bound);
+	}
+
+	ReleaseSysCache(tuple);
 
 	/* Add the parent's quals to the list (if any) */
 	if (parent->rd_rel->relispartition)
@@ -903,7 +913,7 @@ generate_partition_qual(Relation rel)
 	MemoryContextSwitchTo(oldcxt);
 
 	/* Keep the parent locked until commit */
-	heap_close(parent, NoLock);
+	relation_close(parent, NoLock);
 
 	return result;
 }
diff --git a/src/include/utils/lsyscache.h b/src/include/utils/lsyscache.h
index e0eea2a0dc..8c57de77c0 100644
--- a/src/include/utils/lsyscache.h
+++ b/src/include/utils/lsyscache.h
@@ -128,6 +128,7 @@ extern char *get_rel_name(Oid relid);
 extern Oid	get_rel_namespace(Oid relid);
 extern Oid	get_rel_type_id(Oid relid);
 extern char get_rel_relkind(Oid relid);
+extern bool get_rel_relispartition(Oid relid);
 extern Oid	get_rel_tablespace(Oid relid);
 extern char get_rel_persistence(Oid relid);
 extern Oid	get_transform_fromsql(Oid typid, Oid langid, List *trftypes);
diff --git a/src/test/regress/expected/indexing.out b/src/test/regress/expected/indexing.out
index b9297c98d2..8ab543ae31 100644
--- a/src/test/regress/expected/indexing.out
+++ b/src/test/regress/expected/indexing.out
@@ -75,6 +75,25 @@ Indexes:
     "idxpart1_a_idx" btree (a)
     "idxpart1_b_c_idx" btree (b, c)
 
+\d+ idxpart1_a_idx
+                 Index "public.idxpart1_a_idx"
+ Column |  Type   | Key? | Definition | Storage | Stats target 
+--------+---------+------+------------+---------+--------------
+ a      | integer | yes  | a          | plain   | 
+Partition of: idxparti 
+No partition constraint
+btree, for table "public.idxpart1"
+
+\d+ idxpart1_b_c_idx
+                Index "public.idxpart1_b_c_idx"
+ Column |  Type   | Key? | Definition | Storage  | Stats target 
+--------+---------+------+------------+----------+--------------
+ b      | integer | yes  | b          | plain    | 
+ c      | text    | yes  | c          | extended | 
+Partition of: idxparti2 
+No partition constraint
+btree, for table "public.idxpart1"
+
 drop table idxpart;
 -- If a partition already has an index, don't create a duplicative one
 create table idxpart (a int, b int) partition by range (a, b);
diff --git a/src/test/regress/sql/indexing.sql b/src/test/regress/sql/indexing.sql
index 2091a87ff5..48b8853219 100644
--- a/src/test/regress/sql/indexing.sql
+++ b/src/test/regress/sql/indexing.sql
@@ -44,6 +44,8 @@ create table idxpart1 (like idxpart);
 \d idxpart1
 alter table idxpart attach partition idxpart1 for values from (0) to (10);
 \d idxpart1
+\d+ idxpart1_a_idx
+\d+ idxpart1_b_c_idx
 drop table idxpart;
 
 -- If a partition already has an index, don't create a duplicative one