diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml
index 0e9ab2f19e..eb9870d9b3 100644
--- a/doc/src/sgml/ddl.sgml
+++ b/doc/src/sgml/ddl.sgml
@@ -1083,16 +1083,16 @@ CREATE TABLE order_items (
 
    <para>
     A foreign key must reference columns that either are a primary key or
-    form a unique constraint.  This means that the referenced columns always
-    have an index (the one underlying the primary key or unique constraint);
-    so checks on whether a referencing row has a match will be efficient.
-    Since a <command>DELETE</command> of a row from the referenced table
-    or an <command>UPDATE</command> of a referenced column will require
-    a scan of the referencing table for rows matching the old value, it
-    is often a good idea to index the referencing columns too.  Because this
-    is not always needed, and there are many choices available on how
-    to index, declaration of a foreign key constraint does not
-    automatically create an index on the referencing columns.
+    form a unique constraint, or are columns from a non-partial unique index.
+    This means that the referenced columns always have an index to allow
+    efficient lookups on whether a referencing row has a match.  Since a
+    <command>DELETE</command> of a row from the referenced table or an
+    <command>UPDATE</command> of a referenced column will require a scan of
+    the referencing table for rows matching the old value, it is often a good
+    idea to index the referencing columns too.  Because this is not always
+    needed, and there are many choices available on how to index, the
+    declaration of a foreign key constraint does not automatically create an
+    index on the referencing columns.
    </para>
 
    <para>
diff --git a/doc/src/sgml/ref/create_table.sgml b/doc/src/sgml/ref/create_table.sgml
index 1a37922b3a..21279b0788 100644
--- a/doc/src/sgml/ref/create_table.sgml
+++ b/doc/src/sgml/ref/create_table.sgml
@@ -1049,10 +1049,11 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       column(s) of some row of the referenced table.  If the <replaceable
       class="parameter">refcolumn</replaceable> list is omitted, the
       primary key of the <replaceable class="parameter">reftable</replaceable>
-      is used.  The referenced columns must be the columns of a non-deferrable
-      unique or primary key constraint in the referenced table.  The user
-      must have <literal>REFERENCES</literal> permission on the referenced table
-      (either the whole table, or the specific referenced columns).  The
+      is used.  Otherwise, the <replaceable class="parameter">refcolumn</replaceable>
+      list must refer to the columns of a non-deferrable unique or primary key
+      constraint or be the columns of a non-partial unique index.  The user
+      must have <literal>REFERENCES</literal> permission on the referenced
+      table (either the whole table, or the specific referenced columns).  The
       addition of a foreign key constraint requires a
       <literal>SHARE ROW EXCLUSIVE</literal> lock on the referenced table.
       Note that foreign key constraints cannot be defined between temporary
@@ -2177,6 +2178,16 @@ CREATE TABLE cities_partdef
    </para>
   </refsect2>
 
+  <refsect2>
+   <title>Foreign Key Constraints</title>
+
+   <para>
+    It is a <productname>PostgreSQL</productname> extension that a
+    foreign key constraint may reference columns of a unique index instead of
+    columns of a primary key or unique constraint.
+   </para>
+  </refsect2>
+
   <refsect2>
    <title><literal>NULL</literal> <quote>Constraint</quote></title>
 
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index ddc15c31c1..12b46b921a 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -10560,15 +10560,19 @@ transformFkeyGetPrimaryKey(Relation pkrel, Oid *indexOid,
 /*
  * transformFkeyCheckAttrs -
  *
- *	Make sure that the attributes of a referenced table belong to a unique
- *	(or primary key) constraint.  Return the OID of the index supporting
- *	the constraint, as well as the opclasses associated with the index
- *	columns.
+ *	Validate that the 'attnums' columns in the 'pkrel' relation are valid to
+ *	reference as part of a foreign key constraint.
+ *
+ *	Returns the OID of the unique index supporting the constraint and
+ *	populates the caller-provided 'opclasses' array with the opclasses
+ *	associated with the index columns.
+ *
+ *	Raises an ERROR on validation failure.
  */
 static Oid
 transformFkeyCheckAttrs(Relation pkrel,
 						int numattrs, int16 *attnums,
-						Oid *opclasses) /* output parameter */
+						Oid *opclasses)
 {
 	Oid			indexoid = InvalidOid;
 	bool		found = false;