diff --git a/doc/src/sgml/func/func-subquery.sgml b/doc/src/sgml/func/func-subquery.sgml
index a9f2b12e48c..f954f3bf133 100644
--- a/doc/src/sgml/func/func-subquery.sgml
+++ b/doc/src/sgml/func/func-subquery.sgml
@@ -70,8 +70,14 @@ EXISTS (<replaceable>subquery</replaceable>)
    and not on the contents of those rows, the output list of the
    subquery is normally unimportant.  A common coding convention is
    to write all <literal>EXISTS</literal> tests in the form
-   <literal>EXISTS(SELECT 1 WHERE ...)</literal>.  There are exceptions to
-   this rule however, such as subqueries that use <token>INTERSECT</token>.
+   <literal>EXISTS(SELECT * FROM ... WHERE ...)</literal>, another common
+   convention is to write <literal>EXISTS(SELECT 1 FROM ... WHERE
+   ...)</literal> or some other dummy constant.  These conventions are
+   actually equivalent in <productname>PostgreSQL</productname>, which
+   will optimize away evaluation of the subquery's output list altogether
+   when it cannot affect the number of rows returned.  (An example
+   that cannot be optimized away is an output list containing a
+   set-returning function, since the function might return zero rows.)
   </para>
 
   <para>
diff --git a/src/backend/optimizer/plan/subselect.c b/src/backend/optimizer/plan/subselect.c
index e6bc7023562..d7f3cedf3d5 100644
--- a/src/backend/optimizer/plan/subselect.c
+++ b/src/backend/optimizer/plan/subselect.c
@@ -1643,7 +1643,13 @@ convert_EXISTS_sublink_to_join(PlannerInfo *root, SubLink *sublink,
  * Note: by suppressing the targetlist we could cause an observable behavioral
  * change, namely that any errors that might occur in evaluating the tlist
  * won't occur, nor will other side-effects of volatile functions.  This seems
- * unlikely to bother anyone in practice.
+ * unlikely to bother anyone in practice.  Note that any column privileges are
+ * still checked even if the reference is removed here.
+ *
+ * The SQL standard specifies that a SELECT * immediately inside EXISTS
+ * expands to not all columns but an arbitrary literal.  That is kind of the
+ * same idea, but our optimization goes further in that it throws away the
+ * entire targetlist, and not only if it was written as *.
  *
  * Returns true if was able to discard the targetlist, else false.
  */