diff --git a/core/lib/Drupal/Core/Database/Driver/pgsql/Upsert.php b/core/lib/Drupal/Core/Database/Driver/pgsql/Upsert.php
index 29249e57a4..5f60c05fff 100644
--- a/core/lib/Drupal/Core/Database/Driver/pgsql/Upsert.php
+++ b/core/lib/Drupal/Core/Database/Driver/pgsql/Upsert.php
@@ -112,6 +112,8 @@ public function __toString() {
 
     $update = [];
     foreach ($insert_fields as $field) {
+      // The "excluded." prefix causes the field to refer to the value for field
+      // that would have been inserted had there been no conflict.
       $update[] = "$field = EXCLUDED.$field";
     }