diff --git a/core/lib/Drupal/Core/Database/Driver/pgsql/Schema.php b/core/lib/Drupal/Core/Database/Driver/pgsql/Schema.php
index 07f504c..221e5bf 100644
--- a/core/lib/Drupal/Core/Database/Driver/pgsql/Schema.php
+++ b/core/lib/Drupal/Core/Database/Driver/pgsql/Schema.php
@@ -785,6 +785,9 @@ protected function _createKeys($table, $new_keys) {
     }
     if (isset($new_keys['indexes'])) {
       foreach ($new_keys['indexes'] as $name => $fields) {
+        // Even $new_keys is not a full schema it still has 'indexes' and so is
+        // a partial schema. Technically addIndex() doesn't do anything with it
+        // so passing an empty array would work as well.
         $this->addIndex($table, $name, $fields, $new_keys);
       }
     }