And so the leak was then passing the adapter as a constructor argument to the NativePDOIterator, which in place turned not necessary after all.

Author: imota

src/Factory.php

@@ -16,7 +16,7 @@ public function create(\PDO $adapter, $query, $blockSize)
         $adapter->setAttribute(\PDO::ATTR_DEFAULT_FETCH_MODE, \PDO::FETCH_ASSOC);
         $adapter->setAttribute(
             \PDO::ATTR_STATEMENT_CLASS,
-            [NativePDOIterator::class, [$adapter]]
+            [NativePDOIterator::class, []]
         );
         /** @var Iterator $statement */
         $statement = $adapter->query($query);

src/NativePDOIterator.php

@@ -4,10 +4,8 @@
 
 class NativePDOIterator extends \PDOStatement implements Iterator
 {
-    protected $pdo;
-
-    protected function __construct($pdo) {
-        $this->pdo = $pdo;
+    protected function __construct()
+    {
     }
 
     public function count()
@@ -18,6 +16,5 @@ public function count()
     public function close()
     {
         parent::closeCursor();
-        $this->pdo = null;
     }
 }