Index: wp-admin/includes/upgrade.php
===================================================================
--- wp-admin/includes/upgrade.php	(revision 12559)
+++ wp-admin/includes/upgrade.php	(working copy)
@@ -1692,6 +1692,7 @@
 		if ( !empty($delete_options) ) {
 			$delete_options = implode(',', $delete_options);
 			$wpdb->query("DELETE FROM $wpdb->options WHERE option_id IN ($delete_options)");
+			unset( $delete_options );
 		}
 
 		// Drop the old primary key and add the new.