We are going to develop another Action Plugin that will count incrementally archived data. We will truncate piwik_log_link_visit_action table after archiving and accepted incorrect unique visitor number (we will develop another method to count unique vistor).

But there is some case that
$rowFound->sumSubtable(Piwik_DataTable_Manager::getInstance()->getTable($idSubTable)); cause error because we can't find the Table of idSubTable because it's loaded yet.

To overcome this problem I have add a try catch statement surrounding.

$rowFound->sumSubtable(Piwik_DataTable_Manager::getInstance()->getTable($idSubTable));

But the better way could be change the sumSubtable function and it's parameter.