Currently datatables for Actions are indexed with the metrics string (nb_visits, nb_hits, etc.). See code in plugins/Actions/Actions.php archiveDay()
Other datatables are using integer indexes, mapping is defined in Piwik_Archive::$mappingFromIdToName
Actions datatable should use a similar integer mapping to reduce size of Actions datatables, and make querying faster.
thanks to halfdan run of xhprof, we now know that this is indeed very costly, lets fix it
(In ) Page titles report improvements fixes #1898
Improvements to integration tests