Returns a reference to the group-by array for this query.
Because this method returns by reference, alter hooks may edit the group-by array directly to make their changes. If just adding additional grouping fields, however, the use of groupBy() is preferred.
Note that this method must be called by reference as well:
$fields =& $query
->getGroupBy();
A reference to the group-by array structure.
public function &getGroupBy();