I extended datagrid.py and realized two new columns for Dashboard: 
Review Groups and Review People. In our case all requests are assigned 
to a team group. Therefore the People column helps my team to see 
which requests are not yet processed. 

Both columns are sortable. In order to allow sorting I had to disable optimization. This will be only done if People or Group is actually used for sorting.
It turned out the current implementation relies on optimization. I had to make sure that columns are always added - not only with the query is optimized.