How to reference summary colum for same data set in BI Publisher's data model?

 In BI Publisher, it do not support add element by PL/SQL in the data set. In order to add this kind of element (column) in data set, we should create function in the report package and call this function in the SQL of the data set. Now we the function created in above is required to refer the summary column in the same data set. I find that the column summary in the SQL is consider as null.
 For example,
 
 The below is the data model created for test. We use the :CS_PREFIX_COUNT in the SQL of data set to get the value of summary column, CS_PREFIX_COUNT.