The list of functions that are delegated to a source depends on the datasource itself. Virtual DataPort populates the possible analytical, scalar and aggregate functions depends on the database adapter selected. You can check the list of functions that are delegated to a datasource by referring the ‘Source Configuration’ tab of the datasource. Since your source is a delimited file the source cannot perform rownumber() function. Thus, it can be concluded that Virtual DataPort can delegate analytical functions to a database, but cannot execute them. Therefore, if a query uses one of these functions and it cannot be delegated to a database, the query will fail.
Hope this helps!