Only columns which Tableau recognizes are calculated fields in the data source. The original fields are just given a ‘?’ in field type. The original fields consists of both string, dates and doubles.
Though it is possible for me to open the Tableau data source and build a view using the original fields as well, it is not possible to use functions as ‘Ask Data’ as the Tableau Server does not recognize the field types in the data source. If data source is written to the tableau server by Tableau desktop client then there are no problems at all. But obviously I would like to use Knime to write data up.