Pavan,
It is quite complex to get the output as an itab when you use the function module. You could instead - look at this how to paper by Durairaj Athavan Raja
http://wiki.scn.sap.com/wiki/display/BI/Expose+BI+Query+as+RESTful+Service
follow the code - you will find that the code actually merges the tables together. This can be a good starting point.... or you can look at taking the xml output and formatting back to an itab ...?