How to improve derived table performane

Hi,

We are using SAP BusinessObjects Enterprise XI 3.1 SP3  for web intelligence reporting

We have table which has all the assignments log of incidents in our database

Now we want the minimum last modified date of a particular incident. I have created a derived table in the universe withe below query

"Select min(Last_Modified_Date) as Last_Modified_Date,Incident_number from table1 group by Incident_Number"

the query is working fine I am able to get the min last modified date for an incident. But when I use these fields from the derived table along with the other fields from the universe the query takes long time to execute.

Please help me in this issue

Regards

Pavithra P