How do you decide whether you need ti do aggregations at database level or at

It depends upon our requirment only.If you have good processing database you can create aggregation table or view at database level else its better to use informatica. Here i'm explaing why we need to use informatica.
what ever it may be informatica is a thrid party tool, so it will take more time to process aggregation compared to the database, but in Informatica an option we called "Incremental aggregation" which will help you to update the current values with current values +new values. No necessary to process entire values again and again. Unless this can be done if nobody deleted that cache files. If that happend total
aggregation we need to execute on informatica also.
In database we don't have Incremental aggregation facility.