A major extension of LDL++, user-defined aggregates [ZAO93] has been
recently implemented at UCLA.
Rather than waiting for the result till the end of the set, partial results
could be shown to the user using early returns. The following example
returns average every 10
element.
Early returns allow the support of online aggregation in LDL++.
This implementation of user-defined aggregates has shown successful results in terms of ease of use and strong expressive power. A few examples that have been successfully implemented over this extension are:
One can refer to http://www.cs.ucla.edu/ 
czdemo/javademo for details.