Abstract-Data summarization queries that compute aggregates by grouping datasets across several dimensions are essential to help users make sense of very large datasets. In this work, we focus on ROLLUP, an important operator that has been recently added to the Hadoop MapReduce ecosystem. However, its current implementation suffers from very large communication costs, leading to inefficient executions. We thus proceed with the design of a new ROLLUP operator for highlevel languages. Our operator is self-optimizing, which means that it automatically performs load-balancing and determines a suitable operating point to achieve the highest performance. We have implemented our ROLLUP operator for Apache Pig, a popular high-level language in the Hadoop ecosystem. Our experimental results, obtained on both synthetic and real datasets, indicate that our new operator outperforms the current ROLLUP implementation in Pig by at least 50%.