US Patent 7469241 Efficient data aggregation operations using hash tables

Hash aggregation operates in two phases. In the first phase, data read from underlying row source is partitioned on GROUP BY keys with on-the-fly aggregation. It is determined into which partition an entry corresponding to the incoming record should be grouped, and whether a matching entry is already present in the partition, using a hash function and a hash table built for the partition. For aggregation operations, if a match is found, then a measure value from the incoming record is aggregated with a measure value for the entry existing in the partition. If there is not enough system volatile memory to complete the first phase, then a partition is selected for storing persistently. Thus, only one partition at a time is stored persistently, as necessary. In the second phase, one persistently stored partition is processed at a time, whereby aggregation of the data in that partition is completed and results are returned.