A partial group by operator is a group by operator that implements a fallback mechanism. The fallback mechanism is triggered whenever memory pressure reaches a certain threshold. When the fallback mechanism is triggered, a row is included in an output of the partial group by operator without including an aggregation value for a grouping value for the row to an aggregation data structure. A final group by operator computes a final aggregate value of all results, including pre-grouped results and passed through results, from the partial group by operator.