public UnsafeRow getAggregationBuffer(InternalRow groupingKey) {
        final UnsafeRow unsafeGroupingKeyRow = this.groupingKeyProjection.apply(groupingKey);

        return getAggregationBufferFromUnsafeRow(unsafeGroupingKeyRow);
    }
Return the aggregation buffer for the current group. For efficiency, all calls to this method
return the same object. If additional memory could not be allocated, then this method will
signal an error by returning null.
indexr-query-opt/src/main/java/org/apache/spark/sql/execution/UnsafeFixedWidthAggregationMap.java