@Override
            public UnsafeRow getKey() {
                return key;
            }
Returns an iterator over the keys and values in this map. This uses destructive iterator of
BytesToBytesMap. So it is illegal to call any other method on this map after `iterator()` has
been called.

For efficiency, each call returns the same object.
indexr-query-opt/src/main/java/org/apache/spark/sql/execution/UnsafeFixedWidthAggregationMap.java