presto-main/src/main/java/io/prestosql/operator/aggregation/multimapagg/SingleMultimapAggregationState.java/SingleMultimapAggregationState/reset
class SingleMultimapAggregationState:    @Override
    public void reset()
    {
        // Single aggregation state is used as scratch state in group accumulator.
        // Thus reset() will be called for each group (via MultimapAggregationStateSerializer#deserialize)
        keyBlockBuilder = keyBlockBuilder.newBlockBuilderLike(null);
        valueBlockBuilder = valueBlockBuilder.newBlockBuilderLike(null);
    }